I purchased my Prius in February of last year and went through the year worry free until Chicago fall hit. The car battery would be drained a few times on a few of the colder days-30s. I know that the previous owner had switched to a wfh schedule and hadn’t driven the car and suspected the battery but wanted to be sure it wasn’t a phantom draw. I hooked up a voltmeter and pulled fuses and found no drain, so I replaced the battery and everything was back to normal from mid November until this morning’s single digit temps. I walked out and encountered the same shallow buzzing. Because of the holiday I had been home for a few days. Is that enough to send the car into this state so quickly? Is there something else I’m missing?
How often are you driving the car, and when you do: how long are the trips on average? Where do you overnight-park the car, ie: street, driveway, carport, underground parkade, or garage? Do you have secure (or semi-secure) access to a 120 volt AC outlet. There's no real mystery here. Well there "could" be something amiss and fixable, but even if everything is as it should be: Cars steadily impose a small-but-steady draw on the battery. If the car sits idle for multiple days, punctuated by mostly short trips, it's winter, and you don't regularly use a smart-charger, then you will run the battery down.
it is possible that the new battery never had a full charge, and prius more or less keeps it at a steady state. otherwise, it should last a few weeks if you're correct about the quiescent drain being within spec, and the fob is stored far enough away from the vehicle, and the battery wasn't defective.
The charging voltage needs to be checked while the car is running. And it kind of sounds like you didn't do the "phantom drain" test right either.....maybe. Do you have anything plugged into the OBD port all the time ? Or any aftermarket accessories that might be hot all the time.....like a phone charger, radar detector or alarm system ?? And finally......are you sure that your new battery was fully charged when installed ? They often aren't and sometimes city driving won't charge it up all the way.
