Buying Used SOK Batteries

[CampingMike]

Hi Everyone.

Sorry if this has been discussed before, but after searching I could not find what I was looking for. I am looking to upgrade my RV battery to one or more lithium batteries. I am considering buying one or two used SOK batteries listed on Facebook marketplace. My question is what can I do to make sure that I am purchasing a good battery? If the cells have been damaged in some way how can I tell? And the seller claims low cycle count, is there a way for me to test/prove that when I go to inspect them? Is there anything else I should consider? Thank you for your advice!

 

[sunshine_eggo]

Find out how old they are and if the warranty is transferrable. If so, probably safe to take a leap.

The only way you can be reasonably certain is to conduct a full charge/discharge cycle measuring capacity.

 

[Substrate]

Buying used batteries is like buying used underwear or using somebody else's toothbrush.

Ie, you had better know the history, and "low cycle count" does not mean lack of abuse.

Consider this hypothetical:
Those batteries were charged up to 100% SOC and kept that way for years in an inappropriate application like a ups-standby. Fully charged all the time.

Now you get them, and perform a discharge test. And they pass! 5 cycles later they are dead because of the previous owner degrading them sitting at 100% charge for years.

The SEI layer had become so degraded that only a small amount of lithium was able to intercalate for your initial handful of cycles. That is now enough for a full-clog of the SEI layer. Plenty of capacity left, but you can't get to it due to the former owner's degradation. But it has low cycles! BZZT - yeah, but other issues not revealed to you tell the story 5 cycles later.

That's just one example. It's a risk. It is also why if you are starting out, you learn NOTHING from used batteries because your baseline is skewed by the unknown variables of previous ownership.