Is this BMS working?

[Christobel931]

I have a 36V 15ah Lifepo4 pack that seems to be out of balance. The LVC is ~30V however, It cuts out at right on 35v under a load of ~21amps.

Measuring the battery after use I find the cells pretty evenly drained to ~3.3v resting, except for one that is only 3.07v.

During charging this cell comes up, but slower than the others and some others peak at ~3.8v. Then the green LED on the charger comes on and output is shown to be 44.8v at the battery connection. If I measure the voltage at the last BMS wire coming out of the pack, its 41V.

If I unplug the charger, it drops to ~40.6v at the battery connection. I can leave it on for 15 hours, same thing.

As I understand it, the BMS should shunt current away from the "high" cells to top off the others, correct? Shouldn't the "higher" cells LED's come on as current is shunted to lower cells. I haven't seen any LEDs come on at all and the BMS does not get warm at all during charging. I have tried two chargers, both put out just over 44v.

I have read through some of the other battery tech threads, and from what I can tell, the BMS is not doing it should, or the charger is getting a "false" reading?

Any ideas what is going on with this?

 

[dnmun]

nope, that's not how they work. your BMS shuts down the charge when one of the cells reaches the over voltage cutoff point. so the low cell does not get fully charged and you have to continue cycling it through the charge cycles to get the low cell to climb up to the others. make sense?

 

[dogman dan]

The last time I saw my bms working, a year ago now, it would cause the charger to start up again repeatedly. The fets would get hot as they shunted the power to the lower cells or whatever it is they do. ( electronic idiot here) Maybe you need a slightly higer votlage charger? Maybe your bms is supposed to work at a lower voltage but doesn't. I'd unplug the battery, wait a bit, and plug it back in an see what it does. You could also try charging the low group by itself and see if it comes back into balance and stays there.

 

[Christobel931]

Thanks guys,

I see that the BMS is indeed shutting off after one cell gets to 3.8v. That is great that the cell is being is protected, but the low cells aren't being topped off.

I will run it through a few shallow cycles to see if I can get it closer, but that's kinda how I've been riding it until last few rides. The BMS never got warm or, really did anything as far as I can tell. It gets warm during dishcharge, I can tell you that much...

 

[fechter]

What kind of BMS is it?

Most of the chinese BMS circuits simply shut off the charge current as soon as the first cell gets up to the max set point. If you have one low cell, you might try charging it separately with a 3.7v supply.

 

[Christobel931]

What kind of BMS is it?

Most of the chinese BMS circuits simply shut off the charge current as soon as the first cell gets up to the max set point. If you have one low cell, you might try charging it separately with a 3.7v supply.

I don't know what kind it is Fetcher.

My vendor suggests shallow discharge cycles to get it to balance, but if one cell keeps shooting up to 3.8V and shutting everything down, I don't see that happening. I can send it back for a check and manual balancing, but then I have no wheels.

I still don't understand the point of shunts and bypass circuits if they don't engage when one cell gets to a high voltage.

I'll do some more reading and look at single cell power supplies I guess... :?