Did I brick my LiFePO4 batteries? (cold temp charging)

[sheepslinky]

I have 2 100AH renogy lifepo4 batteries -- the older generation, not "smart". The temperature dropped, the batteries went below 0 degrees C, and it's likely the charge controller did not cut off the charge when the temp went below 0C. I don't know the behavior for LiFePO4 batteries when they fail, so I could use some help confirming my belief that they are completely toasted. When I called renogy, nobody could answer whether or not the BMS even protects from low temps. I somehow had assumed that "temperature protection" included cold protection, but I now see that only a high temp cutoff is noted in the manuals.

I put them on the bench, tried to charge them individually with a renogy dc-dc charger. Here's what I found:

The batteries will still discharge, but will not take a charge. I didn't put a heavy load on them, but they will power 10-30W of LED lighting.
The resting voltage on the batteries is 12.6V. When the charger is connected, it sets the charge voltage at 13.5V. However, the current is only a few milliamps (nothing really).

Am I right to assume that the BMS does not protect in cold temps? Is this the expected behavior of batteries when charged at low temps? Is there anything I can salvage from these batteries?

Thanks

 

[Bob B]

I don't know much about the Renogy BMS .... but, If you have brought them to a normal temp .... and they will discharge but not charge .... seems like something in the BMS needs to be reset.

 

[sheepslinky]

Thanks for the response. That's what I initially thought, but the charger should have reset the BMS according to all the documentation. Do most BMS systems shut off current in only one direction, or do they disconnect the load completely when triggered? Maybe I should try loading the battery a bit more to see if the internal resistance has gone up? I do have a good test bench with a scope and good meters...

 

[Terrapin]

I just learned yesterday that some BMS units control the input and output combined and shutting off both during protection. and that some are designed to allow charge or discharge and only disruting power to the needed side as needed by using different terminals for in and output. These may questions only the manufacturer can answer without a teardown. Also look into how to manually short the BMS to reset it. It involves a momentary small wire connection from one of the battery terminals to one of the BMS balance leads.

 

[Nakinto]

That is a loaded question, based on the BMS software. I have not used a Renogy BMS had to look up their settings set for batteries. You said the resting voltage is 12.6? EEEWWW. Renogy says resting 12.6 is going to be a dead battery (almost)(see link below)

https://www.renogy.com/template/files/Specifications/BATT-LFP-12-100.pdf

First thing I would do is an internal resistance test. Lithium batteries tend to have problems with the copper dissolving below freezing, and when it warms up again, the copper isn't where the copper should be and can even short out the battery. That doesn't look like the problem at this time though.

If you can't do a resistance test I would do a discharge and charge test. Discharge one of the 12v 100AH batteries, not to much, don't want to damage them if they aren't already I would say draw out about 6-700 WH then put them back on the charger. If they discharge this much but still won't charge then probably as Bob B said, something is broken with the BMS either it needs a reset or it shorted something. If you don't get that kind of discharge capacity, the batteries are probably shot, sorry to say.

 

[mikefitz]

Renogy says resting 12.6 is going to be a dead battery (almost)(see link below)

No ,12.6 volts resting is around 15% capacity. Most lithium batteries can tolerate a low current charge down to -10c. Why its not accepting charge is odd, I assume the batteries are now at room temperature. Have you an alternative charger to connect?
Mike

 

[Hound dog 78]

Hi, I am fairly new to the channel so take what I say accordingly. I have been doing a lot of research on Lifepo4 batteries and one thing I have learned is that these batteries have a different charging profile than SLA batteries. My understanding is that if you use a regular charger on a Lifepo4 battery, you can cause damage. Anyone have thoughts on this?