Battery pack/BMS questions

[tmho]

I found that the voltage of a certain group of my battery pack (16s5p) is always lower than the other groups. When the pack is fully charged and balanced by the BMS, that group will be the same as the others. But after using the pack, that group will be lower than the others. The difference is about 0.07V. I understand that this may be because that group has a lower capacity than the others. Will that also because the BMS only draws current from that group?

I am planning to charge only that group manually so that its voltage matches with the others. Do I need to unplug the BMS before doing the manual charging to that group?

 

[d8veh]

You need to take the battery right down to LVC to see if there's a problem. It could be that one cell isn't welded properly if it's an array of 18650 cells. You can charge it with or without the BMS connected as long as your charge voltage is safe.

BMSs don't take enough current to make that much difference over one charge from one day to the next. One possibility is that you have a bleed resistor permanently open. After charging, leave the battery overnight to let the bleed resistors do their ballancing, then run your finger along the bleed resistors to see if one is still warm or hot.

 

[tmho]

You need to take the battery right down to LVC to see if there's a problem. It could be that one cell isn't welded properly if it's an array of 18650 cells. You can charge it with or without the BMS connected as long as your charge voltage is safe.

BMSs don't take enough current to make that much difference over one charge from one day to the next. One possibility is that you have a bleed resistor permanently open. After charging, leave the battery overnight to let the bleed resistors do their ballancing, then run your finger along the bleed resistors to see if one is still warm or hot.

d8veh, thanks for your comments. Will try your suggestions later.
May be I did not make myself clear. The 0.07V difference did not happen the next day after fully charged and/or balanced. It just happens when the battery is used for a while (like say down to about half the pack's capacity). I do not use the battery so often, so that might be about 2 or 3 weeks or longer after the full charge.

 

[fechter]

Typical cheap BMS circuits drain a little more from the bottom 3 cells (from battery negative). Over a long time, these cells will tend to run a little lower. Otherwise, I would think maybe it's just those cells are weaker.

 

[d8veh]

You need to take the battery right down to LVC to see if there's a problem. It could be that one cell isn't welded properly if it's an array of 18650 cells. You can charge it with or without the BMS connected as long as your charge voltage is safe.

BMSs don't take enough current to make that much difference over one charge from one day to the next. One possibility is that you have a bleed resistor permanently open. After charging, leave the battery overnight to let the bleed resistors do their ballancing, then run your finger along the bleed resistors to see if one is still warm or hot.

d8veh, thanks for your comments. Will try your suggestions later.
May be I did not make myself clear. The 0.07V difference did not happen the next day after fully charged and/or balanced. It just happens when the battery is used for a while (like say down to about half the pack's capacity). I do not use the battery so often, so that might be about 2 or 3 weeks or longer after the full charge.

That's right, but there's three possibilities that I can think of:
1. The BMS is taking power for itself from that cell.
2. A bleed resistor is stuck open
3. A cell isn't connected.

If you drain a pack a little bit 3 won't show up, but there will be quite a difference when you drain the pack right down.

2 and 3 will both go down constantly without using the pack. Time will differentiate between the two. A bleed resistor drains about 100mA and the BMS may 50 micro-amps