BMS not balancing...

rg12

100 kW
Joined
Jul 26, 2014
Messages
1,584
Hey Guys,

Got this new 20S 20Ah pack built from brand new Samsung 25R cells and the pack came out of balance.
Opened the pack, charged it all the way to the top, then waited for all the BMS channels's blue leds to turn off (lit on led means bleeding down a channel/balancing) and after a few hours when all the lights turned off I measured all the cell rows and got half of the cells at about 4.16V and half of the cells at about 4.09 and one cell was even 4.06V
This is after I changed to a different BMS from the same supplier (exact same model) and it did the same.
I work with this BMS supplier for over a year with no problems.
Is it possible that the BMS is faulty or the balance is too far out for the BMS to balance easily and it will take it too many cycles to balance if ever?

btw, once the charger finished charging and goes to idle mode, does it matter that I disconnect it or does it sometimes draw a bit of current for the balance? because it seems that balance is only happening downwards by bleeding cells down.

I want to avoid manual balance and then giving the pack back and have this happen again.
 
What is that BMS's balance point set to?

What voltage does it turn on the balancers at?

What voltage does it turn off the balancers at?

You can measure this if you leave a voltmeter connected to a cell and note what it reads when the BMS lights come on and go off.

Test each cell group as it happens to see what each channel is set to. This will take as many HVC cycles as there are cell groups. (because the meter has to already be on the cell before HVC occurs, and before balance end occurs, so you can't move it off one cell and onto another until that cell's testing is done).

Remember also that what the BMS may do (probably does) is shut off charging when the first cell reaches HVC, so whatever other cells are at they'll all be lower than that one. Then it wouldn't restart charging until that cell is discharged down below it's cutoff point. In order for them to catch up, the pack must be left on the charger until all the cells are equal, which for a badly imbalanced pack can take hours, days, or even weeks.

If the cells themselves are not all identical, then the pack will always become unbalanced whenever it is used. How far out of balance depends on the differences between cell groups.
 
Start with a piece of paper and write down the voltages like this it will save a lot of time.
1. 4.09v
2. 3.99v
3. 4.18v

20. Xxx volt
 
Charge it, let it balance, then charge it again. Maybe even two or three times, then look at your voltages.

And uhh, if you are sweating it over .01 to .05v, it will take an accurate voltmeter to tell shit. Not sure what you have, but one reason I call within .05v balanced, is my cheap ass voltmeter.
 
Since the balancing shunts are pretty low current, it might take a LOT of cycles to bring up the low cells. If you don't want to individually charge the low cells, then charging the pack repeatedly is another way to do it. With enough cycles, it should eventually balance.

If you have a power supply (Meanwell or bench supply) that does not shut off at end of charge, you could place a resistor in series with it to keep the current under the shunt current (typically around 100mA). Then just let it run until it reaches balance. It could take days.
 
I found out the damn problem!
I left the pack connected to the charger even after it finished charging and instead of the BMS led lights turning off after a few hours they are still on for 3 days now and I checked the balance and it's almost perfectly balanced now.
So, turns out that even when the charge is at idle and the fans are off that it still helps with the balance.
 
Good deal.

Many chargers will just turn off when it reaches end of charge and not come back on until the pack gets discharged some. I guess yours supports balancing. A 'dumb' Meanwell will work the same way.
 
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