How do I know my battery is damaged by the cold?

[rubenscamp]

Hello. I searched the Forum but couldn't find an answer to how could I know if cells were damaged by the cold temperature.

Here's what happened: I have a 12V 560Ah (8 x 280Ah EVE cells) battery bank in my RV. The BMS I am using is a Daly 250A. The batteries are in an box with a small (60W) fan heater controlled by a thermostat to keep it above 10 C. Everything seemed to be working fine until few weeks ago when we had several days with very low temps (-20 C). The RV was not heated. One morning I checked the battery and the voltage meter was beeping with a low voltage alarm. I used my Renogy 2000W inverter/charger to charge the batteries (at this point the thermometer in the enclosure was marking +10C, as expected). While that was happening I notice the BMS couldn't keep the cells balanced and shut itself off with voltage imbalance protection.
Unable to charge the batteries in the RV, I brought them inside, disassemble the pack and began to top charge them using my power source. They were at about 3.4V and I top them off to 3.65V. To my surprise, it took just a few hours to go from 3.4V to 3.65V, which I found suspicious. Anyway, I put the battery bank back together (this time only using 4 batteries, so I could keep a close eye on the balance) and re-connected the BMS. As a quick test, I connected a small 10A charge and notice that, although the BMS was keeping them balanced, the cell voltages dropped very quickly from 3.62 to 3.45V. Again I found it weird considering it's a 280Ah battery.
I am now beginning to suspect that my heated enclosure was just heating the air around the cells, but the cells themselves could be at low temperature (the thermostat temp probe was stuck to one of the cell walls though).

The question is: how can I be certain those cells are OK? Is there a quick test I could perform to find this out?
Thanks in advance.

 

[400bird]

There's almost no energy between 3.4 and 3.65 volts. In fact, when you charge to 3.65 volts the voltage (with no loads) will settle to somewhere close to 3.3.5-3.45 volts

If the temp sensor showed +10 C when the BMS was acting up, I don't think cell temp caused you any issues or damage.

If the enclosure never went below +10c, how could the cells have gotten any colder than +10c?

What is the voltage imbalance threshold?

A capacity test probably isn't needed, but would tell you if any damage was done

 

[rubenscamp]

There's almost no energy between 3.4 and 3.65 volts. In fact, when you charge to 3.65 volts the voltage (with no loads) will settle to somewhere close to 3.3.5-3.45 volts

If the temp sensor showed +10 C when the BMS was acting up, I don't think cell temp caused you any issues or damage.

If the enclosure never went below +10c, how could the cells have gotten any colder than +10c?

What is the voltage imbalance threshold?

A capacity test probably isn't needed, but would tell you if any damage was done

My concern is that the thermostat sensor was picking up the air temperature but not the cell temperature (although the sensor was taped to one of the cells side wall).

Do you think I should keep applying the load on the battery and see if I get most of the amperage?

The balance start voltage is 3.2V and the imbalance open difference is 0.02V

Thank you

 

[400bird]

Start balancing at 3.4 volts.
Is "imbalance open difference" when the alarm sets and BMS opens? Or is that the threshold to start balancing?

It's pretty common to have 0.3 volt deltas at high and low SOC.

 

[rubenscamp]

Start balancing at 3.4 volts.
Is "imbalance open difference" when the alarm sets and BMS opens? Or is that the threshold to start balancing?

It's pretty common to have 0.3 volt deltas at high and low SOC.

My understanding from the app is that the "imbalance open difference" is the threshold to start balancing. The parameter I believe is the one that makes BMS to open is the "diff volt protect" and that is set to 0.25V. This is the one that prevented me from fully charging the battery after the cold days incident I mentioned. The difference between cells went higher than 0.25V and the BMS disconnected the charger.

 

[eemarty]

If you are worried, you could do a capacity test to see if the capacity has been reduced. Do you know how to do that?

 

[rubenscamp]

If you are worried, you could do a capacity test to see if the capacity has been reduced. Do you know how to do that?

I am less worried now. Based on the comments by 400bird, I assumed the BMS had acted up a little bit on that cold day and let the battery connected to a constant 10A load for several hours during the weekend. Although is a small load compared to the battery capacity (280Ah), as soon as the voltage dropped to around 3.3V, things looked more like one'd expect. The voltage was slowly but gradually coming down and the app was reporting the SOC that looked reasonable.
On the other hand, I am suspicious of that BMS now. I have order an JK Smart BMS to use on the other 4 cells and see what happens. If that one looks more reliable, I will replace the Daly on these 4 cells by another JK.

 

[sillared]

How did this turn out? Ive been using the JK B2A8S20P-H with good success. one temp sensor on the top of the battery pack and one on the bottom.