I have had a LiFePO4 battery in my car for several years now without issue. I isolate it after using the car so I very rarely need to charge it even after leaving sitting for months over the winter. This summer however, it has unexpectedly run flat on me twice. I have been able to jump start the car but the protection circuit stops it charging until I connect it to my lithium ctek charger. It's hard to tell what causes the problem; whether it is being drained somehow or there is an occasional surge that triggers the protection circuit. Does anyone have any thoughts on how to diagnose the problem? I don't really know where to start.
I had a similar problem where my Battery drained quickly, It turned out to be the alarm sounder it has a backup battery in it which had corroded and was draining the battery, I believe it's quite common. But at the point mine was particularly bad the alarm didn't sound at all might be worth testing the alarm.
I bought a replacement sounder and new battery pack for the alarm and haven't had any problems since, there was guide somewhere on here on how to replace that backup battery, but given it's now at least 20 years old if it has been replaced might be worth replacing anyway, or disconnecting the sounder to test if it is the culprit
Quote from: steveash on September 15, 2026, 22:25I isolate it after using the car
Quote from: Tomr2 on September 15, 2026, 22:35It turned out to be the alarm sounder
@Tomr2 It can't be anything on the car if the battery is disconnected.
It actually could be something emptying the battery either when the car is sitting before disconnection or even during driving as I have found that the car runs with the battery in its safety cutoff state but it won't start the car or charge. So it could potentially be draining down to the cutoff point during use.
Alternative the battery could be going into its safety cutoff state from a power surge.
The alarm is certainly plausible so I'll order a new battery to see if it helps.
Have you checked the output of the alternator?
I'm guessing you have a DC-DC charger?
Quote from: steveash on September 16, 2026, 09:19It actually could be something emptying the battery either when the car is sitting before disconnection or even during driving as I have found that the car runs with the battery in its safety cutoff state but it won't start the car or charge. So it could potentially be draining down to the cutoff point during use.
Alternative the battery could be going into its safety cutoff state from a power surge.
The alarm is certainly plausible so I'll order a new battery to see if it helps.
There are slightly different part numbers on the alarm sounder, so it's worth double-checking which one you have. I ordered mine from an eBay seller called XCELPLUS — they hadn't seen this particular battery pack in an MR2 before. I'd also had to replace the sounder itself for a second hand one from eBay, as the corroded batteries had damaged the motherboard too. Interestingly, both sounders used the same battery and also both had corroded probably age related. It looks like the seller has since added more detail to their eBay listing: https://ebay.io/m/f02FeR