As is the seemingly the craze in ham radio, I recently picked up several LiFePO4 batteries of varying capacities to use with my Yaesu FT-991a. I was able to get these as part of the Amazon Vine program, so there is no particular reason I chose the models in question: Power Queen 10Ah and SOKERDY 16Ah. For a similar reason, I got a (rather beefy) charger that supports varying chemistries from the SUHU brand (all these links are Amazon affiliate links).
Both batteries have a built-in Battery Management System (BMS) that essentially protects the cells from mistreatment including over and undervoltage. After the 2022 CQ WW VHF contest, which was the first I ran them to empty, I tried to charge them on the LiFePO4 setting on the charger, but it would not detect either battery. After putting a multimeter on them, I realized the BMS cutoff the terminals entirely to protect the low voltage condition. I would have expected this to reset after the load was removed for a certain amount of time but apparently not.
After remembering the battery charging had a “12V Supply” mode (which seemed useless to me when I bought it), I thought it was worth connecting them to the supply to see if this higher voltage would “kickstart” the batteries. Voila! It did! It seems only a second or two of 12-14V power is required to get it out of protection mode.
This is a clunky solution though as the charger requires a long button press to switch modes. It is probably better to not let the battery voltage get this low for battery longevity anyway, which is my solution going forward (and partially why having a simple voltmeter like I desired in my 2022 Field Day Report is handy). But in any case, keep this in mind if your LiFePO4 battery dies and the charger won’t detect it. It probably just needs a high enough voltage to get it out of this protection.
Please do this at your own risk as you are essentially forgoing any sort of controlled charging for the couple seconds it is connected. I take no responsibility for any undesirable result of trying this. Make sure you don’t leave it on too long or you could damage the battery!