# Battery Balance monitoring system



## EVfun (Mar 14, 2010)

Since the Lee Hart Battery Bridge has come up a number of times it has been suggested I start a thread on the topic. 



EVfun said:


> For the Buggy I'm looking into some type of Lee Hart style battery bridge so the car can be shut down easily with no parasitic pack loads. Just disconnecting the bridge pack + and pack - connections would remove the entire load from the system. I've been thinking about 2 different approaches to this.
> 
> One would be a 2 comparators forming a voltage window detection circuit. If the common input was the center of the pack and the references where about 1/4 volt above and below the virtual center of the pack the voltage window would be exited if any one cell swung 1/2 volt out of line. The output of the comparators would drive a optical isolator and then a typical 12v system control setup could drive the alerts. My Clean Power Auto BMS head end board could be used.
> 
> The other idea is to build the system with diodes like Lee designed it but instead of red LEDs use the LEDs in a 2 channel optical isolator. Then the isolated output of those LEDs could drive the warning/shutdown systems. Since this would operate the isolator output transistors is a more analog mode I would guess each channel would require a trim pot to adjust an output stage that would most likely be a TLV431.





GizmoEV said:


> What about starting a thread for something like this. I have a couple of emails from Lee Heart about two other ways to do this. One of them is to use a zero centered ammeter in with a +-50 uA range. The meter could be calibrated to represent any voltage difference desired. The other circuit would be some sort of comparison circuit. A couple of designs could be created with possible source of components for each one.
> 
> What do you think?


It probably is a good idea. Quite a few different approaches have been suggested. I've posted a couple of basic ideas above for how one might be constructed. I'd like to hear more ideas on how to accomplish this, as well as how to use and/or display the results.


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## GizmoEV (Nov 28, 2009)

Below is an email exchange I had with Lee Heart on a modification to his Batt-Bridge.

On 1/4/2011 7:19 PM, David Nelson wrote:Hi Lee,

I was wondering how hard it would be to have some sort of analog
display like a typical auto ammeter or galvanometer with zero at the
center to measure the difference between the two halves of a battery
pack?​That's even easier than the existing circuit. View with a fixed width font:

```
________/\/\__________
  |         R1           |
__|__+                   |
 ___ upper half of pack  |
  |  -      __           <
  |________/  \_________\< R3
  |        \__/ meter   /< trimpot
__|__+                   <
 ___ lower half of pack  |
  |  -                   |
  |________/\/\__________|
            R2
```
Use a zero-center analog meter. R1 and R2 are equal values, about 10 times higher than R3, and limit the meter current in case R3 is turned all the way to one end. R3 is a trimpot to set the balance. Adjust it so the meter reads zero when the two half packs are at exactly the same voltage.

The meter should be something like +/-50 microamps full scale, to allow the resistors to be large values and so not draw much current or dissipate much power. Set the pot *first* and then connect the meter.Another idea I had was to have some sort of comparator circuit
that reads the two halves and outputs a + or - voltage reading to a
display. I would also want this device to be powered directly from the
pack it is measuring and not rely on the 12V system. I really like
that the CycleAnalyst runs entirely off the pack since it won't put an
imbalance on the pack.​That's not hard, either. Connect the LEDs of an optocoupler with a series resistor across each half-pack. Connect the transistor output of the two optocouplers in series. Pick the series resistors for each LED so the voltage between the transistors is half of the power supply when the two half-packs balance. It will then swing up and down as the pack's voltages are different.

With either circuit, you'll have to pick your values experimentally, based on the specific parts you use.
=======================================

Thanks Lee, I think I followed that. Just to be sure see notes below.

On Wed, Jan 5, 2011 at 10:08 AM, Lee Hart <[email protected]> wrote:
> On 1/4/2011 7:19 PM, David Nelson wrote:
>>
>> Hi Lee,
>>
>> I was wondering how hard it would be to have some sort of analog
>> display like a typical auto ammeter or galvanometer with zero at the
>> center to measure the difference between the two halves of a battery
>> pack?
>
> That's even easier than the existing circuit. View with a fixed width font:
>[see circuit diagram above]
> Use a zero-center analog meter. R1 and R2 are equal values, about 10 times
> higher than R3, and limit the meter current in case R3 is turned all the way
> to one end. R3 is a trimpot to set the balance. Adjust it so the meter reads
> zero when the two half packs are at exactly the same voltage.
>
> The meter should be something like +/-50 microamps full scale, to allow the
> resistors to be large values and so not draw much current or dissipate much
> power. Set the pot *first* and then connect the meter.


Ok so the meter is a microamp meter with an internal shunt and is
connected to the wiper of the trimpot. The meter will indirectly
measure voltage difference where the meter scale would be some
constant times the current through the meter depending on R1, R2, R3
and the total pack voltage. So if the parts were selected such that at
a pack voltage of 64V a full swing of 50uA would mean say 2V
difference that with a pack voltage sagging down to 55V a full swing
of 50uA might be something like 2.2V (not calculated, just a guess).

>> Another idea I had was to have some sort of comparator circuit
>> that reads the two halves and outputs a + or - voltage reading to a
>> display. I would also want this device to be powered directly from the
>> pack it is measuring and not rely on the 12V system. I really like
>> that the CycleAnalyst runs entirely off the pack since it won't put an
>> imbalance on the pack.
>
> That's not hard, either. Connect the LEDs of an optocoupler with a series
> resistor across each half-pack. Connect the transistor output of the two
> optocouplers in series. Pick the series resistors for each LED so the
> voltage between the transistors is half of the power supply when the two
> half-packs balance. It will then swing up and down as the pack's voltages
> are different.
>
> With either circuit, you'll have to pick your values experimentally, based
> on the specific parts you use.


So the advantage with this circuit would be that the output would be
the true voltage difference between the two halves regardless of the
changes in total pack voltage assuming that the device was
built/calibrated properly, right? This circuit will take me a little
longer to study out and understand. It is times like this I wish I had
spent more time in my Physical Electronics classes to understand what
really was going on and how everything inter-related.

================================
On 1/6/2011 1:21 AM, David Nelson wrote:Thanks Lee, I think I followed that. Just to be sure see notes below.


Ok so the meter is a microamp meter with an internal shunt
​Yes, though these meters have no internal shunt (and don't need one). Their coil is many turns of very fine wire, having 2K or so total resistance. Full scale is therefore about 50 microamps at 0.1 volts.and is connected to the wiper of the trimpot.​Yes... meter between center tap of pack and wiper of the trimpot. It directly measures the voltage difference between these two points (with full scale being a 0.1v difference. Too sensitive; but you fix that with R1 and R2.So if the parts were selected such that at a pack voltage of 64V a full swing of 50uA would mean say 2V difference that with a pack voltage sagging down to 55V a full swing of 50uA might be something like 2.2V (not calculated, just a guess).​Yes; that's the idea.

To calculate it, start with the equivalent circuit at the wiper. It is a voltage of half the pack, with a series resistance of the parallel combination of the upper and lower resistances (R1 + R3/2 and and R2 + R3/2). Since R1=R2 and R3 is split exactly in half at balance, this is just Req = (R1 + R3/2)/2.

Suppose we want a 2v imbalance to produce full-scale on the meter, and the meter's resistance is 2000 ohms. When one half of the pack is 2v different, the center tap is 1v different from the voltage on the wiper. To get 50ua to flow with a 1v difference, the total resistance must be Rtotal = 1v / 50ua = 20k. The meter has 2k of this, so we need 18k more.

Thus, Req = 18k, so R1 + R3/2 = 36k and R2 + R3/2 = 36k. You can use any combination that adds up to 72k; two 36k and no pot, or two 35k and a 2k pot, etc.

The pot sets the balance point (compensates for R1 and R2 not being identical values). With perfect 0% resistors you wouldn't need it. But as a practical matter, if you use 5% resistors the pot needs to be at least 5% of their value to be able to compensate for the difference between them.

With precision resistors, the meter can be calibrated in volts difference. (Remember that a 1v difference at the meter means a 2v difference in the two half-pack voltages). If you don't use precision resistors, you can add a second pot in series with the meter to adjust the meter so full scale is exactly a 2v difference (or whatever you want).So the advantage with this circuit would be that the output would be
the true voltage difference between the two halves regardless of the
changes in total pack voltage assuming that the device was
built/calibrated properly, right? This circuit will take me a little
longer to study out and understand. It is times like this I wish I had
spent more time in my Physical Electronics classes to understand what
really was going on and how everything inter-related.​The meter circuit measures true voltage difference, regardless of the pack voltages.

The circuit with the optocouplers has an isolated output, but won't be as accurate because the optocouplers are not precision components. Their characteristics change with temperature, with age, and each part is a little different from the others. But it should work "good enough" for spotting a cell in danger.

Warning... remember that this is a very simple circuit; it can't spot a problem is TWO cells are both low in voltage, one in the upper half and one in the lower half of the pack.

-- 
Lee A. Hart | Ring the bells that still can ring
814 8th Ave N | Forget the perfect offering
Sartell MN 56377 | There is a crack in everything
leeahart earthlink.net | That's how the light gets in -- Leonard Cohen


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## GizmoEV (Nov 28, 2009)

Hopefully the above post comes through ok. I had to trim down a few characters because of a forum limitation of 10,000 characters.

I haven't found a ±50 µA meter yet so if someone knows of one not physically too large that would be great. Hopefully we can compile a parts list and/or sources.


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## ZANDARRR (Sep 8, 2010)

I have read and understand the Lee Heart Battery Bridge, but if you are driving and the LED comes on, what do you do? Does this indicate that you need to rebalance that half of the pack? I'm afraid I don't quite understand that aspect.


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## Jan (Oct 5, 2009)

ZANDARRR said:


> I have read and understand the Lee Heart Battery Bridge, but if you are driving and the LED comes on, what do you do? Does this indicate that you need to rebalance that half of the pack? I'm afraid I don't quite understand that aspect.


No, you never (re)balance a part of your pack. Per definition you balance the complete pack. Or all to the same low voltage at the low knee to bottom balance, or to the same high voltage at the top knee. The knees can be seen in charge/discharge curves in the graphs that comes with the brand.

The theory behind this device is that if there occurs an inbalance between the pack halves, you probably have a bad cell. And the half with the slightly lower voltage is the place to look for it.


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## ZANDARRR (Sep 8, 2010)

Ah, that would make sense. Okay, thanks very much!


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## ZANDARRR (Sep 8, 2010)

Just had a quick thought, since this is a relatively simple circuit, why not have the pack divided further, maybe into fourths, to narrow down your search for the underperforming cell?


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## JRP3 (Mar 7, 2008)

It's been discussed but I'm not sure it's worth it. Doesn't take that long to check half your pack with a VM.


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## EVfun (Mar 14, 2010)

I like the meter, but that doesn't fit with the Buggy. I would like a system with a digital output, if you will. That is a normally closed connection isolated from the traction pack (like what is typically seen with cell monitors) when all is good and having that open when the pack half difference exceeds a set value. This could be used to drive one of Dimitri's head end boards just like is currently done with cell monitors. 

I see this type of monitor as a substitute for cell level monitoring (or possibly a backup to it when cell boards have load resistors.) I really think that first the charger and controller should be programmed to take care of the pack (not rely on the BMS.) Then the battery monitoring system should provide an on/off output to insist a charge be stopped or drive be cut short. I don't want to wait until I teakettle a Lithium cell to find out something went wrong. 


```
pack center           
P                   |                p
a           D1 //   |                a
c           --->|---)-------         c
k     R1    |       |      |  R2     k
+ ----^^^------>|------>|----^^^---- -
               \\      \\             
            O1 ___     ___ O2         
               / \     / \            
              |   |----)--|           
              |--------|  |           
         R3   |           |           
+12 -----^^^--|           |           
              | Z1        |           
output  -----|< -------------- -12
```
Pack +, center, and - should be self explanatory. Remember that the pack + and - connections needs fuses at the cell terminals. 

R1 and R2 are precision resistors of the same value to allow the needed current to flow, perhaps 10 milliamps at 2.5 vpc. For a 40 cell pack that would be 5000 ohms each. 

O1 and O2 are a pair of optical isolators, transistor type. The IR LEDs are part of the pack circuit and the isolated photo transistors are part of the vehicle 12 volt system. 

D1 is a red LED with a turn on point about 1/4 volt higher than the IR LEDs. If any one cell drops or raises 1/2 outside of its neighbors one of the photo transistors will be turned on.

Z1 is a TLV431 programmable zener. R3 keeps Z1 on unless one of the photo transistors turns on and grounds Z1 turning it off. This can be used to turn off a common DPDT PC board relay. This provides useful outputs to turn down a controller or turn off a charger or just scream warning alarms or whatever you think a battery monitor should do. 

This is so quick and so crude I'm sure it won't quite work yet. Tear it apart please.


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## ElectriCar (Jun 15, 2008)

Good thread for a good idea. I'd like it to be a little more sensitive, maybe a volt or 1-1.5V per led and have about 9 led total, four each side and center. This way you get a little more data. Ie a high resistance connection will show up sooner with better sensitivity and only when the pack is under load. Let off the accelerator and it would return to normal. As it gets worse more led would light up under load. However a toasted cell would light up most of one side.

I've looked around for an led driver to do such but haven't really found the answer.


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## EVfun (Mar 14, 2010)

I think a display would be great as well. The analog gauge is easy to set up. Using an IC to do this is more difficult because most of the chips cannot take anywhere near pack voltage. The clamping needs to be good while not interfering with measurement. 

The simple on/off single output I was drawing was intended as a "do something" output. The kind of thing that might shut off a charger. The output on the one I drew should alarm at about +/- 0.5 volt (a single cell going up or down by a half volt should move the center tap voltage a quarter volt off from half the pack voltage.)


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## ElectriCar (Jun 15, 2008)

EVfun said:


> I like the meter, but that doesn't fit with the Buggy. I would like a system with a digital output, if you will. That is a normally closed connection isolated from the traction pack (like what is typically seen with cell monitors) when all is good and having that open when the pack half difference exceeds a set value. This could be used to drive one of Dimitri's head end boards just like is currently done with cell monitors.
> 
> I see this type of monitor as a substitute for cell level monitoring (or possibly a backup to it when cell boards have load resistors.) I really think that first the charger and controller should be programmed to take care of the pack (not rely on the BMS.) Then the battery monitoring system should provide an on/off output to insist a charge be stopped or drive be cut short. I don't want to wait until I teakettle a Lithium cell to find out something went wrong.
> 
> ...


 It's time for me to build some variant of this now. I'm off to a good start, I have a wire connected to the center of my new pack. 

With this circuit I think I'm in the neighborhood of what I want to do. It's been over a decade since I worked in electronics daily and I'm unbelievably inept now compared to when I worked with op-amps frequently. How can we connect a comparator in this this circuit in place of the transistors? Or can the transistors be biased so that they turn on in a linear fashion as the voltage imbalance grows? 

As I said earlier, I'd like to light an led ie for each volt or two of imbalance. For example at 1V resolution, a 3V variation would light 3 led's. The Evision meter does that so it can be done, somehow. The Evision is just so frigging expensive, $1000 or so.


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## ElectriCar (Jun 15, 2008)

I wrote Lee last night and got a reply this morning. Here's the text as an FYI.
_"3/31/2011 7:19 PM:
_ _ Hi Lee, I hope this finds you well.
_​_ Very well, thank you. And yourself?

_ _ I got your email from diyelectriccar.com.
I figured I'd contact you about what I'm looking to build... I'm hoping you can help with this.
_ _
We're kicking around ideas and variants over there to your original
led circuit. Metricmind has something like that which has a bank of
led's for each half of a pack. Ie one led may represent 1V imbalance
etc but their product is about $1000!
_ ​ _
Yes, I have one of his EVisions. Nice, but pricey! Also, it is really more of a tool for an engineer than a useful gauge for driving._ _ This way with more resolution, more data can be derived with more led's. I'm thinking it can be done somehow with a comparator circuit but I know there's issues with high pack voltages and op amps. I'm thinking a voltage divider or such may be the answer.
_​_ Here's some background on the Batt-Bridge that may help.

I've been tinkering with EVs for over 30 years. I've lost count of the number of different gauges I've tried in my EVs. I'm an engineer, so most of my gauges read out what I thought was useful information; voltages, currents, watts, amphours, temperatures, etc. They are useful for designing and testing an EV.

But I found they weren't very useful for *driving* an EV. For example, the driver doesn't want to know the pack voltage or amphours in the pack; he wants to know how many miles he can still go (will he make it home), or if there is some problem with the battery pack that requires immediate attention.

A true "fuel" gauge for an EV is a hard problem. There have been many attempts, and almost all of them fail. Most are about as useful for telling how much farther you can drive as a normal car's gas gauge is for telling you how many gallons are in the tank. I.e. they are both "guess gauges".

Since most EVs lack any real driving gauges, it is very common for drivers to murder their batteries. They just keep driving until it won't go any more. This runs at least one of their poor batteries completely dead, which destroys it. They might have a voltmeter that says the pack is at half voltage, but there was no indicator that says "Stop, you idiot!"

The Batt-Bridge was an attempt to provide an "idiot light". When green, it means OK you have power. When it turns red, it means "STOP WHAT YOU'RE DOING NOW!"

The exact voltage differential between the half-packs is not very significant. It doesn't carry any useful information. It is sufficient to know that something has caused 1v to 2v difference between the two half-packs.

*If you want to watch the degree of imbalance between the two halves of the pack, it's easy to replace the LEDs with a voltmeter. Connect two identical value resistors in series across the whole pack, and connect your voltmeter between the center tap of the pack and the center tap between the two resistors. You can watch how it changes as you drive. It normally stays pretty close to zero, and only increases dramatically when something goes wrong (overcharging, or too deep a discharge, or too high a discharge current, or a loose connection somewhere).*

Obviously, you can make the circuit as complicated as you want, with a precision digital readout, or a bargraph of LEDs showing +/-X volts. But I don't think this will add to the usefulness of the gauge."_ 

The bold above is what I'm looking at. Infinitely easier yet gives me actual voltage difference between halves. The polarity will reference which side the issue is on. I have a digital volt meter good to 200V but not sure it will display both polarities.


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## JRP3 (Mar 7, 2008)

Why not use the version that has an analog ammeter in it. When the meter is centered you're good, if it swings one way or the other you have a problem. Quick visual indication of what's happening.


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## ElectriCar (Jun 15, 2008)

Have an example of one? I thought that is what Gizmo was looking for and couldn't find.


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## jackbauer (Jan 12, 2008)

I've been using the ammeter version for charge monitoring this past week. works well. Two resistors , a pot and one 100-0-100uA ammeter in a box.


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## JRP3 (Mar 7, 2008)

ElectriCar said:


> Have an example of one? I thought that is what Gizmo was looking for and couldn't find.


Well you have to make it, looks pretty simple, even to me 
http://www.diyelectriccar.com/forums/showpost.php?p=224908&postcount=2


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## ElectriCar (Jun 15, 2008)

JRP3 said:


> Well you have to make it, looks pretty simple, even to me
> http://www.diyelectriccar.com/forums/showpost.php?p=224908&postcount=2


The meter! The circuit is easy, I'm trying to find a digital dc meter that reads both + and - voltage. I think I've found one on ebay. If it works I'll post a link to it. Based on what Lee said above that's what I'm wanting to do.


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## GizmoEV (Nov 28, 2009)

jackbauer said:


> I've been using the ammeter version for charge monitoring this past week. works well. Two resistors , a pot and one 100-0-100uA ammeter in a box.


Do you have a link to the meter you are referring to?


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## jackbauer (Jan 12, 2008)

http://radionics.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0220790


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## Nick Smith (Oct 26, 2007)

ElectriCar said:


> It's time for me to build some variant of this now. I'm off to a good start, I have a wire connected to the center of my new pack.
> 
> As I said earlier, I'd like to light an led ie for each volt or two of imbalance. For example at 1V resolution, a 3V variation would light 3 led's. The Evision meter does that so it can be done, somehow. The Evision is just so frigging expensive, $1000 or so.


There are schematics for a battery meter on Jerrys site (EVconvert) that i reckon you could adapt easily. It is adjustable for battery voltages so you might just set it for a 6v battery and adjust for centre reading with a pot or resistor. 

I had thought of doing this myself but just haven't gotten around to it.


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## ElectriCar (Jun 15, 2008)

Thanks Nick. I think I'm going with a digital volt meter that will show any voltage difference, negative or positive depending on which side of the pack it's on.


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## ElectriCar (Jun 15, 2008)

I just purchased this meter. It's capable of reading both - and + voltage, unlike a lot of those I looked at. It's also over voltage protected, just not sure how much. I think the meter will be much better than a simple led, much m ore information available.

In the mean time I built the LED version yesterday using two 4700ohm resistors that measured within 2 ohms. Upon installing it I checked it out and it worked like a champ! Just be sure you use the 2 watt resistors he specified. I used 1/4 watt as they were free and I'm only using it temporarily until my volt meter arrives. When playing around moving the center tap, I smelled burning resistor when I moved the tap to about 1/4 of the pack instead of half!


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## jackbauer (Jan 12, 2008)

So that display goes in place of the microammeter in the bridge circuit and basically displays the voltage difference between the half packs?

I've been using the bridge idea for charge monitoring for the past few weeks with great success.


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## ElectriCar (Jun 15, 2008)

jackbauer said:


> So that display goes in place of the microammeter in the bridge circuit and basically displays the voltage difference between the half packs?
> 
> I've been using the bridge idea for charge monitoring for the past few weeks with great success.


Exactly. If you have a cell go out with the meter, you'll read 3.2V difference. With the led display, you have no way of knowing how much difference there is. With the meter you ALWAYS know what's going on. 

If for instance you start out after the first charge on your new pack, all your connections are new and tight. On your first drive you find the voltage varies between 0.10-0.22V for example down to 30%DOD. On the next few trips you take, it pretty much stays in that range. Now you have established the normal voltage variation. 

After some period of time if you see the upper voltage start creeping up to higher values, you know you're likely developing a connection problem somewhere. This will have to continue until it's greater than 1V or higher before the red led kicks in, depending on the resistor values. 

I just like the idea of KNOWING what's normal so I can see anything start to develop. Another example, if you are charging with a replacement cell for the first time or a new pack and one battery starts to peak, you'll see it if you're watching the volt meter. With the led the imbalance could be go as high as 2.5V or so, again depending on the resistors.


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## ElectriCar (Jun 15, 2008)

Hello all. I've completed the install of the balance volt meter mentioned earlier. I used a 50V digital LED display that has auto polarity sensing and over-voltage protection. I provided a link earlier to ebay for the specs for those interested. 

For the resistors, I used two 220K 1/2watt resistors from Radio Shack. That will limit the current to 0.4ma and the watts per resistor to .035 at a pack V of 175V, basically nothing for this resistor. One leg of the meter is connected to the center tap of the pack. The other is connected to the center of the two resistors which are connected in series across the pack.

I moved the center tap up and down one battery and two batteries then took photos. The display reads 00.0 when the pack is evenly balanced. Though one battery is 3.34V currently, the display reads only 2.9V. Not sure why but it doesn't matter truly as I'm only interested in knowing what would be displayed with one defective cell.


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## DIYguy (Sep 18, 2008)

ElectriCar said:


> Hello all. I've completed the install of the balance volt meter mentioned earlier. I used a 50V digital LED display that has auto polarity sensing and over-voltage protection. I provided a link earlier to ebay for the specs for those interested.
> 
> For the resistors, I used two 220K 1/2watt resistors from Radio Shack. That will limit the current to 0.4ma and the watts per resistor to .035 at a pack V of 175V, basically nothing for this resistor. One leg of the meter is connected to the center tap of the pack. The other is connected to the center of the two resistors which are connected in series across the pack.
> 
> I moved the center tap up and down one battery and two batteries then took photos. The display reads 00.0 when the pack is evenly balanced. Though one battery is 3.34V currently, the display reads only 2.9V. Not sure why but it doesn't matter truly as I'm only interested in knowing what would be displayed with one defective cell.


Nice! I like it. Could part of the difference be the voltage drop in the wire? Probably too much...maybe meter calibration. But like you say, it doesn't matter too much as long as its all relative on both sides of zero.


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## tomofreno (Mar 3, 2009)

> I just like the idea of KNOWING what's normal so I can see anything start to develop.


 Yeah, I agree. Seems like a nice diagnostic, like you say, you can see if anything has changed at a glance. After driving with it for a while it should be very obvious if a cell changes voltage significantly or a connection develops higher resistance. Greg said the only cell failures he has seen have been development of high resistance - which is what he speculates was the cause of his fire. This circuit would show a cell developing high resistance while driving. Could divide the pack into quarters for finer resolution, but may not be necessary. Will be interesting to hear reports on it after several months of driving.


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## ElectriCar (Jun 15, 2008)

One tidbit, when I was nearing the end of my initial pack charge, as one cell was climbing quickly, the display was still showing 00.0-00.2V, fluctuating as it had the entire charge. Evidently the remaining cells were close enough to to the knee that the two halves stayed in balance. The one cell was only about .15-.20V higher than the others before I shut it down.


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## GizmoEV (Nov 28, 2009)

ElectriCar said:


> One tidbit, when I was nearing the end of my initial pack charge, as one cell was climbing quickly, the display was still showing 00.0-00.2V, fluctuating as it had the entire charge. Evidently the remaining cells were close enough to to the knee that the two halves stayed in balance. The one cell was only about .15-.20V higher than the others before I shut it down.


What vpc are you charging to?


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## ElectriCar (Jun 15, 2008)

3.45V right now. That may change after I drive it a few times and do some deep discharges to see how closely matched they are. I really would like to bottom balance but it may not be necessary if I don't try and "run it dry". I've only run out of gas twice in my 32 years of driving so I likely will be ok. 

I hope to be able to take it pretty deep and see how close they are so I'll know how low I can go if needed. It just takes a loooong time to charge now if I take it deep, like 6 hours if it were about empty! I've yet to get on the road though, waiting on parts, playing with that balance board etc. Today we cut out the bed and sat it in place but that's for another thread. 

Anyway it occurred to me when I tested the balance meter by moving the center tap over one cell that it was splitting the voltage by half. So 
today I took a wire on one end of the voltage divider resistors and moved it up one cell from the end, which is a true representation of what it would see if a battery shorted. Sure enough it displayed 1.5V instead of the 2.9 the other day. So whatever it reads you know the error is TWICE the display voltage.


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## GizmoEV (Nov 28, 2009)

Last night I got home from a long run where I was pulling 130+A for about 40 minutes straight and then a 400' hill climb for the last couple miles. My battery pack was at 28°C and discharged 174Ah in a 200Ah pack (87%DOD). When I got home I quickly took the battery cover off and went for a drive to see if any batteries BMS LVT lights lit before others. The best I could tell is that one or two were very slightly tripping before the others but nearly all of them lit up under a 1C load. I think that the pack is quite balanced at the bottom even though it was top balanced 9 months ago. The trip voltage is 2.93V. I wish I had a split pack monitor to see better what is going on.

On the split pack voltage monitor couldn't the resistors be chosen in such a way that the display would end up being calibrated for the desired voltage? Maybe the display has to be calibrated instead.


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## ElectriCar (Jun 15, 2008)

What was your outside temp? I don't think I'll run into heating problems based on your data except for 40C days! 

My meter does have a cal pot which I will check out with a known reference soon. Just learned that yesterday. It's very easy to make this thing. You just need an auto-polarity sensing meter. You can get a blank circuit board, approximately 40mm square, from radio shack and a box to mount it in. Solder three joints and you've got it.

Theory wise, I haven't really studied it nor do I fully understand it, just built it per Lee's description but the voltage reading is based on half the pack so I think changing them will only affect current flow, not voltage unless you change the ratio. But the meter displays plus and minus so the resistors must be same size for the meter to have a correct reference point, essentially 0V relative to the center point tap in the pack.

Oh, love the CA BTW. PM me some info on how you installed the speed sensor if you would. I'd like to convert it to use my prox sensor the tach uses if possible. I need to write them if you don't know.


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## dtbaker (Jan 5, 2008)

ElectriCar said:


> Oh, love the CA BTW. PM me some info on how you installed the speed sensor if you would. I'd like to convert it to use my prox sensor the tach uses if possible. I need to write them if you don't know.


on a car you can glue the magnet to any part of the axle rotating at tire speed, and glue/bracket the pickup within 2mm, then set your actual distance per rev in the settings.... its designed for fastening to bike spokes.


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## GizmoEV (Nov 28, 2009)

dtbaker said:


> on a car you can glue the magnet to any part of the axle rotating at tire speed, and glue/bracket the pickup within 2mm, then set your actual distance per rev in the settings.... its designed for fastening to bike spokes.


The manual says that it can handle 1kHz, IIRC. In that case the sensor could be mounted by the drive line like we used to do for adding cruise control to a car. I would mount two magnets to keep the drive-line balanced and use a non-stretching zip tie to hold them. Next, count the number of times a magnet passes the sensor for 1 tire rotation and enter that number as the number of poles and then enter the circumference of the tire, I think in mm. that should do it. Or, if you are as fortunate as I was, just tap into the wire going into the speedometer. My digital speedo uses the exact same input that the CA does so my setup was easy.


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## GizmoEV (Nov 28, 2009)

ElectriCar said:


> What was your outside temp? I don't think I'll run into heating problems based on your data except for 40C days!


I think it was below 10°C. It was cold enough to get my breath to condense.



> My meter does have a cal pot which I will check out with a known reference soon. Just learned that yesterday. It's very easy to make this thing. You just need an auto-polarity sensing meter. You can get a blank circuit board, approximately 40mm square, from radio shack and a box to mount it in. Solder three joints and you've got it.
> 
> Theory wise, I haven't really studied it nor do I fully understand it, just built it per Lee's description but the voltage reading is based on half the pack so I think changing them will only affect current flow, not voltage unless you change the ratio. But the meter displays plus and minus so the resistors must be same size for the meter to have a correct reference point, essentially 0V relative to the center point tap in the pack.


What I haven't figured out is if the current flow has a linear relationship to voltage differences between each half pack. If it does it should be easy to calibrate the meter.


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## tomofreno (Mar 3, 2009)

> What I haven't figured out is if the current flow has a linear relationship to voltage differences between each half pack. If it does it should be easy to calibrate the meter.


 For the circuit electriccar described, two resistors in series across the pack, with a meter between the center of the resistors and a cell in the pack: Let V1 be the sum of cell voltages on one side of the tap, and V2 be the sum of cell voltages on the other side of the tap, then:

I = (V1 + V2)/2*R = Vpack/2R = NVc/2R, where N is the total number of cells and Vc is the cell voltage, and:

V1 = n1*Vc
V2 = n2*Vc
N = n1 + n2

The measured voltage between the two resistors, each of value R is:

V = IR – V2 = IR + V1 – Vp = V1 – Vp/2

" = V1 – N*Vc/2 = n1*Vc/2 – n2*Vc/2 = 0 for n1 = n2 = N/2, tap in center"

Move center tap by p cells from center, then
V = (N/2+p)*Vc/2 – (N/2 – p)*Vc/2 = p*Vc

Edit: I put the one line in quotes because that was the only way I could get it to print it correctly.


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## ElectriCar (Jun 15, 2008)

tomofreno said:


> For the circuit electriccar described, two resistors in series across the pack, with a meter between the center of the resistors and a cell in the pack: Let V1 be the sum of cell voltages on one side of the tap, and V2 be the sum of cell voltages on the other side of the tap, then:
> 
> I = (V1 + V2)/2*R = Vpack/2R = NVc/2R, where N is the total number of cells and Vc is the cell voltage, and:
> 
> ...


Huh? And I went to school for electronics. 

Just build one. It's very simple to build if you know how to solder and it works. If anyone can't solder, contact me. It's easier than riding a bicycle!


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## spdas (Nov 28, 2009)

GizmoEV said:


> Below is an email exchange I had with Lee Heart on a modification to his Batt-Bridge.
> 
> On 1/4/2011 7:19 PM, David Nelson wrote:Hi Lee,
> 
> ...



OK, the DUH bells went off and I saw the light for the half pack method, even for an electronic dimwit as me. So the only difference basically between half pack monitor and each cell monitor, is that in half pack it will show you have a energy flow problem, (bad cell, bad connection, etc.) and you have to use a VOM to find the cell/problem. Seems great, but as I said I am a dimwit, so could you break down the exact parts and values that are needed? thanks
francis


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## ElectriCar (Jun 15, 2008)

R1& R2 are 220k ohm in my setup. No trimpot now though. ebay a 20 Volt DC auto polarity sensing meter. My pack is 160V. A shorted cell will display half the voltage or about 1.65V. Good luck. Dang spell check!


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## spdas (Nov 28, 2009)

ElectriCar said:


> R1& R2 are 220k ohm in my setup. No trimpot now though. ebay a 20 Volt DC auto polarity sensing meter. My pack is 160V. A shorted cell will display half the voltage or about 1.65V. Good luck. Dang spell check!


i guess the trim pot is to zero out the display, which should be zero anyway. Can you give me an ebay example of the meter. I am only finding LED panel meters from 7.5-20v and analog 0-15v meters.

thanks
francis


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## ElectriCar (Jun 15, 2008)

This is similar to the one I have. http://cgi.ebay.com/20VDC-GREEN-DIG...262?pt=LH_DefaultDomain_0&hash=item1c1ce18616 It also has a two year warranty. That would be good for me as I've blown two but the ones I had wasn't over voltage protected I suppose. This time I'm putting two 20V 1W zener diodes back to back across the monitoring leads with a current limiting resistor to prevent that next time.


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## GizmoEV (Nov 28, 2009)

spdas said:


> i guess the trim pot is to zero out the display, which should be zero anyway.


If you buy %5 resistors they could be at either extreme of their specification range which would give you a difference. The trim pot fixes this. Your other option would be to buy precision resistors which I understand are very expensive. Alternatively, you might be able to by a sample of the 5% variety and the measure their resistances until you find two with the same value.

ElectriCar, with using a volt meter, as opposed to an ammeter, in the megaohm range of resistance wouldn't the resistor values be of less importance? For example, in your setup couldn't you go with 440kohm and have the same results with the benefit being lower current drain from the device?


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## ElectriCar (Jun 15, 2008)

I chose those values to keep current drain to a minimum. Megohm would be ok too. I plan to use zener's back to back to limit voltage to the meter but not so sure it will work properly if the resistance in the circuit is so high the zener can't conduct properly. They will be wired with a 1k resistor like this 
-----l<l----l>l--/\/\/\----- with one forward and one reverse biased all the time. Once the voltage exceeds about 21V the reverse biased one should conduct and shunt the over voltage. I think this will work.


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## spdas (Nov 28, 2009)

I have a couple of the panel mount led displays that are 0-200v. Would i not achieve the same results of 1/2 pack monitoring of 46 cells =23 @73.6v each? If i notice one is 72v and the other is 70.8, then i have a problem.

Francis


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## ElectriCar (Jun 15, 2008)

The meter displays both negative and positive values, that's how you determine which half of the pack it's on. Not sure how your led could do that. 

I was at first wanting to use a bar led with the center led lit up at 0V. Each led right and left of center would represent a voltage value like .5V or such but I couldn't figure out a way to make that work.


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## JRP3 (Mar 7, 2008)

He's talking about using two led volt meters, one for each half of the pack. As long as they are isolated it should work and show any imbalance directly by voltage.


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## ElectriCar (Jun 15, 2008)

Yea that would work if one is designed to work on negative values.


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## GizmoEV (Nov 28, 2009)

Why would one have to work with negative values? The "center" tap would be positive for one meter and negative for the other one.


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## spdas (Nov 28, 2009)

Aloha, y'all. I see an advantage of the one gauge and seeing + or- 1.5v as a quick and understandable visual, but is there any other advantage over seeing two gauges, one being 72.5 and the other possibly 74v. The LCD @ 0-200v are the same as the ones described before at 0-20v, but they only go down to .1 not .10 decimal places. 
Since i like lots of visuals, I am going to use one LCD for total pack voltage, 2 for 1/2 pack and one for the 12v side. And leave the Link10 on amperage. Now to find a good place to put them. 
Francis


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## GizmoEV (Nov 28, 2009)

I ordered an LCD volt meter from eBayer clinia and he will be setting the meter to display twice the voltage it reads. It will display from -19.99V to +19.99V with a -10V to +10V input. This should be ample. If my 20 cell pack is 10V off there is definitely a problem!

He said the meter could handle 100V input without damage so I should be just fine with my 64V nominal pack.

The listing was http://cgi.ebay.com/LOW-COST-LCD-PA...110?pt=LH_DefaultDomain_0&hash=item588b92e986

Oh, this runs off of a 9V battery so it definitely has an isolated PS. I have one from him on a 50A shunt and it ran for several months with an old 9V battery out of a smoke detector.


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## ElectriCar (Jun 15, 2008)

Well you can always go with rechargables. I learned that mine has a 5V supply rating. I'm going to use a 7805 3 pin regulator to make it from the 12V. You could also use an adjustable voltage regulator to make 9V if there isn't a 7809.


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## GizmoEV (Nov 28, 2009)

I've blogged about my Battery Balance monitoring system install in my Gizmo. Read about it at http://2003gizmo.blogspot.com/2011/07/battery-pack-balance-monitor.html.


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## Agust Sigurdsson (Oct 25, 2009)

I really like this thread. This battery bridge principle is just genious.

I have the feeling that some kind of alarm, buzzer or warning light would be better suited for getting the driver's attention rather than a digital voltmeter. However the DVM would be useful to analyze the situation once an alarm has appeared.

I am planning to build an electronic alarm, based on this principle. It will have hold/reset function and not draw any significant amount of current from the battery pack (which some of the designs/suggestions in this thread unfortunately do).

I do not own an EV myself but I have a friend who might be tempted to test it out for me.
If it turns out well I will post the results here. 

Agust


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