# DC Wh meter



## etischer (Jun 16, 2008)

Nice find =) Looks like it can even drive an analog gas gauge. Think I might give it a try. Doesn't look like the WHr counting is bi-directional though. Current looks like it is [0 to XXXX] not [-XXXX to +XXXX]


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## dimitri (May 16, 2008)

This is nice, but TBS Gauge is much more professionally looking and fits standard gauge panels. It costs $250 though, but its much better looking device.

Here is the link http://www.evolveelectrics.com/E-Xpert%20Pro.html


Good to have options though, thanks for sharing.


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## etischer (Jun 16, 2008)

Looks like it only goes up to 175 volts. Have you used this (and their 1:5 prescaler voltage divider box) before? I might give them a ring to see what can be done to read 0-400v. 



dimitri said:


> This is nice, but TBS Gauge is much more professionally looking and fits standard gauge panels. It costs $250 though, but its much better looking device.
> 
> Here is the link http://www.evolveelectrics.com/E-Xpert%20Pro.html
> 
> ...


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## dimitri (May 16, 2008)

etischer said:


> Looks like it only goes up to 175 volts. Have you used this (and their 1:5 prescaler voltage divider box) before? I might give them a ring to see what can be done to read 0-400v.


I have not used it myself, but Tomofreno does and I like his feedback in his Swift-E thread.

I am developing similar EV Display to compliment my MiniBMS and I came to a conclusion that pack voltage is basically irrelevant, assuming you have a healthy pack ( BMS will tell you if you don't ), so display can just monitor 4 cells, and be powered by them at the same time, then just use the math to figure pack voltage based on cell count. This greatly simplifies display's electronics, no need for DC-DC or hardware scaling, its all in the software now. Display has a setup screen where user puts cell count and cell size, so I can figure SoC percentage.

I just put my prototype EV Display in my car last night and this morning trip produced excellent results. Calculated pack voltage was precise match with actual pack voltage, Amp in/out reading was perfect and so was AH count. I will keep logging my usual trips, while adding new software features.


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## Travdude (May 11, 2009)

Belktronix has a 10:1 pre scaler for voltage up to 370V.

http://www.belktronix.com/LinkProAdapter.html

I am using one with my Link-pro meter, which appears to be the same thing as the E-expert. Works great.


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## tomofreno (Mar 3, 2009)

> I am using one with my Link-pro meter, which appears to be the same thing as the E-expert


 Correct. Xantrex used to be the only U.S. dealer for TBS and did not sell the voltage adapter. Now there is the dealer Dimitri gave. I really like the TBS, and use it for my fuel gauge. "Empty" can be defined as any soc you like. I set mine as 35% soc. The five bars on the display each represent 1/5 of total charge to this empty soc, so it is similar to watching a gas gauge: 







It also reads out Ah used, soc, and voltage by toggling through the display. 

There is also this: http://www.ebikes.ca/drainbrain.shtml (scroll down to the "Large display/high current version) which reads out Wh as well as Ah. I would like to have Wh since this is the quantity being used to move the vehicle, and varies a bit relative to Ah with soc (charge falls through a greater potential during discharge at higher soc). The change in voltage with soc for LiFePO4 is quite small though so Ah seems to work quite well. I use about 1.6 to 1.8 Ah/mile typically, and I seem to get about 13 miles per bar on the display for mixed highway/secondary driving, with very little variation between high and low soc (not readily noticeable). So about 65 miles to 35% soc.


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## jackbauer (Jan 12, 2008)

Victron bmv-600hs goes from 70 to 350v. Bit pricey but it works very well for me.


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## azdeltawye (Dec 30, 2008)

dimitri said:


> This is nice, but TBS Gauge is much more professionally looking and fits standard gauge panels.....


I agree the form factor is not the best for integrating into a dashboard but the problem with the TBS meter is that it only counts Ah not Wh. Using a Watt-hour (energy) meter as a ‘gas gauge’ is much more accurate that just counting Amp-hours. 

Here is an example using the Thunder Sky 160Ah LiFePO4 published performance curves. Since this data was taken using a constant current load, the power delivered by the battery is proportional to the voltage profile in the graph. The total energy can then be calculated by integrating the power over time. This is shown by the area under the discharge curve. 











You can readily see from the 1C discharge curve that even just using the middle 80% of the total cell capacity demonstrates how much more energy is available at the beginning of the discharge cycle than at the end. This difference gets larger the more you push the cell to its capacity limits, especially if you start out fully charged.

So just using the Amp-hour meter would give you a false indication of how much energy (range) you had left in your pack. By using this graph as an example, when the Ah meter indicated 50% you would have actually used almost 60% of the energy stored in your pack. This could get you into trouble if you were using an Amp-hour gas gauge to determine the point of no return on a long trip…


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## dimitri (May 16, 2008)

> So just using the Amp-hour meter would give you a false indication of how much energy (range) you had left in your pack.


You are correct of course, but with such flat discharge curve the error would be what 5% ? Typically we limit our usage to 70%-80% , so the error is not that significant and can be accounted for by setting a little more conservative SoC range limit.

But, that is one of the reasons I am developing my own display, so I can work with any parameters I want and be able to change the software anytime I find a bug or a new feature. I will implement both AH and WH counting and see which one I like better, both would be available, just flip between screens with a press of a button.


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## Dave Koller (Nov 15, 2008)

dimitri said:


> You are correct of course, but with such flat discharge curve the error would be what 5% ? Typically we limit our usage to 70%-80% , so the error is not that significant and can be accounted for by setting a little more conservative SoC range limit.
> 
> But, that is one of the reasons I am developing my own display, so I can work with any parameters I want and be able to change the software anytime I find a bug or a new feature. I will implement both AH and WH counting and see which one I like better, both would be available, just flip between screens with a press of a button.


That is the basic premise of my software on the Atmel...
Using volts,amps, and time (all both in and out) gives one an idea of consumption and replacement parameters. The algorithm seems to hold up in external tests on bench... Now to see the real world... I agree on being able to do it this way as change is always easy in software...


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## dimitri (May 16, 2008)

Dave Koller said:


> That is the basic premise of my software on the Atmel...
> Using volts,amps, and time (all both in and out) gives one an idea of consumption and replacement parameters. The algorithm seems to hold up in external tests on bench... Now to see the real world... I agree on being able to do it this way as change is always easy in software...


After few weeks of bench testing I also was nervous about EV test since all the EMI issues I had to deal with on PakTrakr. However, it seems that my worry was for nothing, I did several runs today, including partial charges and AH counter appears to be working very well, even exceeding my expectations. Now I just need to make the displays pretty and polish user interface, but data collection and processing is already working well.


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## tomofreno (Mar 3, 2009)

> You can readily see from the 1C discharge curve that even just using the middle 80% of the total cell capacity demonstrates how much more energy is available at the beginning of the discharge cycle than at the end.


 This is a strange looking discharge curve to me. Looks more like a charge curve. Jack Rickard recently posted some 1C discharge curves from TS160Ah, TS200Ah, and SE180Ah on his blog. They look like what I see. The voltage remains pretty flat until down to 25 - 30% soc. So I don't think there is much error using only Ah, though I agree the Wh vary with soc.


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## EVfun (Mar 14, 2010)

A watt hour meter would be nice but I think a simple and reasonably priced amp hour meter that counts both directions would be more important.


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## JRP3 (Mar 7, 2008)

Hall effect too so there are 2 fewer connections


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## Tesseract (Sep 27, 2008)

dimitri said:


> ...
> But, that is one of the reasons I am developing my own display, ...


Ooo... sounds interesting, dimitri! The Dutch dudes are very keen to get a better SoC indication than what we cobbled together inside the Soliton1... Keep us updated for sure.


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## Dave Koller (Nov 15, 2008)

EVfun said:


> A watt hour meter would be nice but I think a simple and reasonably priced amp hour meter that counts both directions would be more important.


Agree - it HAS to be both directions but.. should have voltage figured in the algorithm as an inclusive constant for a good SOC..

I'm not sure of Dimitri's display but with programed output one can go to VB screen displays or simple LCD screens... or even Leds - and changing parameters is all quickly done with software enabled push buttons or more elaborate keypads...

Price is not so high - DIY kit might be nice - Maybe not ..........


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## Dave Koller (Nov 15, 2008)

Lol just saw this on another thread...

"Its based on Arduino and uses 2x8 character LCD display and hall effect sensor to measure AH in and out, along with pack voltage and temperature, so AH can be temperature compensated, to show true fuel gauge in summer and winter.

I already have a working prototype, just polishing some code before I put it in the car for road testing. I'll probably start a thread on it once I have something to show."

So now I know what display you have Dimitri! That is a great platform to use . Arduino has a great little C language.. I use the same Atmel chip and my own kludge of C and assembly...

Easy to wifi from that chip (add on) to a host of devices for telemetry.
Nice serial out for other program monitors...


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## RechargeCar Inc. (Aug 6, 2009)

I should mention that we are working on exactly what you guys are talking about:

http://www.rechargecar.com/product/autoblock-amp

In addition to the 'OpenAPI' USB connection, there will be other interface options so people can hook it up to whatever they want. Sorry for the commercial nature of this post, but we are attempting to drum up interest in a more collaborative approach to EV system development.


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## Dave Koller (Nov 15, 2008)

RechargeCar Inc. said:


> I should mention that we are working on exactly what you guys are talking about:
> 
> http://www.rechargecar.com/product/autoblock-amp
> 
> In addition to the 'OpenAPI' USB connection, there will be other interface options so people can hook it up to whatever they want. Sorry for the commercial nature of this post, but we are attempting to drum up interest in a more collaborative approach to EV system development.


"Plays well with others" ....


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## Tesseract (Sep 27, 2008)

RechargeCar Inc. said:


> ...
> In addition to the 'OpenAPI' USB connection, there will be other interface options so people can hook it up to whatever they want....


My $0.02: USB has no place in a vehicle, much less an electric vehicle... It's a chintzy unipolar synchronous interface "standard" that has undergone, what?, 4 major revisions since it's introduction. It has no tolerance of common mode noise, cable lengths are strictly limited, and the connectors do not positively lock. I pretty much can't stand USB (except on my computer, where it does a great job interfacing my mouse and thumb drives).

Instead, I strongly recommend ethernet (of course) or CAN.


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## JRP3 (Mar 7, 2008)

USB can be pretty snug, and it wouldn't be that hard to make an external positive locking system, a wire bail that provides pressure or something. Of course that doesn't address your other concerns with USB.


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## EVfun (Mar 14, 2010)

Interesting product RechargeCar. Please tell me more about the secret analog output. I think the ability to drive a standard digital panel meter (+/- 200 mv signal) would be slick. 

I have a volt and amp meter for my Datsun but would rather have a volt and amp-hour meter. I don't really use an amp meter, I know what I just did with my right foot.


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## RechargeCar Inc. (Aug 6, 2009)

Thanks Dave! We like to think so. 

Regarding USB: I understand your concern. We went this route due to the ubiquity of interfaces and cables, so software development can happen faster (especially with an Open standard), and so people can build the EV system they want by plugging in whatever 'Blocks' they need to measure specific EV information.

However, going with USB requires diligent electrical and mechanical engineering. After testing our AutoBlock RPM in high EMI environments for quite a while now, we believe we are good to go.

Back to the issue at hand though: Our AutoBlock AMP will have a secondary output - maybe to spoof a current shunt or something. (still working on that part.)

Thanks for the feedback guys,

Josh


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## dimitri (May 16, 2008)

Tesseract said:


> Ooo... sounds interesting, dimitri! The Dutch dudes are very keen to get a better SoC indication than what we cobbled together inside the Soliton1... Keep us updated for sure.


I'm surprised how easy it was to integrate Amp measurements over time using Atmel in the Arduino and how accurate it turned out. If I did it in a couple of weeks, I'm sure Qer can integrate this type of code into Soliton1 without much trouble, unless you are running out of CPU or RAM resources 

I have been running the prototype for 2 days and I already stopped looking at the PakTrakr. Can't wait to rip the Paktrakr from the car


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## Dave Koller (Nov 15, 2008)

dimitri said:


> I'm surprised how easy it was to integrate Amp measurements over time using Atmel in the Arduino and how accurate it turned out. If I did it in a couple of weeks, I'm sure Qer can integrate this type of code into Soliton1 without much trouble, unless you are running out of CPU or RAM resources
> 
> I have been running the prototype for 2 days and I already stopped looking at the PakTrakr. Can't wait to rip the Paktrakr from the car


Great Dimitri! What chip are you using? _ATmega328? it has __32 KB __flash and a great __2KB ram area and _EEPROM 1 KB - At 16 Mhz you have plenty of speed!


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## dimitri (May 16, 2008)

Dave Koller said:


> Great Dimitri! What chip are you using? _ATmega328? it has __32 KB __flash and a great __2KB ram area and _EEPROM 1 KB - At 16 Mhz you have plenty of speed!


Yes, I am using Arduino Nano v3 with Atmega328. It has more than enough resources for my needs. 

Here is a sneak peak at my prototype EV Display. I am idling the motor at 12 Amp and the pack has 154AH remaining. 

My real challenge is how to package the final product. I want it to fit standard automotive gauge pods, that is why I selected 2x8 LCD. I know I can make it small enough, but I need to find appropriate case and make it look good. I'm thinking of finding a cheap gauge just to gut it and put my stuff inside, then I just need to make a new face cover. If anyone has other ideas, I'm all ears.


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## tomofreno (Mar 3, 2009)

Nice work Dimitri! So you won't miss the cell voltage bar graph on the Paktrakr? If it was so easy to integrate current, how about also monitoring voltage, taking the product of it and current, and integrating over time to get Wh? I would like to see how the change in Wh and Ah compare over time.


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## Dave Koller (Nov 15, 2008)

dimitri said:


> Yes, I am using Arduino Nano v3 with Atmega328. It has more than enough resources for my needs.
> 
> Here is a sneak peak at my prototype EV Display. I am idling the motor at 12 Amp and the pack has 154AH remaining.
> 
> ...


I think you could cut out a front plate of plastic and stick it in one of those mounting cases that hang on the side post - I think Tom has one for his TBS...
What are you using for pickups (current, Volts) - I see the small pots on there - It looks nice and small and powerful ... Fun to program... once you get everything you want in it you can program the chip and drop it in a board without USB - bet you can get that down small ...


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## dimitri (May 16, 2008)

tomofreno said:


> Nice work Dimitri! So you won't miss the cell voltage bar graph on the Paktrakr? If it was so easy to integrate current, how about also monitoring voltage, taking the product of it and current, and integrating over time to get Wh? I would like to see how the change in Wh and Ah compare over time.


That is exactly what I am working on now. Now that I have the framework of data collection and processing done, I can focus on various displays and secondary data. I should have it all done in a few days. I will have a screen with WH data and screen with AH data and a screen with voltage and temperature and a screen with SoC bar graph and percentages.


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## dimitri (May 16, 2008)

Dave Koller said:


> I think you could cut out a front plate of plastic and stick it in one of those mounting cases that hang on the side post - I think Tom has one for his TBS...
> What are you using for pickups (current, Volts) - I see the small pots on there - It looks nice and small and powerful ... Fun to program... once you get everything you want in it you can program the chip and drop it in a board without USB - bet you can get that down small ...


Yes, I was planning to just use Atmel alone in a final product, to make it smaller and fit into 2" gauge pod. It would also cut the cost down. I want to make it very affordable for DIY folks.


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## JRP3 (Mar 7, 2008)

dimitri said:


> My real challenge is how to package the final product. I want it to fit standard automotive gauge pods, that is why I selected 2x8 LCD. I know I can make it small enough, but I need to find appropriate case and make it look good. I'm thinking of finding a cheap gauge just to gut it and put my stuff inside, then I just need to make a new face cover. If anyone has other ideas, I'm all ears.


You might be better off just making it as is and let the consumer figure out the mounting if you can't find a cheap housing. Some might want to integrate it into existing panels anyway.


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## Tesseract (Sep 27, 2008)

dimitri said:


> Yes, I was planning to just use Atmel alone in a final product, to make it smaller and fit into 2" gauge pod. It would also cut the cost down. I want to make it very affordable for DIY folks.


Check this company out, D.; they make empty automotive gauges for you to stuff your own junk into:

http://www.imtronics.com/housings.htm


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## dimitri (May 16, 2008)

Tesseract said:


> Check this company out, D.; they make empty automotive gauges for you to stuff your own junk into:
> 
> http://www.imtronics.com/housings.htm


Wow, local company making exactly the product I am looking for! Jeff, you rock as always! Thanks a bunch! I will call them tomorrow.


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## Dave Koller (Nov 15, 2008)

Nice find! Still wondering what you used to interface current to the Arduino ?
I used a Hass 600 and that works well with a minimum of fuss 
I think you have another good product for DIY..... !


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## Mesuge (Mar 6, 2008)

tomofreno said:


> Correct. Xantrex used to be the only U.S. dealer for TBS and did not sell the voltage adapter. Now there is the dealer Dimitri gave. I really like the TBS, and use it for my fuel gauge. "Empty" can be defined as any soc you like. I set mine as 35% soc. The five bars on the display each represent 1/5 of total charge to this empty soc, so it is similar to watching a gas gauge:
> It also reads out Ah used, soc, and voltage by toggling through the display.
> 
> There is also this: http://www.ebikes.ca/drainbrain.shtml (scroll down to the "Large display/high current version) which reads out Wh as well as Ah. I would like to have Wh since this is the quantity being used to move the vehicle, and varies a bit relative to Ah with soc (charge falls through a greater potential during discharge at higher soc). The change in voltage with soc for LiFePO4 is quite small though so Ah seems to work quite well. I use about 1.6 to 1.8 Ah/mile typically, and I seem to get about 13 miles per bar on the display for mixed highway/secondary driving, with very little variation between high and low soc (not readily noticeable). So about 65 miles to 35% soc.


Etischer and others> The major advantage of TBS is that the product line with updates is on the market for more than a decade and has been used in various LV/HV EVs. Moreover it can be connected via serial rs-232 or usb to computer/pda. Software for both stationary (PV solar) and e-drivetrain applications has been written for it, the software can provide additional features based on the incomming data stream. Pls. follow the discussion here:http://www.diyelectriccar.com/forums/showthread.php/swifte-36621p51.html


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## dimitri (May 16, 2008)

Dave Koller said:


> Nice find! Still wondering what you used to interface current to the Arduino ?
> I used a Hass 600 and that works well with a minimum of fuss
> I think you have another good product for DIY..... !


I use Tamura hall effect current sensor and LM60 temperature sensor.


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## dimitri (May 16, 2008)

Added 2 more screens today. One shows Watts in/out and WattHour counter. Another one shows SoC percentile and Fuel percentile. Fuel is the same as SoC, but has user configurable bottom setting, for example I set MinSoC=30%, so Fuel will be at 0% when actual SoC = 30%. Later I will add a bar graph for Fuel reading in addition to percentile counter.

Since Watt and WattHour counters are usually in multiple thousands and change rapidly, I will change that screen to kWatts and kWattHour to make it more readable on a small LCD.

Also spoke to a company that makes custome gauges and gauge housings, they are sending me free samples to see how I can fit my display in them. I should be able to buy their product and produce professionally looking EV Display.


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## EVfun (Mar 14, 2010)

Very cool but wouldn't pack SOC be better tracked by watching amp hours?


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## JRP3 (Mar 7, 2008)

Nice work


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## Dave Koller (Nov 15, 2008)

Yep- outdone yourself again - nice, simple, and to the point!


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## tomofreno (Mar 3, 2009)

Dimitri, How are you using temperature?


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## jtgreeson (Aug 14, 2009)

Great work Dimitri! 

Like many I'm in the market for a SOC / amp / Volt meter for my conversion. I would be incorporating it into a Porsche instrument that I've taken apart so would be interested in yours without a housing. The size that you have apparently designed would work beautifully. 

I would like to throw out some ideas with the thought that you might have considered them already. 

Are you incorporating the half pack voltage monitor like the Lee Hart simple led design or EVision ? 

Would it be possible to upgrade the display with a little color? I'm thinking a simpler / smaller version of:
0100068700_sm.jpg
or:
showbin.jpg
This would just to improve the "wow" factor a little. Or a cool blue and white led display would be nice. Too expensive / not available?

Also, are you planning to separate the components to allow installation flexibility? In other words having the display itself on the smallest possible board connected via cable to the rest of the circuitry.


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## dimitri (May 16, 2008)

EVfun said:


> Very cool but wouldn't pack SOC be better tracked by watching amp hours?


Actually I am tracking SoC by counting amphours. Wattage is calculated by multiplying amps and pack voltage. Wattage is just an informational screen that many people are interested in.



> Dimitri, How are you using temperature?


I will incorporate temperature reading to adjust pack size based on temperature. It will be user selectable in the setup screen, since every EV has different pack insulation and different climate conditions. You will setup percentage point reduction at freezing and the meter will adjust pack size across the temp range based on your selection. There might be some trial and error to find correct value, but it should work well once found.


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## dimitri (May 16, 2008)

> I would like to throw out some ideas with the thought that you might have considered them already.
> 
> Are you incorporating the half pack voltage monitor like the Lee Hart simple led design or EVision ?


I am minimalist, I am not interested in bling or competing with people developing fancy EV screens. As I have been driving EV for over 2 years I learned what data I actually need while driving vs. data that would be nice to have on a PC at home to draw graphs from. This product is designed only to provide minimal battery pack data while driving, no logging, no fancy screens, simple and affordable. If you need data logging, use CellLog8 or something else.

As for pack voltage, since I have MiniBMS installed on my cells, I am not interested in voltage monitoring, as long as BMS is reporting all cells are OK. If BMS reports a cell that is not OK, I need to get off the road and find the problem, I don't need LCD display for that since cell modules have LEDs on them.

Since I am selling MiniBMS I get a lot of requests from customers that simple SoC display would compliment MiniBMS very well, so I am designing this display with the assumption that cell level BMS is present and its not the display's job to care for the cells, only to monitor SoC and data related to real time pack usage.

Sorry for blunt answers, but my goal is simple and affordable display, so I will not be distracted by anything that adds cost and complexity.


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## JRP3 (Mar 7, 2008)

I second that emotion, since I, and probably many others, already have a voltage gauge.


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## EV-propulsion.com (Jun 1, 2009)

Yes, Dimitri,
Keep it simple and just what is needed. (ie lower cost). I LIKE IT.
Alot of gauges and too much info is unecessary.
Mike
www.EV-propulsion.com


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## dimitri (May 16, 2008)

I want to digress a little into theory of SoC counting, the way I understand it ( which may be totally wrong  ), to preempt some criticism which might be coming my way  as usual.

Batteries provide power ( Watts ) and energy ( WattHours ), so from purely scientific point of view we should be counting Watts, not Amps alone. However, battery inefficiency ( lets ignore thermal losses due to resistance for a moment ) comes from the fact that we charge at higher voltage than we discharge ( assuming the same current in and out ), so we always put more Watts in than we get Watts out. LFP curve is flatter than Lead, so they are more efficient, but still there is some energy lost at every cycle. If I was to design EV Display based on Watts, I would have to compensate for battery efficiency to produce effective Fuel gauge.

However, batteries come rated in AmpHours, not WattHours. There is a reason for that. While voltage swing mostly accounts for energy innefficiency, amperage over time appears to be a better indication of useful energy available ( again, ignoring thermal losses ) and it makes amp counting more practical from EV driver point of view, albeit less scientific. Ignoring the voltage in the Fuel gauge automatically provides compensation for battery inefficiency. Due to thermal losses we still need to calibrate SoC at full charge, which is implemented in most battery monitors by setting SoC to 100% when pack voltage is at max charger voltage at the end of charge cycle. This is also implemented in my display.

Now, the tricky part is compensation for low ambient temp. At low temp battery has more voltage sag, which requires EV to pull more amps to maintain the same power level, which in turn drains the pack faster and reduces the range. So, technically EV display based on Amp counting should not need any temp compensation, but I suspect that its more complicated than that. I don't have time or resources to get scientific root cause and proper compensation formulas for ambient temp, so I just want to keep things simple and provide a mechanism for temp compensation and let the user control amount of compensation based on practical experience. It may not be perfect, but it should work OK and be practical enough for day to day needs.

Hope this makes sense, and hope I'm not too far away from the truth.

BTW, I apologize to original poster for hijacking this thread, maybe I should have started a separate one, although we still seem to be on the subject.


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## jtgreeson (Aug 14, 2009)

dimitri said:


> Sorry for blunt answers, but my goal is simple and affordable display, so I will not be distracted by anything that adds cost and complexity.



Thanks for the answers! Real world experience is always better. I'll be looking forward to seeing the final product.


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## azdeltawye (Dec 30, 2008)

I found the Cadillac of EV Wh / Ah meters; The Brusa BCM400. 










Here are some of the features:

Bi-directional Ah and Wh counters
User programmable PWM outputs to drive fuel gauge or analog meters
Auto reset option resets SOC indication when Ah counter goes negative (fully charged battery)
Separate measurement unit and display keeps pack voltages out of the cabin and increases noise immunity
Unfortunately this meter has been out of production for some time according to the folks at Metric Mind.

See attached user’s manual for more info.
View attachment Brusa BCM series EV meter.pdf


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## JRP3 (Mar 7, 2008)

azdeltawye said:


> Unfortunately this meter has been out of production for some time according to the folks at Metric Mind.


Knowing Brusa it's probably not that unfortunate as it was most likely insanely expensive.


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## tomofreno (Mar 3, 2009)

> I will incorporate temperature reading to adjust pack size based on temperature.


 Ok, that is what I was thinking, but not sure how I will determine the correct factor.



> As I have been driving EV for over 2 years I learned what data I actually need while driving


 Agreed. I have found the minibms and TBS current and soc (it reads V also) are sufficient for my needs, as the motor/controller came with a gauge for motor/controller temperature, and pack voltage. The current reading needs to be bidirectional (TBS is) for those who have regen, and it is nice to be able to check when charging. I can see how close I am to end of charging by seeing how much the charging current output of the Manzanita has diminished, and also see if I am getting a full CV phase. I don't want to be looking at too many outputs while driving. I mainly want to know how much charge or energy I have left and how much current I am drawing from the pack. The minibms will let me know if there is a problem with a cell and I am driving it too low in voltage, so I don't need a display of cell voltages.

How often are you multiplying I*V and integrating over time to get Wh? My voltage is constantly fluctuating. From cell log data I would guess it would have to be a period of maybe 1/2 second or less to catch spikes in current and voltage when accelerating.

I have definitely noticed a temperature effect, even though I heated my cells through winter to keep them at 60F while the car was parked. They were usually above 55 F when I returned from a drive. During winter months I was getting about 1.7 to 1.8Ah/mile in mixed driving. Sunday and Monday I drove a total of 41.9 miles, 8 at 60 mph, rest at about 35 mph, and got 1.46Ah/mile. Outside temp was low 70's F. The cells are now around 80 - 85 F upon return from driving, and around 75 - 80 F when charging so they are taking more charge. I am getting about 16 to 18 miles/bar on the TBS battery icon now, whereas in winter I was getting about 13. There are 5 bars, so in winter total range was about 65 miles to 35% soc, and now it is 80 to 90. There is of course also less resistance in the drive train now. 

On the TBS "full charge" is determined by the pack voltage being above some voltage you enter, and the charge current being below some value you enter. It expects some CV mode with decreasing current. You can also just tell it the pack is fully charged.

It will be very interesting to see how much difference you see in Wh used versus Ah used per mile. I think change in Ah per mile will track fairly well with Wh/mile due to small voltage variation with LiFePO4, but of course voltage sag depends on your driving and cell size, and how far you discharge the pack.


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## Dave Koller (Nov 15, 2008)

dimitri said:


> I am minimalist, I am not interested in bling or competing with people developing fancy EV screens. (snip of text....)
> 
> Sorry for blunt answers, but my goal is simple and affordable display, so I will not be distracted by anything that adds cost and complexity.


Right as rain! 
But it would be easy enough to provide a serial (low level - not rs232) out (in code) for all those people who want a fancy screen - let them have the data stream and go from there - it is just two more pins from the arduino..

Then all the fancy displays can be made (by others) to receive all that " not so simple" data - and you do not have to escape from the great minimalist end result... ( or not LOL)


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## dimitri (May 16, 2008)

> How often are you multiplying I*V and integrating over time to get Wh? My voltage is constantly fluctuating. From cell log data I would guess it would have to be a period of maybe 1/2 second or less to catch spikes in current and voltage when accelerating.


I take analog reading 40 times per second, then averaging 40 readings into one second data, then count AmpSeconds and convert to AmpHours. Same goes for Voltage reading, which is then translated to Watts and WattHours.



> I have definitely noticed a temperature effect...


But the real question is, do you pull proportionally more amps on cold days to get the same power at lower voltage? Does reduced range come purely from pulling more amps in shorter time or do you actually get less amphours from the pack? If amphours are reduced at low temp, then we need to compensate for it, but if you get same amphours only pull them faster due to lower voltage, then we don't need to compensate, you just have to watch fuel gauge more carefully.


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## Homer (Aug 4, 2009)

Assuming that is a Arduino Nano 3.0, you could spit out serial data through the same mini-USB port you used to program it with using the "Serial.print(" command. A few lines of code, and no additional GPIO lines. That would allow for the more detailed displays or logging in addition to your little LCD. Just an idea...


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## dimitri (May 16, 2008)

Homer said:


> Assuming that is a Arduino Nano 3.0, you could spit out serial data through the same mini-USB port you used to program it with using the "Serial.print(" command. A few lines of code, and no additional GPIO lines. That would allow for the more detailed displays or logging in addition to your little LCD. Just an idea...


That was my original plan, but in order to fit into 2" gauge pod and provide best possible price, I decided not to use whole Arduino Nano board and instead just use naked Atmel chip.

Dave's suggestion is doable, but honestly I don't see much value in logging this kind of data, this is purely real time data needed while driving, not much useful back at home. I think cell level data is worth logging, but its outside of scope of this display product. Other products like CellLog8 are very good at logging and I have no interest in competing with that.

Thanks for your suggestions though....


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## EVfun (Mar 14, 2010)

dimitri said:


> I want to digress a little into theory of SoC counting, the way I understand it ( which may be totally wrong  ), to preempt some criticism which might be coming my way  as usual.
> 
> Batteries provide power ( Watts ) and energy ( WattHours ), so from purely scientific point of view we should be counting Watts, not Amps alone. However, battery inefficiency {...snip}


My understanding is exactly the opposite. Batteries actually store amp hours. They store potential to move a number of electrons (current) through chemical reaction. The voltage is a measure of the potential difference between the reactants that liberate the electrons. 

The advantage I see to using a strait amp hour counter is that you don't have to make any connection to pack voltage, so a wide range input isn't required to handle the range of pack voltages, and isolation isn't required if current is measured by hall effect. The instrument can be powered by the vehicle 12v system. What the system needs to be aware of is the number of amp hours, the percentage to derate the return of amp hour on charge (from none to about 140% for some NiCad batteries), and some way to be reset to full charge and/or empty.

That said, your the one developing it  I'm just a happy customer and one potential purchaser.


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## tomofreno (Mar 3, 2009)

> But the real question is, do you pull proportionally more amps on cold days to get the same power at lower voltage? Does reduced range come purely from pulling more amps in shorter time or do you actually get less amphours from the pack?


 Well, it is another question anyway.  I haven't carefully quantified it, but I am fairly sure it is the latter. I don't seem to be seeing significantly less voltage sag than I saw this winter, so no reason to suspect I am pulling significantly more amps to get the same energy/unit time (W). It appears more that the cells are taking more charge at the higher temps, and that is the main effect on range.

Another question some have pointed out is: Is tracking charge good enough? Work, or energy, is what moves the car. Energy is stored as charge at some potential, so if voltage varies significantly, charge/unit time leaving the pack is not a good indication of energy/time leaving the pack. I think for the way I drive, and with the Ah SE cells I have, charge works well enough. I won't belabor it further, as you will find the answer soon enough for TS LiFePO4 when you implement your gauge with both Wh and Ah. Sounds very nice!

If someone comes up with a data logging device, you could always offer the usb or serial output later to interface to it. It would be cool to be able to data log current, voltage, and gps data versus time to see how those quantities and power varied with terrain. At least for a month or so, then I would likely never use it again. So I likely wouldn't purchase it from you because I would figure that is what I would do.


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## Dave Koller (Nov 15, 2008)

dimitri said:


> Dave's suggestion is doable, but honestly I don't see much value in logging this kind of data, this is purely real time data needed while driving, not much useful back at home.
> 
> Thanks for your suggestions though....


Yep ,I merely thought it would be nice to be able to use the serial real time to one of those _fancy screens_ - with their fancy graphic gauges - NOT necessarily for data collection - BUT as Tom knows, it is easy to graph data from that real time also .. BUT you do not need to do it Dimitri - just those two pins and NO USB just for the die hards that want to integrate it to any bells and whistles or to those fancy screens that hold all the other data....

Oh does your LCD have back light by the way


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## dimitri (May 16, 2008)

Dave Koller said:


> Oh does your LCD have back light by the way


Yes, it does. I have it programmed to come on when any button is pressed and stay on for 60 minutes, then go off.


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## dimitri (May 16, 2008)

Looking at some temperature data posted by various LFP manufacturers it appears that in addition to voltage sag cell capacity is also reduced at low temps. So my idea of variable temp compensation seems to have some merit. Looks like cell looses 10% capacity at 10C, 20% at 0C and 50% at -10C.

So I will go ahead and implement temp compensation in my display, but make amount user configurable, so people can play with it to find the optimum value for their EV and climate.


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## dimitri (May 16, 2008)

Time for an update. In the past couple of weeks I designed PCBs for production units and ordered a first batch of PCBs, they came in earlier this week. I built guts for first 3 units while I am working with a company that makes custom gauges to house my EV Display in professionally made gauge cases. They sent me a sample, which you can see in the pic, it fits quite nicely.

Meanwhile I am polishing up some software features to make display more versatile. I should have software all completed next week.

Once software is done I will write up the user guide with all the features explained.

As you can probably tell from the pic, sender unit and display unit are connected via standard Cat5e Ethernet cable. I will supply 7ft shielded cable with the display, but if someone needs more distance, just get a longer Ethernet cable.


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## EVfun (Mar 14, 2010)

Looks good! I have a couple of questions.

In addition to the ethernet cable what other wires go to the dash unit? 

Is that gauge case for dash mounting in a 2 inch hole, or for mounting under the dash?


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## JRP3 (Mar 7, 2008)

Sweet. If you've seen my build thread lately you'll realize I need this!


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## Dave Koller (Nov 15, 2008)

looks GREAT ! Keep it coming...


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## tomofreno (Mar 3, 2009)

Looks good! What size is the opening in the current measurement device?


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## jtgreeson (Aug 14, 2009)

Fantastic, just what we need!


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## dimitri (May 16, 2008)

EVfun said:


> Looks good! I have a couple of questions.
> 
> In addition to the ethernet cable what other wires go to the dash unit?
> 
> Is that gauge case for dash mounting in a 2 inch hole, or for mounting under the dash?


No other wires, Ethernet cable is the only wire there is.

The case is industry standard for mounting in 2" gauge pods. Its the same case that most EV Voltmeters and Ammeters come in.



tomofreno said:


> Looks good! What size is the opening in the current measurement device?


Its 0.3" x 0.6" opening in a hall effect sensor. Its not the most convenient size, but its manageable. Its the same sensor used in Paktrakr and can use the same or similar copper or aluminum bars for sensor mounting. I might offer the bar as well if I find a cheap source.


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## JRP3 (Mar 7, 2008)

That means it can't take 2/0 cable  I was hoping to eliminate the 2 extra connections needed with shunts.


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## dimitri (May 16, 2008)

JRP3 said:


> That means it can't take 2/0 cable  I was hoping to eliminate the 2 extra connections needed with shunts.


I know, I am also upset that hall effect opening is so small, but its the best hall effect sensor I could find for the job.

I avoided extra connections in my EV by replacing one of TS copper bus bars between 2 cells with the one I made to hold the sensor. I will post a pic later of how I did it, once I swap the ugly hand made prototype with final version of the sender unit.


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## JRP3 (Mar 7, 2008)

Great idea! Happy again


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## Tesseract (Sep 27, 2008)

I have to say this product looks like a real winner, Dimitri. 

And JRP3, I was kicking around the idea of making a "strap-on" current sensor (ooo, that sounds a bit naughty, eh?) pre-calibrated for 2/0 cable. You literally attach it to the cable with two wire ties. The idea was to be able to turn an ordinary contactor into a smart circuit breaker, but since none of the regulatory agencies approve of such shenanigans I sort of shelved the idea. Could resurrect it as a current sensor for Dimitri's Ah/Wh meter, though.


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## JRP3 (Mar 7, 2008)

I like the idea. Don't know what these cost but the opening is almost an inch in diameter: http://www.sypris.com/filemanager/library/BB150_300_600.pdf


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## Guest (May 16, 2010)

Tesseract said:


> My $0.02: USB has no place in a vehicle, much less an electric vehicle... It's a chintzy unipolar synchronous interface "standard" that has undergone, what?, 4 major revisions since it's introduction. It has no tolerance of common mode noise, cable lengths are strictly limited, and the connectors do not positively lock. I pretty much can't stand USB (except on my computer, where it does a great job interfacing my mouse and thumb drives).
> 
> Instead, I strongly recommend ethernet (of course) or CAN.


Would you believe I agree with you? 

USB is very handy because it is ubiquitous. But this itch won't get scratched until we have CAN. Most of the controllers do CAN. So if an API could be devised that would allow us to add gages and sensors at will, we could tie in controllers and other things. Many modern cars have all sorts of things that are readily available on OBDII - speeds, temperatures, etc that survive an enginectomy. Ultimately, an instrumentation interface is not complete until ti can tie all this together. And that points to CAN.

As you say, CAN is also pretty robust in the face of heavy EMI.

Jack Rickard


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## Guest (May 16, 2010)

dimitri said:


> I am minimalist, I am not interested in bling or competing with people developing fancy EV screens. As I have been driving EV for over 2 years I learned what data I actually need while driving vs. data that would be nice to have on a PC at home to draw graphs from. This product is designed only to provide minimal battery pack data while driving, no logging, no fancy screens, simple and affordable. If you need data logging, use CellLog8 or something else.
> 
> As for pack voltage, since I have MiniBMS installed on my cells, I am not interested in voltage monitoring, as long as BMS is reporting all cells are OK. If BMS reports a cell that is not OK, I need to get off the road and find the problem, I don't need LCD display for that since cell modules have LEDs on them.
> 
> ...



It's clear thinking and good focus Dmitri. 

I like bling. If you could do a little RS-232 serial out, just the pins and little ASCII string with the values, it could be interfaced to larger hardware for bling as they like.

I like pack voltage and the pack split thing myself, but you are quite correct, the AH is the thing.

Jack Rickard


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## Guest (May 16, 2010)

dimitri said:


> I want to digress a little into theory of SoC counting, the way I understand it ( which may be totally wrong  ), to preempt some criticism which might be coming my way  as usual.
> 
> Batteries provide power ( Watts ) and energy ( WattHours ), so from purely scientific point of view we should be counting Watts, not Amps alone. However, battery inefficiency ( lets ignore thermal losses due to resistance for a moment ) comes from the fact that we charge at higher voltage than we discharge ( assuming the same current in and out ), so we always put more Watts in than we get Watts out. LFP curve is flatter than Lead, so they are more efficient, but still there is some energy lost at every cycle. If I was to design EV Display based on Watts, I would have to compensate for battery efficiency to produce effective Fuel gauge.
> 
> ...


I think you're all over it. AH is the most consistent value and the one the batteries are rated in. Voltage is all over the map, and takes kWh with it. A kWh display is a convenience, and easy to do. But I would base it on average voltage and integrated AH.

Your approach to temperature compensation seems inspired. Hook up a temp probe and let them choose the magic number they like. Things are different in Saskatchewan.

Jack Rickard


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## Guest (May 16, 2010)

Dave Koller said:


> Right as rain!
> But it would be easy enough to provide a serial (low level - not rs232) out (in code) for all those people who want a fancy screen - let them have the data stream and go from there - it is just two more pins from the arduino..
> 
> Then all the fancy displays can be made (by others) to receive all that " not so simple" data - and you do not have to escape from the great minimalist end result... ( or not LOL)


The Arduino has RS-232 serial on it. The ATMEL probably doesn't as I recall. There is a softserial library for it though that lets you do RS-232 out any digital I/O pin.

Jack Rickard


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## Guest (May 16, 2010)

dimitri said:


> I take analog reading 40 times per second, then averaging 40 readings into one second data, then count AmpSeconds and convert to AmpHours. Same goes for Voltage reading, which is then translated to Watts and WattHours.


To the question you're interested in, you actually get less AH from the cell in cold temperatures. Your apparent capacity changes. The lower voltage is just a function of an increased IR.

I did this exact thing on an Arduino Dmitri. It is slightly more accurate than what you're doing on the totalization side.

1 There is a millis function in the IDE language.
2. Make a timestamp variable, save millis into it.
3. On your measurement, things are much more stable if you do a very fast loop but do about 200 ADC measurements and take the average.
4. Get currrent millis and subtract time stamp. Multiply by current reading.
5. Amp hours is this value /360000. Sum it into your AH variable.
5. Save millis immediately to timestamp.
6. Exit your current reading loop.

You have a much more incremental integration of the AH by time. But more importantly, you are free. Now you can do other things in the code, take care of housekeeping, send out serial data, measure temperature, whatever. 
When you return to the loop, the time increment is whatever now time is minus the timestamp. 40 per second. 3 per second. 300 per second. It can even be different each time. The value will be accurate as it is going to be. And you are free to add code without worrying about your loop timing.

Here's a couple of other problems I encountered that you might want to be aware of. 

Mostly power supply related, but ALSO some anamolies with hall effect sensors - I used LEM HASS with 5V power from the Arduino so you can see how I got wrapped around an axle.

The power supply is also the Arduino reference voltage for the ADC measurement. You need to be at 5v really, like 5.000 and it has to maintain that under all loads. I wound up having to make my own 5v supply with an automotive grade regulator that would do an amp and a half. The 500 ma thing got loaded and so doing OTHER sensors, like temperature, would load my power supply, throw off the amperage readings and mess up the whole works. 

You have to be constantly checking output during development with some sort of test setup on the bench that will give you a very accurate current reading and do it at a variety of loads - things vary from 10 amps to 300 amps. I actually wound up using two LEM HASS to get the r esolution - a 50 AMP and a 150 AMP. I would read the 50, and if it read above 100, I would switch to the 150 output. I don't know the range or accuracy of the one you are using.

But the accuracy would get thrown off if my 5v supply varied. So I had to kind of massively overbuild it (ok 1.5 amps is not massive, but compared to 500 ma) to stabilize it.

The other problem was the LEM HASS and it may have been tied to the power supply thing. The zero amp index kept moving around. Not a lot, but enough to throw things off particularly between in amps and out amps. You basically have to have a check in there for zero amps and reset your index if it occurs. The problem is, it doesn't REALLY occur much in a car.

I like the automatic reset on charge completion. I'd like to see a manual way to reset AH as well. And it's tricky. If you use current, as you noted, regenerative braking can reset your counter, and you always have afull tank, up to the point where you really really need a miniBMS.. If you do voltage, EMI induced spikes can do the same thing. I was working on a reset at a particular voltage, only occuring during recharge, combined with a specific current reading. This would have to be user defineable as done in the Xantrex.

I have code for all this. While I doubt you'd want to use it, if you want to just read through it to get ideas, you are welcome to it quite gratis and use any or all quite at will. Everything is in objects you can just plug in or not. And one of them uses the Sparkfun VEnus GPS module to derive speed data (along with location etc). I was integrating speed the same as AH to derive MILES TO EMPTY. I've been meaning to put that one up on Arduino anyway, but never got around to just putting it in format.

There is a section to drive analog gages as well. Since the one I was hitting on was nonlinear old Porsche gage, it is hopelessly complex and still doesn't work very well at the top of the gage.

Again I would implore you to set up and test this against a variety of currents with a good current meter. The integration makes errors very cummulative. I found the hall sensors not as linear as I expected, and had to do correction factors for several levels. 


Your welcome to all of this. I had it working quite well actually, but I couldn't manufacture cookies and get anyone fed. I was using a Mega instead of a nano. At nano, I would be surprised if you can get boards made for as little as the nano costs in quantity. But the size would go down of course.

I know we've crossed swords in the past, but I think you're onto something here. Anything I can do to help I'll do. If nothing else, I could do some higher current measurements for you.

Jack Rickard


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## Guest (May 16, 2010)

JRP3 said:


> I like the idea. Don't know what these cost but the opening is almost an inch in diameter: http://www.sypris.com/filemanager/library/BB150_300_600.pdf


Nice unit. Two problems. The first is that it requires both a positive and a negative 15 volt excitation voltage. That sucks.

The second is that it provides a positive voltage for the postive amps, and a negative voltage for the negative amps. The ADC in the ATMEL needs a positive voltage between 1 and 5 volts. There are ways around this, but all are kind of ugly.

The LEM HASS has the same problem in that it has a 10mm x 26mm square opening, so you wind up with a copper bar. But the power supply is 5v and ground. Better, you can use a single resistor to translate the current output to 0 to 5 v, and the midrange, 2.5v will be your zero index. Voltages below that imply increasing negative amps, and voltages above it positive amps. And so it is readily readable with the ATMEL ADC and a single resistor.

By the way, Dmitri, DO clamp your ADC inputs with 5v zeners. Ask me how I know.

jack Rickard


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## azdeltawye (Dec 30, 2008)

Dimitri - Nice job! Looks like you’ve put a lot of thought into this and it shows. 

On the auto-reset function, the way the Brusa BCM meters do it is monitor when the Ah counter goes negative (charge cycle nearing completion). It then resets the Ah and Wh counters when the current goes positive (load current). There is an optional hysteresis function that allows you to tweak the reset threshold to prevent false resets.

Again, good work - It’s quite impressive how quickly you convert ideas into viable products…


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## jackbauer (Jan 12, 2008)

Very nice indeed. I'm working on an arduino based design myself.Got the idea from Jack. It will command the controller (via rs232) to reduce max battery amps at various levels and eventually shutdown the controller and open the main contactor. I've decided on a Lem hass-300s. This has a +/- 0.625v output dependent on direction of current so will clock ah in and ah out. Zero amps is 2.5v so reading neg and pos currents is easy.


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## dimitri (May 16, 2008)

jrickard said:


> I know we've crossed swords in the past, but I think you're onto something here. Anything I can do to help I'll do. If nothing else, I could do some higher current measurements for you.


I don't know what to say, Jack. You insulted me in the past for things that I didn't deserve, now you are offering help. Your attitude swings more than my wife's PMS 

I'm not the one to hold a grudge, so even though I never got an apology for your insults, I will be the first to thank you for your input, although its a little late in the game as I have completed most of the software by now and worked thru all the same problems you have described above. 

I have been doing software and hardware development for 20 years, so please don't be offended that I prefer to write my own code rather than using someone else's, its mostly a pride thing, I'm sure you can relate to it 

Its nice to know that we both independently came to very similar results, its reassuring that I am on the right track here.

My data collection loop is quite similar to what you described and I dealt with Atmel's poor internal 5V ref by simply adding external precision 5V ref IC chip. which feeds from 12V before 5V reg, so it bypasses the issue of 5V dropping as you noted.

I use 600 Amp hall sensor to support larger EVs, but on a flip-side it was a bit challenging to keep zero point from floating. I managed to do it nicely in a software and I will provide setup menu allowing to adjust zero current reference in case it drifts from one EV to another. Picking up 2-3 Amp current with a sensor range of 1200Amp and 10 bit ADC was another challenge, but it seems to be working quite well.

I drive every day with prototype in my EV and I correlate its data with analog ammeter and measurements during charge cycles. Results are better than expected, about 99% accuracy and display resets at every charge, so 99% accuracy remains long term and does not drift.

Every single parameter will be available in the setup menu, so user will be free to play with it.


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## Guest (May 17, 2010)

dimitri said:


> I don't know what to say, Jack. You insulted me in the past for things that I didn't deserve, now you are offering help. Your attitude swings more than my wife's PMS
> 
> I'm not the one to hold a grudge, so even though I never got an apology for your insults, I will be the first to thank you for your input, although its a little late in the game as I have completed most of the software by now and worked thru all the same problems you have described above.
> 
> ...


Still pouting months later. I see. Ok. Have it your way....

On to the next event....

Jack Rickard


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## tomofreno (Mar 3, 2009)

> Looking at some temperature data posted by various LFP manufacturers it appears that in addition to voltage sag cell capacity is also reduced at low temps. So my idea of variable temp compensation seems to have some merit. Looks like cell looses 10% capacity at 10C, 20% at 0C and 50% at -10C.


 Got some data on this. Not as quantitative as I would like but some data. I commented earlier that I noticed my cells taking more charge with warmer weather this spring. Temperatures have been in the low 70's F (low 20's C), and I usually charge at the end of the day when the batteries are warm from driving, so they were typically at 75 to 80 F (24 to 25 C) at start of charging. Last night the temperature got down to 34 F (1 C), and the battery heaters kept the cells at the 60 F set point. When I charged this morning, starting at that temperature I got 11.7Ah less charge.

I would guess this is mainly due to higher internal resistance at lower temperatures, so more voltage required to drive a given charge current into the cells compared to at higher cell temps. The charger then hits the high voltage limit and goes into CV mode before as much charge has been transferred into the cells compared to operation at higher temperatures. Of course in order for the cells to take more charge at higher temperatures and not "fill up" and go into the exponential voltage increase with Ah part of the charge curve where you are mainly adding surface charge (they didn't), they must have increased capacity compared to at lower temperatures. Maybe this is due to increased diffusivity of the lithium ions due to lattice vibrations at higher temps. Anyway, the capacity does increase somewhat with temperature. 

So how to compensate for this in determining when the pack is "full" so you know how much charge (Ah) you actually have? The TBS determines "full" based on two parameters: A pack voltage that must be exceeded, and a value the charge current must decrease below. From the above, you can see that at colder cell temps it will declare "full" at lower Ah added due to higher voltage during charging, causing the charger to hit it's voltage limit, cut back current, and time out earlier. I have watched the charge current drop and the TBS post "full" when the pack was short by over 9 Ah of charge compared to last time it was charged. But now that Ah is defined as "full", even though you don't have as much charge as last time. Conversely, when the cells are warmer, the charger keeps happily pumping more charge in due to lower ir and lower voltage. I have watched the "Ah used" on the TBS drop to zero, and the charger still going at 25 A or more, as it pumps 5 to 10 more Ah into the pack. The cells are all still down around 3.43 to 3.44V during this time. If the charger current drops below the 9A limit I put in the TBS before the charger times out, the TBS will declare the pack full, and that will become the new "full" Ah. If not, the TBS just reads 0.0 for Ah used, and next charge the defined "full" is the same as it was before this charge.

The TBS reads out actual Ah used, but there is always a question of just how much charge was in the cells to begin with. It has never been a problem as I usually recharge at 40% SOC or so, and the std dev in "full" has been less than 10%. Could be a problem for someone who likes to stretch things to the limit. I could decrease this variability somewhat by putting a higher voltage limit in the TBS, and a smaller charge current, so that the cells are always charged to the beginning of the exponential rise part of the charging curve (change in Ah/V is much lower there). The amount of charge required to do this will still vary with temperature though, and "full" will still not always correspond to the same Ah. Something for you to think about.


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## JRP3 (Mar 7, 2008)

I've seen similar results. I think my foray into low voltage territory was partially a result of charging at lower temps the night before, as well as the PFC30 having a lower voltage cutoff when charging from 240VAC compared to 120VAC. That's an annoying problem I have yet to come up with a good solution for, other than using a BMS of course


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## dimitri (May 16, 2008)

Tom,

thanks for detailed data, as always. I think there are 2 separate things involved here. Like you said, the charger is not smart enough to change its voltage based on temperature, so it ends up undercharging the pack somewhat, but not a huge amount. Then, when you drive at cold temps, reduced electro-chemical processes in the battery prevent from removing all the charge from the battery, although, again, if you have smart enough LVC mechanism it would reduce LVC level based on low temp, allowing to get more from the pack. All these things are hard to quantify in DIY world since every EV is unique, so ultimately its up to the owner to tweak things to get the best results for his specific EV and climate.

This week I completed all the software in my EV Display, have been testing for last 3-4 days without any changes. Every single variable will be accessible to the user to tweak if needed.

I am now working on user documentation ( I will post a draft here soon ) while I am waiting for this company to give me a quote for enclosures, they have been dragging feet for a week now. I suppose I am not big enough customer for them to take me seriously, so I end up chasing them every day. Worst part is that all parts except the screen mask are off the shelf, so its frustrating to say the least.

I am also working on a voltage scaler since my display is designed for 12V nominal ( equivalent of 4 LFP cells ), but for the bottom balancing crowd I need to supply the scaler, so the display can be powered by entire pack, otherwise 4 cells will be drained too low over a few months of use. It works fine for me on 4 cells, but I realize that I have to support all combinations to be successful.

The whole project has been challenging, but interesting. Few days ago I got comfortable enough with my prototype that I ripped PakTrakr from the car. It was an occasion worth celebrating  , some of you know how much headache PakTrakr caused me over the past year. I am so much happier now, knowing that the data I get is actual data, not some garbage-in/garbage-out situation.

Stay tuned for more updates....


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## JRP3 (Mar 7, 2008)

Any reason this can't be powered from the 12 volt battery or DC/DC?


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## Dave Koller (Nov 15, 2008)

JRP3 said:


> Any reason this can't be powered from the 12 volt battery or DC/DC?


I believe he is working on the pack voltage from the cells he selected for the 12 volt - he will need to isolate pack and battery (12 volt or DC/DC) and that is - I think what he is looking to scale - but then, I could be wrong...

In any case he needs a good method to read pack isolated from any other DC source....


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## dimitri (May 16, 2008)

JRP3 said:


> Any reason this can't be powered from the 12 volt battery or DC/DC?


It sure can, but you lose ability to accurately track WH , since I am calculating pack voltage to get WH from AH. Some people wanted WH counter, so I provided it, but if you have other means of pack voltage monitoring or you don't really care about ( I find it absolutely meaningless myself as I drive and I never even look at WH screen ), then you can power EV Display from 12V aux source and not worry about voltage scaler or pack imbalance.

That is a very good observation JRP, thanks for reminding me, I will state this in user guide as well.

In fact, assuming that LFP pack voltage doesn't sag THAT much, you can still configure EV Display to show APPROXIMATE WH data, based on static nominal pack voltage, which you can setup in the setup screen.


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## dimitri (May 16, 2008)

Dave Koller said:


> I believe he is working on the pack voltage from the cells he selected for the 12 volt - he will need to isolate pack and battery (12 volt or DC/DC) and that is - I think what he is looking to scale - but then, I could be wrong...
> 
> In any case he needs a good method to read pack isolated from any other DC source....


That would be one way of doing it, but it would be complicated and expensive. And you know how I like to keep things cheap and simple .

It will come to basic choices:

1. Power display with 12V aux and ignore WH data, or just understand that its approximate and doesn't account for voltage sag.
2. Get a voltage scaler to power the display from the main pack and read actual pack voltage and actual WH data.
3. If you top balance the pack with BMS, then BMS will counteract imbalance on 4 cells caused by extra load of the EV Display, which eats 30mA, so that comes to 0.72AH per day. If you are OK with this, power display from any adjacent 4 cells in the pack.
4. If all above fails, then move on and buy some other EV Display 

I respect anyone's wish to know precise WH data, but I personally find it useless. All I really need is AH data, which my display seems to track very well.


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## EVfun (Mar 14, 2010)

I vote for the first option. Will it be easy to just have it display battery "fuel gauge" and amp hours?

I'm very interested in this new creation of yours. I've been wanting a good amp hour counter with full pack reset that is suited to the automotive environment. The e-meter is such overkill for what I want to know.


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## dimitri (May 16, 2008)

EVfun said:


> I vote for the first option. Will it be easy to just have it display battery "fuel gauge" and amp hours?
> 
> I'm very interested in this new creation of yours. I've been wanting a good amp hour counter with full pack reset that is suited to the automotive environment. The e-meter is such overkill for what I want to know.


Yes, that is the primary function of the device. I also wanted such display for 2 years now and since I couldn't find anything I liked or could afford, I finally decided to build my own.


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## jtgreeson (Aug 14, 2009)

dimitri said:


> It will come to basic choices:
> 
> 2. Get a voltage scaler to power the display from the main pack and read actual pack voltage and actual WH data.


I'm leaning towards #2 if it's the only way to also get pack voltage. WH data is not required. Is pack voltage useful in your experience in a vehicle? I have only test driven my project a mile and do not have instrumentation (amps, volts, SOC, etc.) yet. For me having a total pack voltage would be useful during charging and after a drive. It would keep my dash less cluttered and eliminate buying another gauge. My only experience with a similar system is 5 years with a boat electrical system. Multiple battery banks, several different sources of charge (solar,alternator) and discharge (12v,110v), managed via a Link battery monitor (like the old e-meter). You are right that all you need is a running total of Amps in and out. (What extra info does WHs give you?) I like the e-meter/Link feature that after a full charge and you get back into positive amps it zeroes total amps out automatically when you start to draw from the batteries. Having total battery voltage has been very useful also.

I hope will we be able to monitor charging amps with this meter. Another gauge purchase eliminated if it does. 

My conversion has a small 12 volt sli battery (20ah). My Zilla controller draws 70 mA at standby and I had to rewire so it would not drain the aux sli 12v battery dead if the car sat for a week. So maybe another reason to power the meter off the traction pack. Will it be able to monitor the sli 12v battery voltage?

I can wait til you post the documentation for the answers to these questions. I hope this input is helpful.


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## dimitri (May 16, 2008)

Unfortunately there isn't a single clear cut solution, each one has benefits and drawbacks. I can't even pick myself how I want mine set long term.

No matter how good AH counter is, it will always drift long term, so you need a reference point to set it to 100% every full charge cycle. This is best done by monitoring pack voltage and set AH counter to full when certain voltage is reached, typically it will be CV level of your charger, or something right under it.

However, if you power display from 12V aux battery, you lose this ability to reset AH counter.

Powering display from 4 cells works great, but only if you top balance your pack with BMS, so its not an option for those who don't top balance.

Best option is voltage scaler, but it means an extra expense and extra device to wire in the car. I just started working on a scaler design, so I can't even report anything working yet, hopefully I will pull it off.

So, as you see, there are still some challenges and some more work must be done.

JTGreeson, I already said earlier, IMHO monitoring pack voltage during driving is useless if you have AH counter and some sort of LVC protection either from controller or BMS or both. It jumps up and down and doesn't really tell you anything until you are empty. Of course if you have nothing else, then voltmeter would be the bare minimum you'd put in.


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## tomofreno (Mar 3, 2009)

The difference in pack rest (several hours after charge) voltage in the two charges where I had an 11.7Ah difference was 120.6 - 120.1 = 0.5, or about 0.0139V/cell.


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## EVfun (Mar 14, 2010)

I guess that brings up 2 questions Dimitri. 

1. Is there some type of manual reset that is easily accessed? Perhaps I could just rig a button that I press when I know the pack is full. 

2. Is the charge efficiency of Lithium low enough that the amp hours returned is consistently greater than the amp hour used? If so, amp hours going above zero could be used to reset to zero (basically, reject any positive amp hour number in software.) The e-meter used to display the positive amp hour number until the first load after a charge. At that time it dropped to zero and began counting down the amp hours. Since sealed lead has a charge efficiency a little over 90% there was always positive amp hours every full charge. That may not be the case with Lithium Iron Phosphate batteries.


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## dimitri (May 16, 2008)

EVfun said:


> 2. Is the charge efficiency of Lithium low enough that the amp hours returned is consistently greater than the amp hour used? If so, amp hours going above zero could be used to reset to zero (basically, reject any positive amp hour number in software.)


I think you nailed it, one of software changes I did recently is not to allow AH counter to "overflow", just like you mention.

I set my "full voltage" artificially too high so it won't interfere with testing and I am currently testing to see if counter consistently hits 100% during charge.

If this works, then it will bring the option of powering from 12V aux back to frontrunner for me.


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## dimitri (May 16, 2008)

I really don't want to introduce manual reset, since it would add the risk of losing track of AH on the road if someone does it unintentionally.

If you really want it, just add NC button in the power line to reset the display by breaking the power line.


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## tomofreno (Mar 3, 2009)

> Is there some type of manual reset that is easily accessed?


 The TBS permits this. You push the two "arrow" buttons simultaneously to set it to "full". I have been doing this as the weather warms, increasing what Ah corresponds to "full". When cold weather returns I will do the opposite, adjusting "full" back down. I define "full" as when my lowest capacity cell gets to 3.44 to 3.45V since this is the beginning of the exponential voltage rise part of the charge curve for SE cells, and I don't like to charge any cell beyond that point, as I think it may stress them more (purely a guess). But of course in addition to this you have to keep in mind that "full" is not the same Ah in cold versus warm weather, and adjust your expectation of range accordingly. I could plan on 60 mile range typically in winter (cells heated to 60F) for mixed driving and about 35-40% SOC, in warmer spring weather it seems to be 70 miles or more (well, it was warmer, 23F here this morning!).


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## dimitri (May 16, 2008)

Here is the first draft of the user guide. It was created in a hurry, so apologies for formatting and syntax errors if any.

Your feedback to improve the guide is appreciated.

Later, there will be installation section with close up pictures of the sender unit and back of the display.

View attachment EV Display User Guide.pdf


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## JRP3 (Mar 7, 2008)

A quick look through seems pretty straight forward. I'm excited to get one in my hot little hands


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## jackbauer (Jan 12, 2008)

Any info on costing and availability?


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## dimitri (May 16, 2008)

jackbauer said:


> Any info on costing and availability?


I just ordered gauge enclosures, but my order was too small for this company to make custom screen mask for me, so I need to find a way to make good looking screen mask. I attached the drawing here to give an idea of what it looks like. 

I think it would have to be stamped from some thin black or blackened material, like plastic or aluminum, then silkscreen white edges around button and screen holes to contrast them and pehaps a line of text "EV Display" over the top part.

The mask would sit behind the lexan clear lense that comes with these units.







Maybe I can find a way to make stickers that could be applied directly to the back of the lense, and serve as a mask?

It has to be inexpensive and look professional. Any ideas?

As for cost, retail price will likely to be $180, which includes display unit, sender unit and 7ft cable. Voltage scaler would be optional since not everyone will want or need one. I don't have any info on voltage scaler yet, its still in development phase.

I ordered all parts I need for first batch of units, so if I can figure out the screen mask issue, I should be able to ship first units sometime in June.


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## EV-propulsion.com (Jun 1, 2009)

A competent sign shop should be able to make a computer generated decent looking face. Probably not your local sign a rama, but a design shop. Should easily be made in small numbers.
Mike
www.EV-propulsion.com


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## EVfun (Mar 14, 2010)

The manual reads quite well. I think you write up a manual better than my friend Rich Rudman.

I was a little disappointed that 3 of the 5 display screens involve pack voltage (showing either voltage or kWh.) I was kinda hoping one displayed the "fuel level" bar graph and actual amp hours. A few extra display modes with the bar graph and something else would likely suit a lot of users (with volts, amp hour, kilowatt hours, and amps that you already provide.) Energy available at a glance and more detail. With the current options I would most likely find myself using the actual amp hours and temp display. I have no intention of hooking this thing up to my pack, just passing it through the hall sensor.


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## dimitri (May 16, 2008)

EVfun said:


> The manual reads quite well. I think you write up a manual better than my friend Rich Rudman.
> 
> I was a little disappointed that 3 of the 5 display screens involve pack voltage (showing either voltage or kWh.) I was kinda hoping one displayed the "fuel level" bar graph and actual amp hours. A few extra display modes with the bar graph and something else would likely suit a lot of users (with volts, amp hour, kilowatt hours, and amps that you already provide.) Energy available at a glance and more detail. With the current options I would most likely find myself using the actual amp hours and temp display. I have no intention of hooking this thing up to my pack, just passing it through the hall sensor.


I think you misunderstood a little. Upper line and lower line are independently controlled, upper line rotates thru available values via left button, lower line with right button. My screens were just random combinations of 2 lines. 

However, I designated more dynamic values (volts, amps, watts, temp) to lower line and less dynamic ( fuel bar, ah , wh, soc ) to upper line, but your example calls for fuel bar and ah or soc to be on the same screen, but then you will not have any dynamic data on the screen.

I personally find it critical to observe amps as I drive, so I always keep it in lower line and fuel bar or AH or SOC on the upper line.

I suppose I could make it so any value is available on either line, you could even display the same value on both lines if you really wanted to


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## jtgreeson (Aug 14, 2009)

Dimitri, The manual looks good. At the risk of having misread it here's a few random comments that might help if you haven's already considered them.

The "Curr Dir" option should help many installations. Super.

The two buttons each controlling a display line results in great user flexibility. Good idea! I think maybe EVFun missed that. 

If the data is available please suggest a "TempComp" percentage that users could start with. This might be difficult due to all the geographical and battery type variables but would give the user a starting point. Maybe updating this data later from actual users might be an option.

I still like the pack voltage option via a scaler for the sole purpose of monitoring in the garage during charging. It would be a convenient way to help in trouble shooting problems, calibrating chargers, etc. 

A volt ("AuxVolt"?) display of the 12v aux battery would be nice also but that might be to late to include. One could double check the battery health and the operation of the 12v aux battery charger via this voltage. Might be useful.

Please consider those that might want to integrate the display into their current dash. Is there an option to remote the switches? Would it be easy to solder parallel wires or is accessibility on your pcb an issue?

I think the manual might be clearer on what resets the Ah to zero in different conditions. Powered via aux bat versus 4 cells versus scaler. Also what should the Ah display show during charging after getting to zero. You mentioned in a post but I didn't see it in the manual.

The relay idea is good. You got to love the "idiot light" concept when dealing with humans.









I noticed you display "total Ah". I didn't notice an "Ah used" display like in the e-meter. Your way is probably a better way to do it. I do, however, use the e-meter's "Ah used" to quickly check how far along the charging process is. Maybe have both? (lazy guys like me wouldn't have to do the subtraction in our heads)

Overall , great job.


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## tomofreno (Mar 3, 2009)

> Here is the first draft of the user guide.


 Looks good Dimitri. I am partial to powering the gauge from the pack with a scaler since it then tracks actual pack voltage. With the other powering options pack voltage and kWh are just estimates as you clearly explain in the manual. How good an estimate will depend on how balanced the pack is, and proportion of driving time when higher current is drawn (larger V sag). 

Sounds like there is some flexibility in mounting the temperature sensor. Is it a separate wire from the cable to the Hall sensor? Ideal would be to attach it directly to a cell of course. In winter my cells are kept heated to 60 F in their insulated boxes when the car is parked in the garage. They heat up a bit while driving, and cool a bit if the car sits outside for several hours. Temperature measurement anywhere but on a cell will have large error in these conditions, as it may be 10 F outside when cells are at 65 F.


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## dimitri (May 16, 2008)

tomofreno said:


> Sounds like there is some flexibility in mounting the temperature sensor. Is it a separate wire from the cable to the Hall sensor? Ideal would be to attach it directly to a cell of course. In winter my cells are kept heated to 60 F in their insulated boxes when the car is parked in the garage. They heat up a bit while driving, and cool a bit if the car sits outside for several hours. Temperature measurement anywhere but on a cell will have large error in these conditions, as it may be 10 F outside when cells are at 65 F.


Unfortunately there is little flexibility since temp sensor is a tiny SMD chip next to the hall sensor. The idea was that hall sensor would be close to the battery pack and so temp sensor would be close too. That is why I recommend to install sender unit inside insulated battery box if you have one.

I want to make it very clear that I am trying to keep things simple and temp compensation feature will require some adjustments and may not work as precise as you may wish, but its better than nothing, right?

JT, to answer your related question, I would start with 10% temp compensation in the climate zone where your BATTERY can reach freezing temp. I stress BATTERY temp, not ambient temp, since I assume that any smart EV builder would insulate the battery in such climate zone and try to keep it above freezing. If 10% proves too little, then try 15%-20%. If 20% is not enough, then you are either north of polar circle or you are in need of serious battery heating, or both


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## jtgreeson (Aug 14, 2009)

Ha Ha! Polar ice cap! Luckily I'm just south of you in Ft Myers..... Just thinking of others in the great white north.

Use .01% correction for me?


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## EVfun (Mar 14, 2010)

dimitri said:


> I think you misunderstood a little. Upper line and lower line are independently controlled, upper line rotates thru available values via left button, lower line with right button. My screens were just random combinations of 2 lines.
> 
> However, I designated more dynamic values (volts, amps, watts, temp) to lower line and less dynamic ( fuel bar, ah , wh, soc ) to upper line, but your example calls for fuel bar and ah or soc to be on the same screen, but then you will not have any dynamic data on the screen.


Both lines being independently controllable is a nice touch. I'm looking at this for the amp hour functions. I already have volt and amp meters, but with Lithium I can't drive by those alone. With Lead, you can quickly learn how to tell the state of charge by voltage sag under load. Perhaps swapping temp and SOC would be more useful for those who choose not to connect to pack voltage? I bet the temperature on the board changes more slowly than SOC with me driving 

It's just a thought. This is shaping up to be a nice product.


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## JRP3 (Mar 7, 2008)

EVfun said:


> I already have volt and amp meters, but with Lithium I can't drive by those alone.


Uh oh, no one told me that, and that's what I've been doing  I know when I see 113-112V (36 cell pack) off throttle I need be aiming for an outlet. I zero my trip odometer at every charge, I know I have about a 35-40 mile range on a full charge, I watch my amp draw and know if I see a lot of 400+ pulls my range will be less. It's really quite workable if you stay a bit conservative. That said, the Ah counter will be much nicer and will eliminate some guesswork.


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## dimitri (May 16, 2008)

Based on feedback I got from last few posts I made hopefully last changes to the software before initial release 

1. All 8 data values are available on either top or bottom display line and can be set in any possible combination by using 2 buttons, each scrolling thru 8 values on its corresponding display line.

2. Holding both buttons for 5 sec will initiate zero current calibration and also reset AH counter to FULL, just in case display doesn't reach FULL on its own once in a while during charging.

I am still testing to ensure that display consistently gets to FULL after full charge, so far so good. This will allow most simple option of powering display from 12V aux battery.


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## EVfun (Mar 14, 2010)

JRP3 said:


> Uh oh, no one told me that, and that's what I've been doing I know when I see 113-112V (36 cell pack) off throttle I need be aiming for an outlet.


Is this with Thundersky cells? The discharge graphs look so flat that I thought it would not be viable to use voltage. It figured the warning would come to late.


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## JRP3 (Mar 7, 2008)

EVfun said:


> Is this with Thundersky cells? The discharge graphs look so flat that I thought it would not be viable to use voltage. It figured the warning would come to late.


SE cells actually, but yes the difference between 113 volts, 3.14 per cell, and 108 volts, 3 per cell, is enough to give useful information and give you some extra leeway on the bottom, in conjunction with your trip odometer.


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## MN Driver (Sep 29, 2009)

EVfun said:


> Is this with Thundersky cells? The discharge graphs look so flat that I thought it would not be viable to use voltage. It figured the warning would come to late.


They are flat until the end when unloaded. With CALB/Sky Energy you find out when you have about 10% left. If someone was driving at a constant load and saw the voltage starting to drop at a quick rate without regard to an increased load, you've got to find a place to plug in mighty quick unless you've got 100 miles of range in the first place aren't far from home, otherwise it's time to park in a safe place and arrange a tow.
http://3.bp.blogspot.com/_i_c2BM_uB...AABTg/hpmMRUpxA0w/s1600-h/SkyEnergy1Cload.jpg
The Thunder Sky cells provide a little more warning it seems as you would notice it dropping under load somewhere around about 70-75%
http://2.bp.blogspot.com/_i_c2BM_uB...ABTo/FAK2c8X9BpU/s1600-h/thundersky1Cload.jpg
These are out of Jack Rickard's evtv.me blog

I personally have no plans of ever running batteries like these that low due to the cycle life considerations. The battery is under more stress to provide those amps when running low on power which becomes apparent by the spike in heat production and drop in voltage with the rise in internal resistance, it's essentially like pushing more current through the battery as the chemistry scrounges harder at pulling less of what is inside out. The RC community chooses cells that have less internal resistance since are convinced they will last longer. I agree with them on that aspect regarding cycle life. Although I don't really know if the Yttrium will do what Thunder Sky promises or not with the cycle life advantages it seems to be alluding to in its specsheets.

Then again I'm also looking to push 1mOhm CALB cells at 1.5C constant current on average with bursts of 2.5C in between. Quite abusive but it's the application. The CALB 40Ah cells are rated at 10C at 400 amps for 10 seconds now. The 180Ah cells are rated at 1000 amps(5.5C) for 10 seconds now. 100Ah cells rated at 800 amps(8c) for 10 seconds now. It also looks like we might be able to expect a new pack design that is gray without the ribbed sides soon too.


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## dimitri (May 16, 2008)

I promised a picture of how I mounted sender unit with hall effect sensor.
I bent U shaped bus bar from aluminum stock such that hall sensor slides in the middle with enough clearance for sender board to sit on top of the cells. Once bus bar is bolted to the terminals, you can fill the hole with silicone to prevent vibration, but in my case it's tight as is.










I got all parts and cases in stock now, I have an appointment at a local sticker shop tomorrow to make a screen mask. If that works, then I should be able to offer EV Display sometime this month.

Also, I have been using my unit powered from 12V battery for over a week now and it consistently gets to 100% at the end of charge based on charge current alone, so if you don't care for precise voltage and wattage data you can simply power the unit from 12V battery.

I am still planning to offer optional voltage scaler, but I have been too busy lately and have not had a chance to work on it yet.


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## EV-propulsion.com (Jun 1, 2009)

Looks good Dimitri, since the sensor is similiar in size to the paktrakr, what I have done with those might work here as an alternative: I have stripped a little extra insulation off the welding cable at the end where the lug/terminal goes. Slide the sensor on, then the lug, and crimp. Of course this was with 1/0 welding cable, it may not work with larger gauge. Unfortunately the sensor can't be removed without cutting the cable, but like your method also, it does eliminate extra connections for a copper pass through bar.
Mike
www.EV-propulsion.com


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## JRP3 (Mar 7, 2008)

I tried it with bare 2/0, no go.


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## jtgreeson (Aug 14, 2009)

Dimitri, That bus bar looks like a elegant solution. Is the small opening in the sensor a limitation in finding a common material to fabricate a bus? Would you think about supplying a length of copper bus bar material with the sensor? Users could bend, cut & drill to suit their installation. Suppling the material with the max dimentions that fit the limited sensor opening would save end users a bunch of time searching. Hopefully adding it wouldn't be a big time cost to you. There might be a temptation to use 1/0 cable or an undersized bus that might cause a path of higher resistance for some installations.


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## JRP3 (Mar 7, 2008)

For such a short piece 1/0 would be fine, but an included piece of copper would be nice, at least as an option.


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## Dave Koller (Nov 15, 2008)

JRP3 said:


> I tried it with bare 2/0, no go.


Experimenting with bus-bars and sensors the thickness has a bit of effect and if it is placed in the middle, up to the top (inside) or bottom slightly different setups BUT nothing serious - Dimitri has done it in a nice, simple (minimalist ) way.... Nice...


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## dimitri (May 16, 2008)

jtgreeson said:


> Dimitri, That bus bar looks like a elegant solution. Is the small opening in the sensor a limitation in finding a common material to fabricate a bus? Would you think about supplying a length of copper bus bar material with the sensor? Users could bend, cut & drill to suit their installation. Suppling the material with the max dimentions that fit the limited sensor opening would save end users a bunch of time searching. Hopefully adding it wouldn't be a big time cost to you. There might be a temptation to use 1/0 cable or an undersized bus that might cause a path of higher resistance for some installations.


I can't find appropriate size copper stock in local shops, but aluminum stock you see in the pic is plenty at every hardware store, like Ace, Home Depot, Lowes, etc.

For short bus bars I prefer aluminum since its available, easy to cut, bend, drill, etc. Copper is a bitch to drill, I almost lost a finger once


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## JRP3 (Mar 7, 2008)

Dimitri, what is the cross sectional area of that aluminum and what is the current carrying capacity? From the picture it looks to be about 1/2"x1/8", or .5x.125, which gives an area of .0625. My cell interconnects are about .1"x.8", .08 area, and copper as opposed to aluminum. 1/0 cable has a .08289 inch area.
http://www.allaboutcircuits.com/vol_5/chpt_3/1.html


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## dimitri (May 16, 2008)

JRP3 said:


> Dimitri, what is the cross sectional area of that aluminum and what is the current carrying capacity? From the picture it looks to be about 1/2"x1/8", or .5x.125, which gives an area of .0625. My cell interconnects are about .1"x.8", .08 area, and copper as opposed to aluminum. 1/0 cable has a .08289 inch area.
> http://www.allaboutcircuits.com/vol_5/chpt_3/1.html


Your numbers are correct, it does seem a bit too small, but it doesn't get hot and seems to carry my 450Amp acceleration current just fine.

The hole in the sensor allows to put 2 of 1/8" strips in, so if you are worried about it, make 2 bars and stack them.


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## jtgreeson (Aug 14, 2009)

dimitri said:


> Its 0.3" x 0.6" opening in a hall effect sensor. Its not the most convenient size, but its manageable.... I might offer the bar as well if I find a cheap source.


Here's Dimitri's quoted dimensions from several pages back on this topic. So is it correct an aluminum or copper bar that is .5 in by 1/4 in (.25) would yield .125 sq in area and be more than 2/0 cable? If so I think I'll start looking for that size at the local hardware store.

Another thought: Using ideas from this forum I made some interconnects from flattened copper pipe. I first heated it up to cherry red with a propane torch and let it cool slow to anneal it. It was softer and easy to drill and shape. 1/2 inch pipe came out to an area very close to 2/0. It's working so far but I'm monotoring it. Would this method work for these hall sensors if we folded the pipe over once? Any downsides?


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## jtgreeson (Aug 14, 2009)

A question for you Dimitri. Is the location of the current sensor critical? With the Link/E-meter and the EVision I believe they wanted it at the most Negative terminal. Can yours be placed in the middle of the pack?


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## JRP3 (Mar 7, 2008)

The only problem with 1/4 inch bar is it's not wide enough to make good contact on the battery posts and there isn't much left after you drill mounting holes. *Edit, realized you were talking about 1/4 inch thick, not wide. Never mind.
I was considering flattened copper pipe too, but I think I'll just use a short piece of stripped 1/0 and lugs.


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## dimitri (May 16, 2008)

jtgreeson said:


> A question for you Dimitri. Is the location of the current sensor critical? With the Link/E-meter and the EVision I believe they wanted it at the most Negative terminal. Can yours be placed in the middle of the pack?


Link/E-meter and EVision use shunts, not hall effect sensors, so they must be on the Negative side of the pack since they feed the display from the same pack and must have common ground on the power and shunt sensor circuits.

Since I am using hall effect, which is totally isolated, you can put it anywhere in the battery current loop.


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## dimitri (May 16, 2008)

Finally got masks from the print shop. Here is the completed product.

If you look close enough, you may notice minor imperfections in the screen mask and cutout hole for the display, clearly its not a mass produced factory product, but if you look from a foot away and pay attention to the screen data, its pretty damn good.

I will finish user guide and add everything to my Web store at 

http://minibms.mybigcommerce.com/categories/EV-Display/

in the next day or two.


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## 9852 (Jan 17, 2010)

dimitri said:


> Finally got masks from the print shop. Here is the completed product.
> 
> If you look close enough, you may notice minor imperfections in the screen mask and cutout hole for the display, clearly its not a mass produced factory product, but if you look from a foot away and pay attention to the screen data, its pretty damn good.
> 
> ...


wow, that looks good to me,what will you be asking for it.


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## Dave Koller (Nov 15, 2008)

Well done Dimitri!


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## dimitri (May 16, 2008)

billhac said:


> wow, that looks good to me,what will you be asking for it.


 


Dave Koller said:


> Well done Dimitri!


Thanks guys!

The first batch is done and ready for shipping. It can be purchased at my store at http://minibms.mybigcommerce.com/

Price is $180

First 3 units already sold today, less than 24 hours since I posted the product on the Web site 

Updated user guide is posted on the store's product support page at http://minibms.mybigcommerce.com/pages/Product-Support.html


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## azdeltawye (Dec 30, 2008)

tomofreno said:


> ..….I would like to see how the change in Wh and Ah compare over time.





EVfun said:


> Very cool but wouldn't pack SOC be better tracked by watching amp hours?





dimitri said:


> I want to digress a little into theory of SoC counting, the way I understand it ( which may be totally wrong ), to preempt some criticism which might be coming my way as usual.





dimitri said:


> Batteries provide power ( Watts ) and energy ( WattHours ), so from purely scientific point of view we should be counting Watts, not Amps alone. However, battery inefficiency ( lets ignore thermal losses due to resistance for a moment ) comes from the fact that we charge at higher voltage than we discharge ( assuming the same current in and out ), so we always put more Watts in than we get Watts out. LFP curve is flatter than Lead, so they are more efficient, but still there is some energy lost at every cycle. If I was to design EV Display based on Watts, I would have to compensate for battery efficiency to produce effective Fuel gauge.
> 
> However, batteries come rated in AmpHours, not WattHours. There is a reason for that. While voltage swing mostly accounts for energy innefficiency, amperage over time appears to be a better indication of useful energy available ( again, ignoring thermal losses ) and it makes amp counting more practical from EV driver point of view, albeit less scientific. Ignoring the voltage in the Fuel gauge automatically provides compensation for battery inefficiency. Due to thermal losses we still need to calibrate SoC at full charge, which is implemented in most battery monitors by setting SoC to 100% when pack voltage is at max charger voltage at the end of charge cycle. This is also implemented in my display.
> …….


 
Shortly after I started this thread I went ahead and purchased the Brusa BCM400 Ah/Wh meter for the Jetta conversion to gauge battery state of charge. Like I had mentioned earlier, Metric Mind didn’t carry the meter anymore. However I was able to get one from Brusa directly, for an arm and a leg… After installation I verified the Amp and volt readings were right on the mark with the fluke handheld and clamp-on meters (less than 1% error).

Over the past 500 miles I’ve been taking meticulous data after each and every charge cycle: Wh in & out, Ah in and out, mile, charge voltage etc… Much to my surprise I found virtually no difference in state of charge calculation whether I used Ah or Wh. See attached spreadsheet: The column on the far right is the battery state of charge using Watt-hours and the column next to that is SOC using Amp-hours. These SOC calculations were based on a nominal battery voltage of 128V (40 TS cells) and battery capacity of 180 Amp-hours which gives a nominal energy capacity of 23.04 kWh. As you can see from the SOC comparison there was less than a 0.3% difference in the SOC calculation for the limited data I charted. 

I recently added 5 more cells to my pack to bring the nominal voltage up to 144V and capacity up to 25.92 kWh. I re-baselined my spreadsheet to reflect the new battery capacity and will continue to take data. Since that’s what engineers do…


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## dimitri (May 16, 2008)

Darren,

this is excellent data! Very similar to what I am seeing with my EV Display too.

Thanks for sharing it!


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## EVfun (Mar 14, 2010)

azdeltawye said:


> Over the past 500 miles I’ve been taking meticulous data after each and every charge cycle: Wh in & out, Ah in and out, mile, charge voltage etc… Much to my surprise I found virtually no difference in state of charge calculation whether I used Ah or Wh. See attached spreadsheet: The column on the far right is the battery state of charge using Watt-hours and the column next to that is SOC using Amp-hours. These SOC calculations were based on a nominal battery voltage of 128V (40 TS cells) and battery capacity of 180 Amp-hours which gives a nominal energy capacity of 23.04 kWh. As you can see from the SOC comparison there was less than a 0.3% difference in the SOC calculation for the limited data I charted.


What is interesting to me is the kWh in and out and Ah in and out (charge and trip.) It shows near 100% charge efficiency based on amp hours. Only 0.3 amp hour more was used to charge the pack over nearly 1000 amp hours delivered! How are you charging your pack? It looks to me like you are doing something right.


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## azdeltawye (Dec 30, 2008)

EVfun said:


> What is interesting to me is the kWh in and out and Ah in and out (charge and trip.) It shows near 100% charge efficiency based on amp hours. Only 0.3 amp hour more was used to charge the pack over nearly 1000 amp hours delivered! How are you charging your pack? It looks to me like you are doing something right.


I'm undercharging my pack. I don’t have my BMS shunt modules installed yet so I set my charger CC/CV knee at 3.55V per cell (Manzanita charger). I wanted to reduce the risk of overcharging the weaker cells while I’m running around without any BMS (aka bareback). I had the CV threshold set a little higher at first but noticed some of the cells were up in the 3.8V range at the end of the charge cycle. At this point my pack is neither top balanced nor bottom balanced. More like middle balanced…

I imagine that once I install the shunt boards and crank up the charger voltage my charging efficiencies will drop. Although I question if there is any value in increasing the charge voltage since there doesn’t seem to be very much energy storage above 3.6V in the TS cells…


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## dimitri (May 16, 2008)

azdeltawye said:


> I imagine that once I install the shunt boards and crank up the charger voltage my charging efficiencies will drop. Although I question if there is any value in increasing the charge voltage since there doesn’t seem to be very much energy storage above 3.6V in the TS cells…


It won't drop much since there will only be a few minutes of CC charging between 3.6V and 3.8V. There isn't much useful energy at this point, but if you top balance the pack, this area allows you to line up all cells without having to shunt too much energy.

The downside in setting CC/CV knee too low is that the charger will spend too much time in CV phase since current is dropping, but there is still room in the cells to suck it in, so it will drag on and on until the charger times out. With CC/CV knee set higher in the curve, CV phase will be shorter and more efficient.


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