# BMS and battery balancing



## GE11 (Oct 24, 2011)

I have a couple of questions...

I have been reading about battery balancing and trying to understand a few things... you what they called too balancing and bottom balancing. So if I take for example the 12s3p Chevy volt pack and “bottom balance” it do I need a BMS system? Again if I do a bottom balance on This pack would I ever need to this bottom balance it again?
So when I charge the pack what keeps it on balance? What’s to say one cell doesn’t charge correctly and cause the pack to go unbalanced??!

Thanks for your help


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## Halu (Feb 16, 2018)

I also have a question, and instead of making another thread I will ask it here. If i use nissan leaf modules, I dont have to take them apart to the individual cells to use a bms right?


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## brian_ (Feb 7, 2017)

Halu said:


> I also have a question, and instead of making another thread I will ask it here. If i use nissan leaf modules, I dont have to take them apart to the individual cells to use a bms right?


No, you don't. The Leaf modules have cells paralleled in pairs, then the pairs connected in series. They have three terminals: positive and negative for the whole module, plus one more (with a smaller stud) which connects to the point between the pairs, so the BMS has access at the lowest level. That's for two cells at a time, but they are paralleled cells - there's no lower level of connection.


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## onegreenev (May 18, 2012)

GE11 said:


> I have a couple of questions...
> 
> I have been reading about battery balancing and trying to understand a few things... you what they called too balancing and bottom balancing. So if I take for example the 12s3p Chevy volt pack and “bottom balance” it do I need a BMS system? Again if I do a bottom balance on This pack would I ever need to this bottom balance it again?
> So when I charge the pack what keeps it on balance? What’s to say one cell doesn’t charge correctly and cause the pack to go unbalanced??!
> ...


Just sent you a PM.


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## Moltenmetal (Mar 20, 2014)

If you're re-using an OEM pack, what you'll notice is that every OEM has a BMS. Nobody other than DIYers who want to risk acting as their OWN BMS actually uses vehicle-scale high energy density Li ion batteries without a BMS. 

Voltage alarming on each cell (each group of cells in parallel) is the primary, most important function of a BMS. If you don't have some means of voltage alarming, you run the risk of over-charging or over-discharging a cell. Both conditions are potentially dangerous to the longevity of the pack, but over-charging is the most dangerous and can lead to fire.

Balancing is a different matter. BMSs all balance at the top of charge, though some permit you to switch this function off. Balancing itself is not essential unless you are not willing to rely on the BMS high alarm on a particular cell to arrest the charge as either a primary or secondary means to stop charging. If you need to use the entire capacity of the pack to meet your range needs, balancing is essential, but if you are OK sacrificing some of the pack's capacity then balancing beyond a coarse level isn't essential- again, as long as you have a BMS with at least the ability to arrest the charge when any cell goes above your high voltage cutoff and to at least alert you when any cell goes below its low voltage cutoff.

It is possible to bottom balance and then arrest charge at a low enough total pack voltage that the pack will remain comparatively safe- until something happens to damage a particular cell. While many people here are confident acting as their own BMS, and most find that their cells stay in balance fairly well over time, I personally had a LiFePO4 cell lose a substantial amount of capacity for no apparent reason- I suspect it just wasn't made well, or was damaged in shipment. My BMS protected me in this case - the BMS warned me so I could avoid over-discharging and reversing current through the cell. While some will argue that the loss of capacity of only one cell is improbable and that may well be, it is in fact possible for a cell to lose capacity (it happened to me). It's also possible for that low capacity cell to reach its HVC point before all the others in the pack. If you don't have a BMS or any other means to detect imbalance during a charge and automatically arrest charge, the risk of severely over-charging a cell does exist, and that can lead to a fire. You really, really don't want that to happen.

Think of a BMS as comparatively cheap insurance to protect a comparatively expensive pack. Or think of it as I do- as minimally necessary safety equipment when using a high energy density battery.


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## Duncan (Dec 8, 2008)

To echo Molten's point any pack can have a cell fail

And the non OEM ones have a failure rate that almost ensures that one of YOUR cells will fail

You *NEED* some way of identifying that this has happened

A "Proper BMS" will warn you that the cell is *failing* - I use a BattBridge - which will tell me when a cell has *failed*!

But you *NEED* something to tell you


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## GE11 (Oct 24, 2011)

Thanks man for your help I am looking at using the Chevy volt pack do a gokart project. But first I plan on using 6 golf cart batteries... I have not finished it yet, but boy the thing with all these 6V batteries is very awkward as best to describe it. I would like to be confident in using a Chevy volt 48V pack and have real confidence in it.

Let me ask this, on the Chevy volt 2KWh pack, what is the max charge current and the max discharge current?

Thanks for all your help!


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## GE11 (Oct 24, 2011)

Moltenmetal said:


> If you're re-using an OEM pack, what you'll notice is that every OEM has a BMS. Nobody other than DIYers who want to risk acting as their OWN BMS actually uses vehicle-scale high energy density Li ion batteries without a BMS.
> 
> Voltage alarming on each cell (each group of cells in parallel) is the primary, most important function of a BMS. If you don't have some means of voltage alarming, you run the risk of over-charging or over-discharging a cell. Both conditions are potentially dangerous to the longevity of the pack, but over-charging is the most dangerous and can lead to fire.
> 
> ...



What do you mean by an OEM pack?
What is your view on using the Chevy Volt pack?
If you were to use a BMS what would you recommend?
Then if I didn’t use a BMS, how often would you do the bottom balancing ?


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## Functional Artist (Aug 8, 2016)

GE11 said:


> What do you mean by an OEM pack?
> What is your view on using the Chevy Volt pack?
> If you were to use a BMS what would you recommend?
> Then if I didn’t use a BMS, how often would you do the bottom balancing ?


OEM = (Original Equipment Manufacturer) 

...refers to an "original" part, right from the automaker

...like what came on the car originally, not an "aftermarket" part 


* I have done a lot of research on Chevy Volt battery packs 

...they seem very well designed & manufactured


For a BMS, it depends 

...on how many bells & whistles you want (features) some do everything for you

...& on how much you want to spend (cash) some cost hundreds of dollars


Bottom Balancing = draining all cells evenly (wastes time & energy)


* I have read that Duncan, Yabert & others have successfully used Volt battery packs, without a BMS, for a while now

They use a Batt-Bridge's to monitor the pack balance

...& use a proper charger specifically set for the 2Kwh Volt battery pack


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## GE11 (Oct 24, 2011)

So what is this batt bridge people talk about?? How does it work? Is there a Batt bridge on every cell? So for the volt pack there would be 12 batt bringers...
Where do get these bat bridges?


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## Duncan (Dec 8, 2008)

GE11 said:


> So what is this batt bridge people talk about?? How does it work? Is there a Batt bridge on every cell? So for the volt pack there would be 12 batt bringers...
> Where do get these bat bridges?


http://www.evdl.org/pages/battbridge.html

It's an incredibly simple way of detecting in a cell has failed 

The electronics is so simple even I can understand it


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## Functional Artist (Aug 8, 2016)

Hey Duncan,

How sensitive are your Batt-Bridges? (within 1 volt of difference?)

I have gotten very inconsistent results with the ones I have made & tested.

&

I have noticed that Mr. Hart has updated the Batt-Bridge diagram, instructions & description a little bit

http://www.sunrise-ev.com/LeesEVs.htm#battbridge


The description used to read:

"The Batt-Bridge is about as simple as you can get; that's why it is so inexpensive. If all you want is an 'idiot' light to say, "Stop driving, your batteries are dead," I can't imagine anything any simpler. You really don't need dozens of ICs and hundreds of components just to light a light.
The Batt-Bridge divides the pack in half, and compares the voltage of each half. It lights an LED when one of them is 1v less than the other.

If a cell dies somewhere in the pack, it typically causes a 2 volt change. So the Batt-Bridge warns you that a cell went dead. There are two LEDs, so they indicate which half-pack contains the bad cell. 

R1 and R2 are chosen to draw about 10-20ma from the pack. For example, if you have a 120v pack, R1 and R2 each have about 60v across them. At 15ma, they would be R = 60v / 0.015a = 4k ohms. They need to be identical values (1% or hand picked or trimmed). And they must be power resistors; 60v x 0.015a = 0.9 watts, so use at least a 2 watt resistor.

Use an ordinary low brightness green LED. Its purpose is just to indicate that power is on, and to act as a low-voltage 2.4v "zener" diode. However, the red LEDs should be high brightness types -- the brighter the better, so you can see them even in daylight.

Here's how it works. All voltages are relative to the pack center tap. If +pack == -(-pack), then the green LED lights. The green LED's anode is at +1.2v, and its cathode is at -1.4v. The red LEDs don't light because they only have 1.2 volts across them (they need over 1.5v to light).

Now, suppose you have a dead cell in the upper half of the pack. Then +pack is 2v less than -pack. R1 and R2 form a voltage divider, so both ends of the green LED are 1v more negative; its anode is at +0.2v, and its cathode is at -2.4v. This means there is now 2.4v across the lower red LED; so it lights! Likewise, if the dead cell is in the lower half, then the upper red LED lights.

The total resistance of R1 and R2 sets the sensitivity, and the ratio of these resistors sets the desired center-tap voltage of the pack. If both LEDs light, then the resistors are too low a value; increase the resistance of both of them proportionately. Ten milliamps through the resistors is low sensitivity (over 2v difference to light an LED); 20ma is normal sensitivity; 40ma gives you high sensitivity (less than 1v difference to light an LED).

If one LED lights when the half-pack voltages are correct, then adjust the value of one of the resistors. This is also how you deal with packs with an odd number of batteries, where the "center tap" is off by one."

__________________________________________________________

Here is the updated version:

"An EV's pack consists of many cells or batteries. In theory, they are all identical. In practice, they aren't. There will always be a "weak link" somewhere in the pack. That's the cell that limits your range, and limits how much you can charge before damage begins.
But it is difficult to know if you have a weak cell. Total pack voltage won't tell you until too late. The amount of circuitry needed to monitor every single cell can get very complicated and expensive!
The *Batt-Bridge* is a quick-n-dirty "idiot light" to give you a good/bad warning when any cell in the pack goes undervoltage (dead) or overvoltage (overcharged). It works with all types of batteries; lead-acid, lithium, nicad, or nimh. If it lights up red, back off on the current until it goes out. If it stays on even at zero current, *stop driving or charging* until you find the problem and fix it!

*How it works*: The Batt-Bridge divides the pack in half, and compares the voltage of each half. When the two halves are equal (within 1 volt or less), a green LED is lit. If a cell goes dead or begins overcharging somewhere in the pack, its voltage typically changes by more than a volt. This imbalance lights one of two bright red LEDs to tell you which half of the pack is low.
D1 is a standard brightness green LED. D2 and D3 should be ultrabright red LEDs for best visibility. R1 and R2 should be identical resistors, chosen to provide about 10ma at your pack voltage. The current sets the sensitivity and brightness of the LEDs."


It looks like the math used to determine the proper resistors needed, depending on the pack voltage, has been changed a bit too.


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## brian_ (Feb 7, 2017)

Functional Artist said:


> I have noticed that Mr. Hart has updated the Batt-Bridge diagram, instructions & description a little bit
> 
> http://www.sunrise-ev.com/LeesEVs.htm#battbridge
> 
> ...


I think the only difference - other than just omitting some of the detail - is that the recommended current for "normal" sensitivity has changed from 20 mA to 10 mA.

The original mentions that a dead cell causes a voltage change of two volts. This corresponds to lead-acid chemistry; with lithium it would be reasonable to expect closer to four volts. That may be why the current recommendation is lower (for less sensitivity), but at the same time, isn't a completely dead cell too late to be triggering a warning?


It certainly looks like Lee Hart and his associates put a lot of work into making EVs workable in an era before there were commercially available products... and some of it is even still useful. The Renault 5 (LeCar) photo on that web page is trip back in time that brings a smile to an old car guy.


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## Functional Artist (Aug 8, 2016)

brian_ said:


> I think the only difference - other than just omitting some of the detail - is that the recommended current for "normal" sensitivity has changed from 20 mA to 10 mA.
> 
> The original mentions that a dead cell causes a voltage change of two volts. This corresponds to lead-acid chemistry; with lithium it would be reasonable to expect closer to four volts. That may be why the current recommendation is lower (for less sensitivity), but at the same time, isn't a completely dead cell too late to be triggering a warning?
> 
> ...


Mr. Hart has made a lot of kool stuff. 

Yes, I kinda wondered about the "late" warning thing too

...that's what my tests showed (sometimes) a 2 - 4 Volt imbalance

...I figured, I must be doing something wrong


The first Batt-Bridges I made (going by the original version):

I divided the 48V pack voltage in half. (48V /2 = 24V) 

24V / .015 = 1,600 Ohm

24V x .015 = .36 W 
(then doubled the wattage to be able to handle the whole pack voltage)

So, I used 

...(2) 1.5Kohm 2W 1% resistors (because I already had them)

...(2) standard brightness red LED's

...& (1) standard brightness green LED


But, it didn't seem to work right

So, I made another (this time going "by the book") & used

(2) 1.6Kohm 1W 1% power resistors

(2) ultra bright red LED's

(1) standard green LED


But, it still didn't seem to work right.


While going over everything, double checking & trying to figure this thing out.

I noticed that that the description has changed, the web site, a little bit.


Now, it says,

R = packV / 0.02A Ohms 1% (48V / 0.02A = 2,400 Ohm 1%)
P = packV 0.01A Watt (48V x 0.01A = .48W)


So, does this mean that I should have used (2) 2,400Ohm .5W 1% resistors?


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## john61ct (Feb 25, 2017)

There were comments made on a past thread that the old text implied some sort of SoC warning functionality. 

I sent him a link and he corresponded by email with me, very responsive.


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## Functional Artist (Aug 8, 2016)

john61ct said:


> There were comments made on a past thread that the old text implied some sort of SoC warning functionality.
> 
> I sent him a link and he corresponded by email with me, very responsive.


 
Yes sir, that was a question on one of my threads (that may be why he updated the description)

http://www.diyelectriccar.com/forum...idge-battery-pack-balance-monitor-191482.html


Yes, I have messaged back & forth with Mr. Hart, he seems very nice.



* I explained the issues that I was having & asked if I should be using 

...1.6Kohm 1W 1% resistors (old version)

or 

...2.4Kohm .5W 1% resistors (the new version) 


His response:

"That all sounds correct. What problem are you having?

The LED current is not critical. Anything from 10ma to 20ma is usually 
fine, and the resistors can vary over a range of values -- they just 
have to be the *same* value.

I would suggest testing your batt-bridge. This is pretty easy if you 
have (or can throw together) a small adjustable-voltage power supply. 
With your setup, I would use a 24v battery (any kind; it only has to be 
able to supply 10-20ma), and an adjustable 24v power supply. Connect the 
24v battery and 24v supply in series. Connect the Batt-bridge to it as 
if it is your 48v pack.

The green LED should always be on. it just tells you there is power. 
When you adjust the 24v supply, you should see

23v - upper red LED on (and green LED)
24v - only the green LED
25v - lower red LED on (and green LED)

The precise threshold where the red LED begins to glow is adjusted by 
the choice of LEDs. Picking red LEDs with a higher voltage makes the 
+/-1v threshold get bigger. Picking green LEDs with a higher voltage 
makes the +/-1v threshold get smaller.

The resistor values basically set the brightness. You want it bright 
enough to notice in the daytime; but not so bright that it's painful at 
night. You also don't want to use up a lot of battery current 
continuously (if the Batt-Bridge is on all the time -- though I think 
you were going to use a switch or relay to turn it off when parked?)

Does this help?"


I have done a lot of "Batt-Bridge" testing

...the results were inconclusive

That's why I was doing more research & asking more questions

But,

...it was an "either" or "question"  

& 

...800 Ohm's is quite a difference


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## brian_ (Feb 7, 2017)

Functional Artist said:


> ...it was an "either" or "question"
> 
> &
> 
> ...800 Ohm's is quite a difference


It was an "either/or" question, but the answer was that it doesn't matter. What matters more is matching the two resistors to each other.


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## Moltenmetal (Mar 20, 2014)

My battery bridge was made carefully using closely matched resistors. I actually had two of them, separate for the cells in my front pack and in my rear pack.

It failed to notice the one cell in my pack which mysteriously lost capacity- it vented electrolyte with no reason (I suspect it was made poorly and had a leaky relief valve/vent valve).

My BMS caught it and allowed me to limp to somewhere to charge without reversing the cell.

I now have cheap LCD voltmeters which I can turn on and off with a switch. Three pairs. If my BMS alarms, I can check the voltages without having to go from cell to cell with my multimeter. I can check balance now to 0.1V on every charge and at the end of every long drive.


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