Bought a 13' Avalon 3.5l a few years ago (New Battery). Was left to sit due to COVID shutdowns and the battery would die in about a week if not turned on. I started using it again recently and it slowly got worse until it started dying overnight. I got a battery that was known to be good (not brand new) and worked great for a few days (held charge for more than a day) but now does not work. Today, the battery was at 12.8V, and could not start the car without using a jump pack. After turning on, the voltage was 13.4V and the car ran like normal for a 30 minutes drive. But after shutdown, it immediately dropped to 10.3V. Did a voltage drop test for the alternator on both positive and negative where no issue was found. Did a parasitic draw test and found a 0.7A draw where removing the ECU B1 fuse drops it down to 0.3A. Unsure if the problem is the alternator, battery, or something else like the parasitic draw. Lights did flicker yesterday when testing everything (alternator?). Thank you.
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Hey, my name is Dusty and I'm one of the auto experts with RepairPal's Car Genius team here to help!
It sounds like the issue is with the batteries.
If the output voltage at the alternator was 13.4V and remained stable for 30 minutes without any issues, it's safe to say that the alternator is operating fine.
However, if the voltage dropped to 10.3V when the vehicle was turned off, it indicates that the battery may not be capable of accepting a charge.
I recommend starting by replacing the battery with a new one before retesting anything, especially a parasitic draw test.
It's important to note that parasitic draw is a separate issue to consider, and if not tested properly, it can lead to skewed results.
Before testing for parasitic draw the battery must be in good condition, weak or dying batteries aren't aren't stable enough to allow the vehicle to go into 'sleep' mode.
Then ensure that all doors, trunk, and hood latches are 'closed' and 'locked', and allow the vehicle systems sufficient time to go to sleep before beginning testing.
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