48V LiFePo4 16s - 1 cell low voltage - Help

[FrozenStash]

Hi everyone,

First time poster here, but I have been lurking for a little over a year. I am having an issue with one of my 48V LiFePo4 packs; 1 cell going much lower than the other 15.

I have 2 identical 48V100Ah packs at my off grid cabin, purchased from SolarPowerStore Canada. I have had the packs installed for around 9 months now and have not had issues up until 2 weeks ago, when I went out to the cabin and the packs were out on low-volt cutoff. 1 cell in pack #2 was down to around 900mV while the other 15 cells were sitting at around 3200mV. Pack #1 was the same with all roughly sitting within 20mV of each other right around 3200mV/cell.

I pulled pack #2 out so that I could still run my lights + heater off of pack 1 + the generator to charge it when needed - every other day if I leave the heat on.

After pulling pack #1 apart, I realized that the cells are not user serviceable as I had anticipated. The cells and jumpers are all soldered together, and not studs with nuts as I had hoped. I have attached a picture of the pack opened up so you can get an idea of what I mean. I have on order a 60V 5A DC power supply as I was hoping to pull the cell and top it up, and then top balance all the cells in the pack before putting it back into operation with pack #2. When testing the cells with my DMM a few days later, cell #7 was reading 1.9V, so it rebounded...

My question is this. As I am not easily able to remove cell #7, would I be damaging or somehow otherwise hurting the entire pack if I were to hook up the DC power supply to the cell while it is attached to the other cells? As in, try to charge the 1 cell while it is still hooked up in series with the others. I have thought of ways to remove the cell if I must, but it would kind of be a PITA to do, unless there is a way that is easier that someone could recommend.

I was hoping to top up the single cell to around the same voltage as the other cells, and then putting it back onto the charge controller to top the entire pack up and then try to top balance it using the charge controller. I know this is not the correct way, but being the pack is technically not user serviceable, i dont see many other options.

I live in the Arctic, so currently we get about 4 hours of daylight at the moment - saying it isn't overcast, which it has been every day - so solar cannot charge them, hence the generator for the winter months. It is not financially reasonable to send the battery back for warranty as it would cost me around $1500CAD for shipping and logistics as I live in a fly-in only community.

Any and all help is appreciated. Thanks,
FS.

 

[SeaGal]

My question is this. As I am not easily able to remove cell #7, would I be damaging or somehow otherwise hurting the entire pack if I were to hook up the DC power supply to the cell while it is attached to the other cells? As in, try to charge the 1 cell while it is still hooked up in series with the others. I have thought of ways to remove the cell if I must, but it would kind of be a PITA to do, unless there is a way that is easier that someone could recommend.

That is fine to do if it's an isolated PSU (most bench ones are) and disconnected from the inverter. Many have done that on here, including me. I think you need to charge very slowly if it got that low, but I'll leave that input to someone more knowledgeable.

 

[BentleyJ]

If cell #7 was truly down to 900mV, there is a good chance its damaged beyond repair. I've ruined several 18650 cells at voltages above 1 but less than 2. This is why the BMS is usually set to disconnect at 2.5V.

There is no harm in trying to charge cell 7 while its still connected in the pack. When the BMS is balancing individual cells its using the voltage sense wires. If you connect alligator clips from an isolated source to the bus bars for cell 7 + & - that will work. MAYBE if the cell will hold charge.
I've made up several low cost, low wattage power supplies using a HVAC or door bell transformer, rectifier, filter caps and DCDC Buck or Boost modules from Ebay.

or something like this connected to a 12V automotive battery.

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[FrozenStash]

Bentley, thank you for the quick reply. I am going to attempt to charge cell #7 when my power supply comes in exactly as you said, with some alligator clips and bring it to 3.3V to match the others.
The BMS did cut it off, so I don't understand how it got so low on the 1 cell.
If it does not hold the charge and fails again, is there anything special I should pay attention to when buying a single new cell. All the same technical specs as the other cells and just swap it out?

 

[BentleyJ]

How long were you away and is there a means of keeping the batteries charged during that time.

A BMS can disconnect the batteries from the inverter but the BMS itself still pulls a small amount of current unless it has a sleep mode. In this case since its only the one cell, seems more likely that cell went bad or the BMS developed a low resistance path on that one cell only and unevenly discharged it.

 

[FrozenStash]

It was about 3 months with no visit (sept 14 - end of Nov), but it was still fine in early December. It was literally 1 day fine, then 2 days later I went and it was low-volt cutoff. I disconnected the battery right away. We use solar to charge the batteries normally, except now during the short winter days we use generator.

I read that the BMS does about a 2%/day load on the batteries when powered on.

If the issue is a low resistance path on the 1 cell, is that an issue with the BMS or the cell?