I have a 7S 4P Li-Ion battery pack with a built-in BMS. The battery won't charge or supply power.
One of the groups of 4 cells reads 0.2V. Could this be a BMS problem or is there a duff cell in this group?

[edit] The other groups of 4 cells are around 3.6V.

 

If the voltage is only 0.2V on one group of cells, all four are discharged beyond the point of recovery.

 

BigClive would disagree:

 

He may disagree but I notice that he just talks about recovering a cell without actually doing it. I have tried all the ways I can think of but have never had any success with Li-Po batteries below about 2V.

 

I remember that video., I've recovered many cells over the years - as long as you are sensible and carefully monitor the cell through a couple of charge/discharge cycles there's no reason to just junk a cell that's been over-discharged. I once bought a job lot of 100 x 18650 cells, over 80% were recoverable and most are still in use today.

 

I have a 7S 4P Li-Ion battery pack with a built-in BMS. The battery won't charge or supply power.
One of the groups of 4 cells reads 0.2V. Could this be a BMS problem or is there a duff cell in this group?

[edit] The other groups of 4 cells are around 3.6V.

I just made my 7s6P lithium ion battery pack and I have seen all kind of problems with chinese bms, why not d/c the bms, check the bad series, if there is a bad cell the total should not be 0.2 on that series cells, can you share the bms you are using?

 

I just made my 7s6P lithium ion battery pack and I have seen all kind of problems with chinese bms, why not d/c the bms, check the bad series, if there is a bad cell the total should not be 0.2 on that series cells, can you share the bms you are using?

If one cell in a set of parallel cells becomes shorted, it will discharge the rest of the parallel cells to a voltage below recovery point.

 

If one cell in a set of parallel cells becomes shorted, it will discharge the rest of the parallel cells to a voltage below recovery point.

ofcourse because they are all connected with each other as one, same as shorting every single cell in that group

 

If one cell in a set of parallel cells becomes shorted, it will discharge the rest of the parallel cells to a voltage below recovery point.

I have follower the procedure set out by BigClive. Trickle chsrge up to 3V and then charged to 4.2V. So far so good. The next part is to watch the voltage over a few days to see if there is excessive self-discharge. So far no problem but it has only been a couple of days

Next I will check capacity and cycle a few times.

 

can you share the bms you are using?

It is the BMS that was built into the battery. As the affected cell seem to be OK I presume the problem was caused by the BMS.
I have ordered one of these: https://www.ebay.co.uk/itm/265837379606?var=565652985827

 

It is the BMS that was built into the battery. As the affected cell seem to be OK I presume the problem was caused by the BMS.
I have ordered one of these: https://www.ebay.co.uk/itm/265837379606?var=565652985827

I also thought the same issue, it is very rare to short cells to 0.2v
btw i suggest you to buy a bms with an active balancer, very important for charging/discharging
btw can u plz take a look at my pinoneer amplifier issue, i am stuck there! thanks

 

Not all those "BMS" boards include cell balancing. And some are prone to running down one tier of cells to run the protection/monitoring circuits.

As for fully discharged cells, I've salvaged a few packs with cells like that. The low cells often recovered and had nearly identical capacities to the other cells.

 

I have follower the procedure set out by BigClive. Trickle chsrge up to 3V and then charged to 4.2V. So far so good. The next part is to watch the voltage over a few days to see if there is excessive self-discharge. So far no problem but it has only been a couple of days

Next I will check capacity and cycle a few times.

And after some more days it is still at 4.17V so it seems to be passing the self-discharge test.

 

It's most likely not the BMS from my experience. One or more of the cells in parallel has gone faulty and discharged the rest. Best fix is to replace all the parallel cells with the same type then balance across the 7 series groups.

 

It's most likely not the BMS from my experience. One or more of the cells in parallel has gone faulty and discharged the rest. Best fix is to replace all the parallel cells with the same type then balance across the 7 series groups.

Self-discharge across a few days was negligible so I don't think that was the problem.
That group of four is now being discharged and measuring the capacity and its looking good so far.

 

Can take weeks to be noticeable particularly if you have a balancing type BMS which will drop voltages across all cells to prop up the faulty bank and keep balance. Another little trick I do is charge up the faulty parallel group (4 cells in this case) and then disconnect the negative side of the cells in the suspect group. I find this shows up the problem cell quicker as it will discharge much quicker and the good cells in the group will hold up on voltage.

 

First discharge gave 6611mAh (4 cells marked at 2500mAh). This battery is a long way from new so I wasn't expecting the full 10,000mAh. I will see what happens on future cycles.

 

Second cycle 6750mAh

 

Second cycle 6750mAh

This is my experience too. You may never get the 10,000mAh back (if it was ever that) but over a few gentle charge/discharge cycles you will see it increase.

BTW what rate are you charging/discharging? That original 2500mAh/cell would probably have been spec'd at the 20hour discharge rate, ie 125mA/cell (500mA/pack) over a 20h period. Are you measuring your 6600mAh (1688mAh/cell) at the same discharge rate?

 

The discharge uses an 8 ohm resistor.
The charger runs from a standard USB charger so definitely less than 1A.

After the capacity reaches a stable value I will do the same test on the other blocks of four cells to see how close they are.