Battery issue - Output battery different from BMS

[marcoredit]

Hi, I've got a lion battery 14s 10p 48 v with a Signalab BMS, the battery full charged is 54.6v, but the output of the BMS on the motor supply cables is 41.8v.

I replaced the BMS but the output is 41.8v, I Checked the battery from + to - and is 54.6v.

When I charge the battery, the charger indicate that is full. When I check with the voltmeter, the battery is ok but the supply cable sign 41.8v.

Do you have any suggestion how can I fix the issue? Do I need to replace any cables?

Thanks.
Mark

 

[amberwolf]

Sounds like the BMS is shutting down, most likely due to a low cell.

What are the individual cell group voltages, listed in order from most negative to most positive?

 

[marcoredit]

3.89v is the lowest cell, the others are 4.1v

Do you think I have do change the 3.89v cell?

 

[amberwolf]

Just leave it on the charger for a few days (or weeks), if the BMS is a balancing type, and the low cell(s) will be brought up.

If the BMS has LEDs on it to indicate balancing, then they'll probably light up red for low cells and green for full ones.

If the charger is the type that just shuts off after the BMS cuts off charge due to a high cell, then you might have to keep unplugging and re-connecting the charger to restart balancing.


If after a few days or weeks this doesnt' fix the issue, then you might have damaged cell(s) that need replacing.

Or manually charge the low cells yourself, with a single-cell charger or Laboratory PSU (lab psu) that is adjustable current-limit and voltage. If it won't charge teh cells, then you might need to replace them.

 

[molybdenum]

4.1V x 13 + 3.1 is 56.4V, and much higher than the 41.8V the OP reported. Something else must be going on, though I'll agree a parallel group is unbalanced.

 

[amberwolf]

Yes, it is higher, but a BMS in shutdown due to LVC or HVC can have a lower voltage on it's output than the actual pack voltage, which will drop to zero when enough load is applied (but can keep the voltage as long as only a minimal load is on it).


While none of the reported cell voltages is out of normal range for a BMS's LVC or HVC, and it *shouldn't* be triggered by that, re-balancing the cells will correct the difference and then it's eliminated as a possibility, at least.


EDIT: also, it isn't 3.1v reported on the low cell, it is 3.89v, which isn't that far off in the capacity/discharge curve of typical cells from the 4.1v the other 13 cells are reported at. That totals up to 57.19V.

So the actual pack voltage is higher than the meter reads (54.6v according to the first post), by at least one whole cell group, which means there's yet another problem somewhere (most likely a measurement error by the OP).

 

[molybdenum]

c- is the negative charger lead.

P- is the power negative going to the controller.

B- goes to the negative terminal of the pack.

Since the voltage reads 41.8 from C-, could the charger only be connected to 10 of 14 balance leads? Could a few of the leads not be well connected to either BMS or the battery between the parallel groups?

Is the harness pushed into the BMS all the way and making good contact?

 

[marcoredit]

Thanks for all the advises. In a few days I will try all the tests.

Thanks again.
Mark

 

[Punx0r]

I've had this but can't remember what fixed it. It could have been as simple as initially connecting the power/charge/balance leads in the correct sequence. I'd check the cell voltages at the PCB while it's plugged into the BMS to rule out a bad pin in the connector. It shouldn't be the temperature sensor at fault if this is the second BMS you've tried with the same fault. Something is causing the BMS to shut off the output...