I Think I froze my battery

[ChrisK]

I have a 12V, 300ah pack that I built out of (8) 150 ah cells. My BMS is a Solardacus SBMS0, which controls my solar charge controller and low voltage disconnect. I have a temperature probe connected to prevent charging below 5c, but a few days ago I checked and it was charging at -3c. My system is in my rv, which I was fulltime in until a month ago, so the pack was always well above freezing. Basically, I'm worried that it charged too fast below freezing and killed a cell or two. I was using it yesterday to run a charger and it hit the low voltage disconnect out of nowhere, and one cell fell all the way to 2.2v. It recovered to 2.8, and I let the solar system charge it all day today. When I checked this afternoon, two cells were at 3.3v, one was at 2.8v, and one was at 2v. Does this mean the cell is dead? I'm not sure what the symptoms are of a battery that charged too cold.

Also, what are some good choices for replacement? Could I just replace the (4) individual cells that are dead, or do I need to replace the entire pack?

[Charlie G]

I recently had a similar issue, just a different cause. My batt pack is 24v @ 300ah using the boue 150ah cells

You will need to break the pack down into individual cells. Then do the following. 

1) measure and record Voltage and Resistance of each cell. Make sure to use tape to identify the cells. 

2) check for physical deformation, swelling, of each cell. Compare cells with Low Voltage or Low Resistance to the High Voltage Cells. 

3) remove all cells that are physically damaged or have low resistance. 

4) connect all cells in parallel using coper wire (use some ground wire from household electrical cause it's cheap). Let sit for a day or two for voltages to stabilize. 

5) put a power supply on the pack and set voltage to 3.4v. takes a couple days to charge. Remove power supply once done. 

Recheck voltage and resistance of each cell where their isolated. Then order new cells as required. 

[Dave Festing]

2) check for physical deformation, swelling, of each cell. Compare cells with Low Voltage or Low Resistance to the High Voltage Cells.

Low resistance or high resistance ... I expected bad cells to be high resistance

[Dave Festing]

Wrong quote ... try 3)

[Dacian Todea]

ChrisK,

I doubt your problem is related to low temperature. Something else is wrong in that setup.  Photos and details about setup will help diagnose the problem.

[ChrisK]

Sorry about the late response, for some reason groups was automatically deleting my response every time I tried posting.

My setup is: 2p4s 150ah BLS prismatic cells for a total pack capacity of 300ah 12v. I use the SBMS0 to as the bms controlling a victron 150/60 MPPT charger and a victron battery disconnect. Unfortunately my inverter/charger has no low voltage protection, and I was using it without monitoring when it fell below the low voltage disconnect of the bms. After doing some more reading, I'm now wondering if I just had a cell with lower capacity than the others and it's voltage fell rapidly once the pack approached 0 SOC. I've never actually drained my pack down past 35% SOC before, so I don't know it's behavior at lower voltages. Usually I stay above 75%.

Yesterday I disconnected all the cells and measured voltages:

BMS ID	Cell	Voltage
4	8	3.195
4	7	3.195
3	6	3.192
3	5	3.192
2	4	2.07
2	3	2.066
1	2	2.452
1	1	2.452

I then wired them in parallel until they equalized around 3.078 Volts. I disconnected them for the night because I wouldn't be able to monitor them. Should I rebuild the pack and charge them while monitoring to ensure voltages stay close? I have a variable power supply coming in sometime today, but at just 10 amps it will take me forever to charge all the cells up with it.

[Oberon Robinson]

Chris, you may find this guide on top-balancing helpful: https://diysolarforum.com/resources/top-balancing-lifepo4-cells-using-a-low-cost-benchtop-power-supply.65/  (you may need to register on the forum to be able to download).

[Kohala Jim]

Ensure you heed all the potential pitfalls of power supply usage, including reverse current flow, walking away, etc...
I think that thread has all the concerns/caveats.

[Dacian Todea]

ChrisK,

That is an empty battery. While it may seem like there is a large difference between 2V, 2.5V and 3.19V all those are very close to fully discharged.

There may be maybe 5 to 6% SOC left in those 3.19V cells but not that significant.

You just need to solve the problem with the inverter else this will happen again and can be worse. Yes if voltage only dropped to 2V you can just reconnect the pack and charge it 

 

[Dacian Todea]

ChrisK,

I seen your two photos of the BMS not sure why they are deleted. The battery is just fully discharged and the low temperature has no connection with this. You just used more energy than you had available form solar or maybe you set a temperature limit the battery dropped below that and charging was not enabled during the day so you used energy but received no charge.

If you will be having battery in a cold environment the only proper solution is to have the battery in an insulated box and have a small heater 10 to 15W controlled with a thermostat to maintain the battery above freezing at all times.