Battery problem

[Thrillhouse]

Recently built a 10 series lion pack out of 18650 cells. 3 cells per cell group for a total of 30 cells. This is the info for the bms

my problem is that when the battery is discharged down to 36 volts it cuts out. If I measure it with a voltmeter it will usually read something strange like 15 volts and if I hook it up to carger it will immediately read 36 volts again and then charge up to 42 volts normally. It's cold where I live now but I doubt that's the problem because the battery is warm to the touch after discharging. Does anyone have an idea of what's going on?

 

[999zip999]

Where did you get the cells and what brand a link maybe ? After cutoff check the cell voltage and write down like this for state of charge.
1. 3.2v
2. 2.8v
3. 3.1v


10. xxx volts
charger voltage Xxx volts

 

[Hwy89]

The BMS will shut down if one of the cell groups drops below 2.1volts. Check each of the cell groups to see if one group is lower than the rest. If so, manually balance the pack.

 

[Thrillhouse]

Would I need a 4.2 volt charger to balance individual cells

 

[999zip999]

Yes an old phone charger. But must check it's voltage and seat and watch as to never over charge. Must monitor closely . Or buy a cheap R/C balance charger Best to write down on paper. What cells are they ? Old used computer cells ? Why does it say lifepo4 ? A different battery cell.

 

[Thrillhouse]

Also shouldn't the bms do that?

 

[999zip999]

What ampage you using pack at ? As 3p is a small pack.

 

[billvon]

Also shouldn't the bms do that?

A typical resistive top balancer will only correct for minor imbalances - and only when held at 100% charge (4.2VPC) for a while. If you used old (disparate) laptop cells, for example, no amount of top balancing will give you good performance. Some cells will leak and drain on their own, some cells will have significantly lower capacity and some cells will have a higher ESR (thus triggering the protection function early during high rate discharge.) You will never see performance better than the weakest cell group, and sometimes even less than that (if you have a leaky cell.)

If you have a bunch of disparate cells you need something like a balancing charger and/or a full time balancer. Those are more expensive and complex.