# LiFePO4 and a full charge



## GizmoEV (Nov 28, 2009)

That is similar to my observations. I got lower numbers than you but then I charged the cell to a particular voltage and let the current drop to a very low value and then charged to a higher voltage also letting the current drop very low. On my 100Ah cells I got about 1Ah above 3.5V.

I'm hoping in a couple weeks or so to test a couple of TS 40Ah cells to see what the ending current should be if charging to a voltage lower than 4.00V. My TS book says to charge to 4.00V and hold until the current drops to 0.015CA. My hypothesis is that charging to a lower ending voltage along with a lower ending current will still fill the cell. I hope to map the two values.


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## spdas (Nov 28, 2009)

"110 volts (3.43 vpc) to 111 volts (3.47 vpc) took 8 minutes. That is about *1.6 amp hours* added (2.7%.)"
This is where I charge to.

TS I guess has similar voltages to the Calb, but still I do not go anywhere near the 3.6, 3.7 being talked about in other threads. I charge my Calbs at 3.478 per cell. My Solar charge controller is set to that voltage and when that voltage is reached it goes into float mode and starts tapering the amp right to "0" as the cells have their fill. I tried up to 3.7v and get less than 2ah more on my 180ah cells, so that is my safety margin. And since Lith cell have such low resistance, there is no need to charge considerably higher rate to get your ending voltage, like a lead battery.

Francis


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## EVfun (Mar 14, 2010)

spdas said:


> TS I guess has similar voltages to the Calb, but still I do not go anywhere near the 3.6, 3.7 being talked about in other threads. I charge my Calbs at 3.478 per cell. My Solar charge controller is set to that voltage and when that voltage is reached it goes into float mode and starts tapering the amp right to "0" as the cells have their fill. I tried up to 3.7v and get less than 2ah more on my 180ah cells, so that is my safety margin. And since Lith cell have such low resistance, there is no need to charge considerably higher rate to get your ending voltage, like a lead battery.
> 
> Francis


I have done some work charging to only 3.5 vpc, but until I pull the BMS cell modules I can't keep it up for more than about 2 weeks at a time. I've done this 2 times and I have some cell drift that causes the spread to go from 3.49-3.51 vpc to 3.44-3.56 vpc by the 2 week mark. I measured my cell modules (not the installed units, but the other 13 I have) and found the standby current varies by up to 82 micro-amps (0.000082 amp.) That is about 7 amp seconds per day or 0.7 amp hours per year.

At the end of one of these test runs I connected a 5 ohm resistor to 2 of the highest cells for 120 seconds each. That removed 84 amp seconds while they where recharging, right after the charger hit 3.5 vpc (112 volts for 32 cells.) I let the charger time out normally, it runs for 1 hour after 3.5 vpc. During the next charge cycle, after a good drive, those 2 cells only went to 3.44 volts at the end of the charge cycle. They both dropped 0.12 volts for just 84 amp seconds! If ending voltage matches charge level with no drift this almost exactly matches the results of testing modules, up to 7 amp seconds per day variance. 

I'm thinking about going up to 34 or 35 cells without the cell modules and then only charge to 3.5 vpc to keep the DC to DC converter well under it's max input voltage of 130 volts. 35 cells at 3.5 volts would be 122.5 volts, 3.65 vpc would be 127.8 -- very close to the upper limit to the point where ripple would concern me.


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## T1 Terry (Jan 29, 2011)

I have 3.45vpc limit and at the moment using solar charging the cell drift seems to be going the other way, they seem to be coming together rather than drifting apart. It's early days yet but the pack has a cell logger attached so after a mth it should be fairly conclusive if these cells can self balance. I think charging to an excessively high voltage causes problems without any benefits, if the cells are fully charged at 3.4v 0.5v is enough over voltage to create a current flow.

T1 Terry


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## Guest (Aug 12, 2011)

3.65 is only a figure and not something set in stone. It is so you can be sure your on the upswing before terminating the charge cycle. Using the 3.45 or so is fine too because it can be on the low end of the upper rise but I'd say not quite on the steep upper curve. At 3.65 your on that steep curve but not near the top. My Leaf charges to full and with the gauge that lets me know how much I have left will drop fast in the first 1.4 miles. For the energy expended into the cells it is not required to go to that level. At 3.65 for just about any of the LiFePO4 cells will work just fine. Even my old Hi-Power which still hold over the rated 100 AH. Now if you have one or more cells in your pack that go past 4 volts per cell by the time the charge terminates you may want to check how well you bottom balanced your pack and you may want to lower your upper voltage limits. I charged my lithiums to 3.65 per cell but by the time the charge would terminate I had a couple up in the 3.7 volt range. Not an issue there either.

Pete 

I have a visual of the drop in mileage that correlates to the battery charge curve. It is kinda cool to have that sort of real on the road visual of how fast that top charge burns off showing that there is little usable energy at those upper levels. Granted my Leaf cells are not LiFePO4 Cells but they react the same. Want me to post a video? I have been doing a few videos of my first 1.5 miles to show consistent drop and not some fluke. When I get my 220 charger I will be setting my charge to 95% rather than 100%. No need to go lower than that. 70 real miles at freeway speeds is excellent and that includes blasting air in the hot weather.


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## EVfun (Mar 14, 2010)

gottdi said:


> 3.65 is only a figure and not something set in stone. It is so you can be sure your on the upswing before terminating the charge cycle. Using the 3.45 or so is fine too because it can be on the low end of the upper rise but I'd say not quite on the steep upper curve. At 3.65 your on that steep curve but not near the top.


Very true. I would be quite happy charging to only 3.5 volts per cell but until I remove the BMS modules I need to get all the shunts turned on each charge. If I don't then the drift caused by their very slight difference in standby current (less than 0.1 milliamp) will slowly pull the pack out of balance. I was surprised to find that just 84 amp seconds (0.02 amp hour) worth of deliberately created imbalance created a 0.12 volt difference in charge voltage the next cycle when I was charging to stay below the regs (to 112 volts for 32 cells.) 

I only hold 3.65 vpc for 15 minutes right now (only 20 minutes per charge above 3.5 vpc) and my cell range when the charger turns off is 3.62 to 3.68 volts. The BMS has no control over the charging process, just 1/2 amp shunts that slide on between 3.61 and 3.64 volts. The BMS monitoring loop isn't even powered when charging. 

The nice amp hour counter was discussed by JR on EVTV right after I had discussed it in another thread, so course it was sold out. I have one now and it is installed, but I'm having to much fun driving to want to take the time to mess with the wiring more than a few minutes at a time. So, right now I rely on the BMS cell loop to keep from pulling any cell below 2.5 volts. I haven't touched that point on any cell yet despite regular 6C blasts.


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## GizmoEV (Nov 28, 2009)

EVfun said:


> I have done some work charging to only 3.5 vpc, but until I pull the BMS cell modules I can't keep it up for more than about 2 weeks at a time. I've done this 2 times and I have some cell drift that causes the spread to go from 3.49-3.51 vpc to 3.44-3.56 vpc by the 2 week mark. I measured my cell modules (not the installed units, but the other 13 I have) and found the standby current varies by up to 82 micro-amps (0.000082 amp.) That is about 7 amp seconds per day or 0.7 amp hours per year.


Wow, that is fast. I went for 11 months before my spread was 0.1V. The extreme cells were at 3.441V and 3.544V. On this pack the first 6400Ah was on 18 cell pairs and charging was to 4.00Vpc. Just prior to starting my "drift test" I installed the remaining two cell pairs for a total of 20 cell pairs. I charged to 3.485Vpc with the new configuration. I ran 11,852Ah (not including regen Ah which is ~3%) through my 200Ah pack before reaching 0.1V spread.

How old is your pack and what charging voltage had you been charging to?



EVfun said:


> Very true. I would be quite happy charging to only 3.5 volts per cell but until I remove the BMS modules I need to get all the shunts turned on each charge. If I don't then the drift caused by their very slight difference in standby current (less than 0.1 milliamp) will slowly pull the pack out of balance. I was surprised to find that just 84 amp seconds (0.02 amp hour) worth of deliberately created imbalance created a 0.12 volt difference in charge voltage the next cycle when I was charging to stay below the regs (to 112 volts for 32 cells.)


It looks like I may have misread your post. You didn't do the charging to 3.5vpc with no BMS modules, right?


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## EVfun (Mar 14, 2010)

GizmoEV said:


> It looks like I may have misread your post. You didn't do the charging to 3.5vpc with no BMS modules, right?


Correct. I have not yet taken off the cell modules. I thought I would try charging at a lower level where none of them would ever turn on the shunts. I thought I could treat the pack like I didn't have a BMS by keeping the voltage down. That led to drift. I'm confident that drift would be much less or nonexistent if it wasn't for the very slight variations in the standby current of the BMS modules.


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## GizmoEV (Nov 28, 2009)

EVfun said:


> The nice amp hour counter was discussed by JR on EVTV right after I had discussed it in another thread, so course it was sold out.


Is this the Manzanita one?

I need a standalone Ah counter I can use to do some cell testing and it needs to be bidirectional. Anyone know of such a device that isn't too expensive? FWIW, this question isn't OT as the testing I want to do has to do with how to get a full charge at some voltage below the factory charge procedure.


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## GizmoEV (Nov 28, 2009)

EVfun said:


> Correct. I have not yet taken off the cell modules. I thought I would try charging at a lower level where none of them would ever turn on the shunts. I thought I could treat the pack like I didn't have a BMS by keeping the voltage down. That led to drift. I'm confident that drift would be much less or nonexistent if it wasn't for the very slight variations in the standby current of the BMS modules.


I guess that also points out how close my BMS modules were to each other in current draw because I left them on until just last month. Black Sheep Technology's BMS boards aren't the cheapest but I think the design is sound and consistent. I rebalanced the pack at the top and now only have a batt-bridge device in place.


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## EVfun (Mar 14, 2010)

Have you considered checking a number of cell modules for variations in standby current? I put a number of cells in parallel and then hooked up the cell modules, one at a time, with a 100 ohm resistor between the positive cell terminal and the positive BMS module terminal. Each 1 millivolt across the resistor is 10 microamps of current. Mine where running in a range around 280 millivolts. 

Have you checked to see how removing a few amp seconds from a cell effects the finish voltage the next charge? It sounds like your regs are very tightly matched. Mine closer than I would consider to be an issue for any other kind of electronics. It seems that just 1/50th amp hour is 1/10th volt at the end of charge. Perhaps it's a little less if charging to a lower voltage.


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## spdas (Nov 28, 2009)

"off topic, but related". What do you use to bring down the voltage in a high cell?

Francis


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## EVfun (Mar 14, 2010)

Time, usually. I try hard to avoid a high cell. My DC to DC pulls about 40 milliamps and if I want to pull down faster I hit the headlights, bringing the discharge to a bit over 1 amp. There is so little energy stored above 3.45 volts that after the charge is stopped it takes almost nothing to pull them back down under 3.4 volts resting. 

If I want to knock down a single cell in a string, when bringing them into initial balance, I use a 5 ohm power resistor with the end of an old battery charger cord spliced on for the wire and terminal spring clamps. If you know which ones are high you can take some charge out of them at any time. In other words, you can knock the whole pack back with the lights and then remove a little from the high cells after they are all down into the safe range.


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## spdas (Nov 28, 2009)

5 ohm power resistor......how many watts?

like this:?
http://cgi.ebay.com/1-New-Milwaukee...036?pt=LH_DefaultDomain_0&hash=item3a4c389a0c

thanks
Francis


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## EVfun (Mar 14, 2010)

10 watts at least. A power resistor run at over 1/2 its rating gets very hot. I think this is the resistor I use, it looks the same.


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## Guest (Aug 12, 2011)

Charging to the top will net you about 1.5 miles extra per charge. This is another reason to not bother to waste the time or expense to top balance and to charge to the top of the curve. Nissan does but in reality is not needed. It gains about 1.5 miles extra for what? Maybe a loss in battery life? Maybe. But the gain is minimal if you decide to do so. The video is uploading to youtube at this time but the graph below corresponds to charging to the top of the battery curve then discharging to the flatter portion of the curve. 

We start by showing 90 miles to drive.
We begin the graph at .4 miles 
We start out and climb to 65 mph. 
1 minute later and 1 mile down the road we show 81 miles remaining. 

This is consistent and I have been videoing the event every morning for a few days to show it is not some fluke or some abnormality. It does in fact happen and it does correlate with the curve of a lithium battery fully charged and discharging. So in one mile and in one minute at 65 mph we loose that top charge quickly. 

I measured in 5 second intervals for a full minute. Total distance traveled was 1.5 miles. I began the video and measurements at .4 miles.


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## spdas (Nov 28, 2009)

If you let it rest for a couple of minutes, how many miles will it come back to?

And after how many minutes or miles does it "Flatten" out and stick at a certain milage? 

francis


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## Guest (Aug 12, 2011)

If I back off my speed while driving I can get it to ease back and show a mile or more gain. Never just drove it 1.5 miles then let it sit for awhile. I presume it would go up a tiny bit. Computer calculations are also based on current speed. It is not directly volts and AH. Your also seeing mileage compared to directly voltage which happens to be quite high. I am sure you'd see a much greater drop in voltage. You must also incorporate sag into the equation. See, if I back off a bit and the mileage bounces up a tiny bit I can see that correlation to less under load sag which you can see in amps and voltage while driving if that is all you have. It is an under load change. 

After 1.5 miles your already getting into the flat part of the charge curve. The upper end of the flat but remember the curve is not totally flat. It always points downhill. 

Pete 

Just thought it would be a fun little video.


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## Guest (Aug 12, 2011)

Don't forget that this is the first minute (60 seconds) of the drive. The curve would look much steeper if I were to include more miles on the graph. 

Pete


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## lowcrawler (Jun 27, 2011)

So my take-away from all this is: Don't worry about trying to get the most charge into your battery -- there isn't much at the top and by trying to get it, you are possibly risking your batteries.


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## jddcircuit (Mar 18, 2010)

Evfun,
Thank you very much for the valuable data.

With such low Ahs above the knee does this tell me that if my pack is not TOP balanced to within a couple of Ah then it is theoretically possible that I could overvolt a single cell during charging if I am only monitoring the full pack voltage or is there enough room for error that this could not happen.

If single cell overvolting is possible with relatively small top imbalance and overvolting matters to the life of the cell, then I guess bottom balancing with any type of cell capacity variation could be a mistake if I am charging based on pack voltage.

I guess I want to control my charge profile based on my highest SOC cell and not the pack voltage but I need to be aware that single cell level monitoring circuitry could be causing the top imbalance thus creating the problem it is trying to detect.

much to think about
Thanks
jeff


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## EVfun (Mar 14, 2010)

jddcircuit said:


> Evfun,
> Thank you very much for the valuable data.
> 
> With such low Ahs above the knee does this tell me that if my pack is not TOP balanced to within a couple of Ah then it is theoretically possible that I could overvolt a single cell during charging if I am only monitoring the full pack voltage or is there enough room for error that this could not happen.
> ...


I have not tried a bottom balanced pack but I am a bit concerned that peak charge voltage would need to be tailored to when the first cell starts to take off with upward voltage. I was surprised, and concerned, that just 0.02 amp hour (84 amp seconds) of deliberate imbalance was changing the end of charge voltage on the next cycle by 0.12 volts. 

I had not heard anyone taking about cell modules causing imbalance but my pack showed some drift and testing a sample of my cell modules showed they could imbalance the pack by up to 7 amp seconds a day. I should have continued charging to 3.5 vpc for a few more cycles after that, but instead rebalanced the cells at 3.65 volts. Once rebalanced I set the charger voltage limit back to 116.5 volts and current limit up to 12 amps for future cycles.

Perhaps some other members with bottom balanced packs will chime in with end of charge profiles and cell voltage variation data.


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## Guest (Aug 12, 2011)

If you don't use a BMS to keep the pack charged and balanced at the top then you want to bottom balance and not use the BMS to try to keep the top balanced. Your pack should be balanced enough to be able to charge your pack to 3.65 v per cell but that is taking into account the entire pack. You will most likely have a few up above 3.65 but you don't want any above 4 v. If you do at the end of charge then you may have trouble but it should not be terrible. It's not like having a BMS stop working and your charger keep charging long after it's full. A few cells in the upper limits is usually no trouble. But to put the whole pack there is more or less a waste. You burn through that very fast. So a 3.65 terminal cut off is a good point. No doubt. 

If you have a bms and do shunting to keep the pack fully balanced at the top you will burn off that top bit in no time. Why then bother. 

The problem with cells charging near the top is that the rise in voltage is extreme and fast. Just like it is extreme and fast on the discharge until the level stabilizes. 

Knowing the knee on top and bottom and staying between does not really need a BMS. But if you must then by all means do so. If you can control your BMS to have a lower upper limit then that would be good to set that. 

I don't see too much problem with life of the cell if charged up to like 4 volts every time. It is if you charge up to 5 v that you may have life troubles. Up beyond that your itching for trouble. 

Also don't over discharge your cells. Same issue at the bottom but at the bottom you will most likely be driving and pumping mega amps through the cells. Go to low and you ruin a cell or more in the blink of an eye. My controller can keep me from doing that. Even my Leaf keeps me off the bottom. I can keep it off the top too. I won't do 80% but I will do 95%. 

Pete


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## drgrieve (Apr 14, 2011)

I've found that the EVTV videos on this topic very informative.

He's done all the leg work for us.

This is an old one, but I can;t find the more recent one I watched.

http://www.projectooc.com/evtv/index.php?showid=5


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## steven4601 (Nov 11, 2010)

Don't consider this post topic worthy if you know better, but here is what I have to say about bottom balancing & no cell voltage protection circuit.

How do you / the charger decide when to stop charging when bottom balancing? Ah Counting? About all chargers are CC/CV chargers. In my honest understanding, setting the charger max voltage low enough to prevent the 'fullest' cell from over-charging in a bottom balanced pack is a bit too 'tricksey' . The fullest (or lowest capacity one) cell could be amp-hours away from the others... Charging it with a CC/CV charger can literally cook it. 

To put it bluntly, bottom balancing without an Amp-hour counter is worse than any other option. 

Top balancing is the safest way for (dumb) CC/CV chargers. 

Like I said, you do not have to take my advice, not telling to do otherwise. 
​


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## rwaudio (May 22, 2008)

steven4601 said:


> Don't consider this post topic worthy if you know better, but here is what I have to say about bottom balancing & no cell voltage protection circuit.
> 
> How do you / the charger decide when to stop charging when bottom balancing? Ah Counting? About all chargers are CC/CV chargers. In my honest understanding, setting the charger max voltage low enough to prevent the 'fullest' cell from over-charging in a bottom balanced pack is a bit too 'tricksey' . The fullest (or lowest capacity one) cell could be amp-hours away from the others... Charging it with a CC/CV charger can literally cook it.
> 
> ...


I agree with Steven on this one for a few simple reasons. There are two easy ways to kill a LiFePO4 cell, over discharge, and over charge. If you over discharge a cell it becomes a brick, no lives lost, no harm done, except for the poor cell(s). Over charge a cell too far and you could get a brick or a fire. 

Then I look at it from the perspective of how often I get to one of the two potentially dangerous ends. How often do you discharge past 80% DOD? In my design, I hope that's very seldom. How often do you charge your car to what you consider "full"? For me this will be daily. I would rather be safer at the top where the outcome could be worse and I go there daily than safe at the bottom where I seldom go.

The key is to consider what everyone else says and make YOUR OWN DECISION on top/bottom balancing, bms/no bms, just putting the cells in the car as is like JR has suggested many times before and hope for the best.


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## EVfun (Mar 14, 2010)

drgrieve said:


> I've found that the EVTV videos on this topic very informative.
> 
> He's done all the leg work for us.
> 
> ...


I am watching that one now. I do want to point out an error which confused the heck out of me for a bit. Between 11:20 and 11:30 in the video he explains how LiFePO4 cells charge. He has it backwards, the Lithium ions travel to and incalate with the graphite anode material. When they leave the graphite on discharge the electrons of the outer shell do not go with their Lithium atoms (that is why they are ions) but instead go through our EV circuits. 

It appears that the meat of the video starts at the 1 hour mark (it is a 102 minute video.) On the other hand, it is sunny outside so I will have to get back to it later. Seattle has lacked sunshine this summer so I am going to enjoy it!


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## GizmoEV (Nov 28, 2009)

steven4601 said:


> How do you / the charger decide when to stop charging when bottom balancing?​


By having the cells closely matched in the first place. If the cells are close enough and do not drift in capacity and/or SOC everything works just fine. That is what JR has shown over a couple of years. We don't have data to support what we hope happens as the pack ages.

My 20 cell pairs (TS-LFP100AHA buddy paired), starting with them top balanced at 4.00V, took 11 months to get 0.1V apart at the end of charge when charged to 3.485vpc. This was with Black Sheep Technology BMS boards in place. These particular boards are currently set to shunt at 4.00V so no balancing was done during the 11 months.

I think it is as rwaudio basically says, look at all the data and info and determine what risks you are willing to live with. Given that BMS boards have killed cells and cells have been killed without a BMS what risk to you want to take. In any case, make sure your charger will shut off on its own even if the BMS doesn't tell it to. Have a backup plan.


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## Guest (Aug 14, 2011)

GizmoEV said:


> By having the cells closely matched in the first place. If the cells are close enough and do not drift in capacity and/or SOC everything works just fine. That is what JR has shown over a couple of years. We don't have data to support what we hope happens as the pack ages.
> 
> My 20 cell pairs (TS-LFP100AHA buddy paired), starting with them top balanced at 4.00V, took 11 months to get 0.1V apart at the end of charge when charged to 3.485vpc. This was with Black Sheep Technology BMS boards in place. These particular boards are currently set to shunt at 4.00V so no balancing was done during the 11 months.
> 
> I think it is as rwaudio basically says, look at all the data and info and determine what risks you are willing to live with. Given that BMS boards have killed cells and cells have been killed without a BMS what risk to you want to take. In any case, make sure your charger will shut off on its own even if the BMS doesn't tell it to. Have a backup plan.


Properly set up I don't yet know of any directly related to not having a BMS installed. Top balancing then driving with no BMS however HAS resulted in loss of cells at the bottom. 

Im pretty impressed with the results from over 3 years worth of information. I guess the first generation must fully pass muster before any nay sayers back off. Do what ever you wish to make your head happy. It's your money and your car and your time. 

Pete


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## Guest (Aug 14, 2011)

Testing cells and charging and discharging in the lab or garage don't count. It is the installed in the car and driven on the road that is the true test. Mistakes in testing will occur. Which should be done long before putting them in the car but you know we can't just do that. We just gotta have them in the car. Would not be a risk unless we did. Risk taking is in our nature.


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## Guest (Aug 14, 2011)

Doesn't count for damaged or destroyed cells.


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## EVfun (Mar 14, 2010)

But your Gizmo is top balanced, right? Having watched the EVTV video from 12-18-09 I see JR did briefly address the variation at the end of charge with cells that are NOT top balanced. It sounds like +/- 0.1 volts from target was about to be expected when shooting for 3.5 vpc average. He suggested the variation would get larger if a higher target voltage was selected. 



> In any case, make sure your charger will shut off on its own even if the BMS doesn't tell it to.


I think that is very important. If you run a BMS you have to realize that everything has a failure rate. The charger should be able to handle itself despite a failure of the BMS. My charger and cell modules do not talk. The charger is set to take care of the pack. It will only go to 116.8 volts at just milliamps of current (32 cells, 3.65vpc.) It will only run at 116.2+ volts for 15 minutes before shutting off. I think I'm leaving only 1 or 2 percent "on the table."


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## GizmoEV (Nov 28, 2009)

EVfun said:


> But your Gizmo is top balanced, right? Having watched the EVTV video from 12-18-09 I see JR did briefly address the variation at the end of charge with cells that are NOT top balanced. It sounds like +/- 0.1 volts from target was about to be expected when shooting for 3.5 vpc average. He suggested the variation would get larger if a higher target voltage was selected.


Yes, they are top balanced. I was thinking that since I left my BMS boards on they would alert me to a low cell so I didn't run it in the ground and it would make comparing voltages at the end of charge much easier to see if there was any drift.

I was thinking that the cell variation I saw after 11 months might have been due to differences in the BMS module current draw but I'm thinking that isn't the case. I just tested each one at 3.400V and the range was from 1.010mA to 1.021mA. Even after a year this would only be about 0.1Ah on a 200Ah cell. I don't think this would account for the 0.1V difference at 3.485vpc. Besides the low cell had a 1.018mA board and the high cell had a 1.012mA board.

I do agree that as the ending voltage goes up the variation between cells will be more pronounced.



> I think that is very important. If you run a BMS you have to realize that everything has a failure rate. The charger should be able to handle itself despite a failure of the BMS. My charger and cell modules do not talk. The charger is set to take care of the pack. It will only go to 116.8 volts at just milliamps of current (32 cells, 3.65vpc.) It will only run at 116.2+ volts for 15 minutes before shutting off. I think I'm leaving only 1 or 2 percent "on the table."


I don't think you are even leaving that much on the table. If you are ending with just milliamps of current and going to 3.63vpc I would be willing to bet you are at much less than 0.5%.

I am still looking for a bidirectional Ah counter to test ending voltage vs ending current to see how they are related for the same end of charge SOC. Anyone have a suggestion?


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## GizmoEV (Nov 28, 2009)

gottdi said:


> Properly set up I don't yet know of any directly related to not having a BMS installed. Top balancing then driving with no BMS however HAS resulted in loss of cells at the bottom.


The key words are "Properly set up." Too often it seems that people are unwilling to admit they made a mistake and just blame something else or don't think some piece of info is important so leave it out. This gives a false idea of what really went wrong.



> Im pretty impressed with the results from over 3 years worth of information.


Are you referring to your own data or the collective data from the past 3 years?



> I guess the first generation must fully pass muster before any nay sayers back off. Do what ever you wish to make your head happy. It's your money and your car and your time.
> 
> Pete


But then they will say the new cells are different so you can't use the old cells as a benchmark.


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## EVfun (Mar 14, 2010)

GizmoEV said:


> I was thinking that the cell variation I saw after 11 months might have been due to differences in the BMS module current draw but I'm thinking that isn't the case. I just tested each one at 3.400V and the range was from 1.010mA to 1.021mA. Even after a year this would only be about 0.1Ah on a 200Ah cell. I don't think this would account for the 0.1V difference at 3.485vpc. Besides the low cell had a 1.018mA board and the high cell had a 1.012mA board.
> 
> [snip]
> 
> I don't think you are even leaving that much on the table. If you are ending with just milliamps of current and going to 3.63vpc I would be willing to bet you are at much less than 0.5%.


That would be 6 micro amps for 11 months, about 0.048 amp hours. I removed about 0.023 amp hours (84 amp seconds / 3600 to get amp hours) to create a 0.12 volt difference on the next charge cycle. So roughly, 0.019 amp hours would have been about 0.1 volt on my pack, but mine are only 60 amp hour cells. I would roughly expect the difference would have been enough to make a 0.1 volt drop in a 150 amp hour cell -- not far off what you got. 

Sorry, I wasn't very clear on that. I am only getting down to about 2.5 amps (and dropping fast) when the charger shuts off. I can't get down to just milliamps when charging to 116.6 volts because the regs will draw about 1/2 amp at 3.65 volts. I set the maximum voltage on my Manzanita Micro charger with a 7 watt light bulb for a load, about 60 milliamps.


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## DIYguy (Sep 18, 2008)

GizmoEV said:


> I am still looking for a bidirectional Ah counter to test ending voltage vs ending current to see how they are related for the same end of charge SOC. Anyone have a suggestion?


Cycle Analyst. As long as you have the charger connected after the shunt, it will count up when driving and back down again when charging. Mine finishes very close to zero each time. I did calibrate it to be accurate at lower currents, so it may be out a bit at the higher end.
I do have a Zeva fuel gauge, but I haven't connected it yet. I use the CA amp hour counter as a fuel gauge, and that works great.
Of course the CA does a host of other math calcs as well.
Edit; wrt ending current, you can see it on that same screen as the AH counting down.


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## EVfun (Mar 14, 2010)

I did some range testing in the EV Buggy today. When asked at shows or when driving it about I've stated the range is 30 miles, even though I had never gone farther than 19 miles. It's just a little pack of 32, 60 amp hour Thunder Sky cells. I was being conservative. Today I have gone 34 miles so far. I have just managed to pull it just below 87 volts (2.71 vpc average) at 6C, briefly blinking the Zilla low battery voltage indicator. The cells have been allowed to rest long enough that I can measure all of them without the first ones creeping up more than 0.001 volt (perhaps 10 minutes.) They are resting between 3.197 and 3.216 for a range of 0.019 volts (not bad for a top balanced pack.) No cell hit 2.5 volts during the drive, the BMS has to be good for something.

The best my EV Buggy ever did with lead (10 Optima groupe 34 yellow tops) is 21 miles. I had to drive like there was an egg under my right foot the whole time to stretch it that far. The sag under load was horrible for the last 2 miles, effectively limiting me to about 200 amps for acceleration. Now I've gone 34 miles and can still drive it like I stole it. Lithium kicks butt!

Hey DIYguy, thanks for the tip on the cycle analyst. Here is a link to the home page for it. It can run on up to a 150 volt pack and can be special ordered for up to 350 volts input. It powers itself from the full pack so it doesn't cause any pack imbalance. It offer a shunt based amp hour count in addition to a number of other features.


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## MalcolmB (Jun 10, 2008)

Hi EVfun. I just wanted to say many thanks for posting the info on your voltage tests. Those are really useful figures.

Looking at the range you're getting, I'd also be interested to hear what your buggy weighs, to give me a rough idea what I can expect from my mini.

Malcolm


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## EVfun (Mar 14, 2010)

Your very welcome, Malcolm. I try to share whatever I learn about EVs. I have not weighed the buggy since converting it to Lithium, but based on what was removed and what was installed I estimate the weight at 1100 lb. That happens to be what it weighed when it was gas powered. With the lead pack it weighed 1420 lb. 

I drove it a couple more miles, for a total of 36 miles on the odometer. But wait, I forgot about those undersize front tires. The actual distance is 88% of indicated. So in total about 32 miles and the last mile I could feel the minimum battery voltage limit kick in and cut back the amps to keep the pack to at least 84 volts (3.625 vpc average.) I when I got home I turned the headlights on to put a little load (about 1.4 amps total) on the pack. The voltage stabilizes more quickly (quits creeping up sooner after parking) with just a little load. I measured the cells and found a low of 3.062 and a high of 3.164 volts. The pack is at its limit and they all stayed above 2.5 volts under load. I'm thinking that with a pretty well matched pack and a smart controller the top balance vs. bottom balance issue may be moot. 

The pack is receiving a well earned charge now.


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## drgrieve (Apr 14, 2011)

I've found a more recent video - that last one was in 2009 - this one is July 8 2011.
Also tests A123 batteries in this video. 

http://www.youtube.com/user/marionrickard#p/u/4/BasB7E5PUGI

You can also find videos dicussing the merits of bottom balancing versus top balancing.

One point to make is that voltage under sag does not damage the battery (you can sag a battery to 1 volt if you want) but the resting voltage around 2.85 is pretty much empty. 2.5 volts resting is pretty close to death! (This for Thundersky, Calb has a flatter discharge curve and steeper knee.)


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## MN Driver (Sep 29, 2009)

drgrieve said:


> I've found a more recent video - that last one was in 2009 - this one is July 8 2011.
> Also tests A123 batteries in this video.
> 
> http://www.youtube.com/user/marionrickard#p/u/4/BasB7E5PUGI
> ...


Not quite, sagging a battery to 1 volt had better be a VERY short term duration or that battery is likely going to have a very short life with how hot it will be internally. Say you've got a 180Ah cell that sags to 2.5v at 1000amps, that 2.5v sag and 1000 amps is turning that battery into a 700 watt heater where that heat is not quickly escaping that plastic case, its trapped in there for awhile. That is 700 watts of heat with minimal dissipation in a 5.6kg(12.3lbs) package that is very small for that much heat at 275x182x71mm. I couldn't imagine what kind of current you'd have to pull if your battery was at half SOC or more to sag it to 1v and I wouldn't know why someone would stress their batteries that much when they are low either.

Seriously, what reason do you have to sag that low? In a racing application I could see someone making very short runs at 2 volts like what some people are doing with Headway cells but for anything on the street I'm not seeing why you'd ever need to pull below 2.5v at normal temperatures.

The only time I'd think a valid consideration for sagging lower than 2.5v on a pack someone wants to last a long time would be in cold temperatures as that self-heating helps you a bit but once the battery is hot that sag goes away. Otherwise I would keep that sag to a very short time period.

Heat damages batteries, Jack demonstrated that in his own video with the CALB cells pulling 4C and at the 30% SOC mark the batteries were beyond hot enough to damage them, especially since he was measuring the plastic and not the terminal temperature where it would read hotter. He vented those cells with that experiment, which he didn't deny that it was partially his intent while testing to see their limits but 4C isn't a constant current for those, 3C constant, maybe at a moderate or cooler ambient temp.

My point is that more sag is more internal heat. 1v would require a ton of amps and I can't imagine the duration that they might accept that sag or the amp level you'd be pulling while those cells are permanently damaged.


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## GizmoEV (Nov 28, 2009)

DIYguy said:


> Cycle Analyst. As long as you have the charger connected after the shunt, it will count up when driving and back down again when charging......
> Edit; wrt ending current, you can see it on that same screen as the AH counting down.


The CA is a great unit. I have one on my Gizmo. The problem is I want to test one cell and the CA needs more than that voltage to work. That is why I need a similar device which uses a separate power supply. Maybe I can get GrinTech to make a special one for me.


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## DIYguy (Sep 18, 2008)

What about the Manzanita, State of Charge Head like JR was using lately? Not sure how low it will go in voltage . . But, I think Jack was doing 4 cells maybe?? Can't recall for sure, but it wasn't a whole pack. 

Regards,
Gary


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## EVfun (Mar 14, 2010)

The Watt's up or Doc Wattson meters may be of interest to you. They are made by RC electronics and have been around for some time.


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## GizmoEV (Nov 28, 2009)

Thanks for the suggestions.

Wow, the SOC head is expensive. I don't understand why it is $450. Maybe it does more than the website says.

I looked at the Watt's up an DocWattson meters but it isn't clear if it is bidirectional. I want to be able to monitor the discharge of a battery and then charge it and have the meter run in reverse. It looks like the meters have to be disconnected and turned around to go from discharge to charge. They do list it for e-bike use and some of those have regen so maybe it will work. I'll have to contact them and find out.

I just checked some more on the website and it says:

*"About measuring current flow in both directions (e.g. bi-directional measurement)?*

The "Watt's Up" and Doc Wattson meters only measure current flowing in the black (negative) wire from their LOAD side to their SOURCE side as labeled on the meter's case. Current flowing in the opposite direction is not measured and does not affect accumulated or peak readings."


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## EVfun (Mar 14, 2010)

I found the bi-directional question hard to find an answer too. I found it in the FAQ and the unit is NOT bi-directional. For bench testing that isn't a huge deal, just swap the leads and count up both ways or use 2 meters. At work we use 2 meters so we can walk away from a battery and come back as couple days later to see the the in and out amp hours. The FAQ also provides a link to modify the units for more amps with an external shunt.


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