Unbalanced cells in year-old DIY battery

[Flat Badger]

Hi all,

Posting for friends, but as I helped them set up their system I feel very invested. If there is a thread with a similar issue, I'm happy to be pointed towards that, thanks.

These friends have an 8s 200AH lifepo4 back, Daly BMS and Victron everything else. The cells were top balanced before installation, and everything has been running great for about 18 months.

In the last week, they have been waking up to everything switched off. I asked them to check the BMS, and they sent through these photos. They confirmed with a multimeter that the readings are correct.

Nothing about the system has changed recently, and the connections seem fine (although we haven't pulled it apart and put it back together, just checked the bolts).

Is there anything I/they can do to possibly remedy the situation, or is it clear that a cell or two are caput? Any further diagnostics we can do?

Thanks!

 

[TorC]

First, you're only showing us differences in the low knee. Unless you have very well matched cells, they are pretty much expected to diverge here, if perhaps not to that degree. What do you see for cell divergence around 3.45-3.55V/cell? If they're balanced there, the cells are still top balanced. If the same high cells at low are also high at the high knee, then balance is off. If the cells showing drained first also hit 3.45V or so first, then it's likely you have a cell or two with low capacity.

While you look at what happens with the cells at full charge, start investigating for possible increased power usage in the house or decreased input from PV/etc.. See what you can find for reports, and how far back you can get them for. Check all the devices (mostly the other solar system stuff) that may store history and see what you can pull from them.

 

[Eoin]

Relatively new to this build but it appears from your pics that the charging of the cells is quite low as the maximum charge lists as only 3.197v over 102 charge cycles.

What I've noticed while capacity testing my 16 cells (after a full lengthy slow charge top Balancing) was that when the cells are at low voltage they gained cell imbalance.

Is it possible that a similar situation has occurred here with your friends situation. Also only one of the pics displayed that active Balancing is switched on.

Hopefully someone with more experience will pop along soon enough!

 

[Luxpower_Gilbert]

Check for loss connection

 

[timselectric]

Charge the battery.

 

[chrisski]

I agree with the others that the top balanced numbers are more important.

I would:

1) charge the battery
2) take the connections off the battery
3) clean all connections with a wire brush, alcohol, and then put something like a dab of oxguard on the aluminum connections.
4) top balance the cells individually
5) reassemble the connections and torque

I have a problem where battery #2, cell 8 is .05 volts off after a loose connection. If after letting it sit for several days the BMS’s built in balancer does not rebalance it, that is how I’ve decided to proceed. What my BMS balances at is a huge mystery.

Also, with your friend, does how low does he typically go with the battery? Also, how often does he get as low as the pics show? Going to 2.9% will cut into the life of the cells.

Is anything changed in your friend’s usage? A guest brought over an installer one day which pulled up to 1000 watts and I did not watch consumption and that used more power than I wanted. I was also sleeping as the alarm beeped and no one woke me up to deal with the alarm.

 

[Flat Badger]

Thanks everyone for your replies. Definitely a few things to try now.

 

[740GLE]

How are they charging the battery, my guess is they’ve used more than was generated from PV and have slowly drained the battery flat.

Do you know the last time the bank reached 3.45v per cell?

 

[chrisski]

I want you to know I was able to leave the BMSs balancing on all the time instead of charging only. I did not use any loads on the system in this time, and the cell that was off did come within specs and load testing showed no differences from the other cells. Sat 5 days with no loads

Except for the one loose nut, I did not remove anything. I only cleaned that one cable. I torqued all nuts ton25 inch pounds.