I am looking for resources around buying used LiFePo4 batteries and how to determine if they are in good shape or damaged prior to purchase. A local seller on Craigslist has 130AH Valence XP U27-12XP Series Batteries which they say were charged but never used as a part of a UPS electric prototype vehicle.
Seller is asking $650 per or a discount for multiples. Seems to be a decent deal for 130ah if they are functional.
Below is a long post about my experience with these batteries. I'll sum it up in this one first paragraph for you in case you're in a hurry. even when heavily-used they are very strong batteries that will last decades. They are the smartest batteries and can do things you've never even knew about.
I Bought hundreds of those U27-12XP with the same history as the ones you're looking at. I hooked up the communication cable to my laptop and looked at the batteries history. This enabled me to pull out lots of useful data from the batteries on board computer. Being in the electric vehicle and also having no cooling system like a Tesla would have. it was a high demand situation and it showed. With only five hundred cycles they were already down to being able to hold 85% of original rated capacity. They had as much as 300 amps surge coming out of the batteries and the peak charge rate was 166 amps. the whole truck was consuming 125,000 Watts that's 160 horsepower.
Now they are in a solar installation and they'll be used very lightly with very shallow cycles. I expect the discharge curve from the manufacturer to hold accurate with my usage. In other words I'll be using them for over 10 years probably 20 years. I put them through lots of tests before I bought so many and I'm very impressed. They're very well matched to each other. There's aftermarket bms's designed for keeping them matched to each other as well. Each battery has its own internal mosfet and if the master BMS decides One battery should drop out it can basically turn off the cells and short is terminals to drop out. Even better than that if its capacity is running lower than the others for some reason the master BMS will command it to put out a lower percentage of power compared to the others so it will run out of power together at the same time as all the others. As long as your system is tolerant to the change it is the most versatile system you can get. Solar systems are very tolerant especially when you're dealing with the higher voltage systems.
valence has a couple different versions. One version does not require the master BMS , it has internal high and low voltage disconnects for safety and battery protection. and the batteries communicate directly to each other but it has severe limitations on what do you can do with a series parallel set up. It's good for small solar systems.
If you're going to use the model number you you showed it needs to have a master BMS or at least a module balancer one of those cheap $60 ones from Amazon. The larger battery Bank you have the more likely it's worth buying the $500 BMS. And make sure your charge controller knows when to stop and your inverter knows when to shut down or else you will also need a battery capacity meter with relay controls & You'll have to get a high current relay so that you can protect the batteries because they do not have any internal voltage protection.
I've got a friend that was using hundreds of these and he turned me on to them. He said he likes to buy the ones that the distributor deems as bad. Sometimes there dang near zero volts. He revives them gently and he's measured their capacity and it's still very good. he's managed to amass a gigantic battery bank with mismatched ages / capacities /voltages and it's actually working quite well for many years. You could never do that with lead acid batteries.
You asked how you can tell if they're good? Well with lithium iron phosphate pretty much if they haven't dropped into dangerously low voltages you're definitely going to do just fine. The voltage won't tell you anything about its capacity or abilities. The amount of capacity remaining is definitely related to their past abuse. if you can pump in tons of amps without the voltage spiking and suck them back out without the voltage sagging bad then you're good to go.
