I was at the track today and throughout my last run my battery voltage dropped gradually from around 13.5 to 5v and the car stalled after the session. I know it dropped from the Megasquirt log (file*)

Now, I would assume that this is an alternator issue however a similar thing happened back in May at another track day where my voltage slowly rose eventually hitting 20.1v!! (log file*) I assumed that this was an alternator issue so replaced the alternator with a rebuilt unit.

A few weeks ago my battery died (a Shorai LiFePo - that has been left drained a few times and delivered 20v as said above) so I replaced it with the same model.

Both times at the track the voltage problems got worse quickly after the run (during the cool-down) and seemed to correct themselves (temporarily) after turning the car off for a while.

I find this issue quite odd and, like I said, I would assume that it's another alternator issue but I'm really not sure - perhaps bad coils (I'm running Toyota COPs) or another electrical issue - so I wanted to see if anyone has any other insights/experience.

Thanks in advance

*Log files were too big to upload as attachments so I've shared them from my Google Drive

 

Sounds like a flaky regulator.
Intenal or homemade/ecu?

 

Its a rebuilt NA alternator with internal regulator.

 

This may have absolutely nothing to do with your situation but.....I had a situation where my voltage would vary from high 12's to high 13's while running. My car is an '03, meaning alternator regulated by the ECU. I cleaned every ground thoroughly and the problem persisted. I assumed it had to be the alternator and was ready to swap,it out when a friend offered me a one year old AGM battery for $25. Even though my car turned over and started without a problem, the battery was 8 yrs. old. I installed the "new" battery. Voila, no more voltage swings. Steady at 13.7 volts while running and sits about 12.9 when shut off. I don't know why.

Maybe borrow a battery and see and see if it has any effect on your situation.

Paul