2nd Dead 12v Battery in less than 1 month

Hello,

I've scoured the net and can't really find much of an explanation to my troubles. We have a 2007 Prius (no nav, yes JBL). A few weeks back we were mucking around in the car, and left the radio on for some tunes and had the windows up and down a few times, without the car on (but yes accessory mode?). Surely enough the 12v battery was dead within a half hour. When we investigated, we found it to be the original 2007 battery - quite impressive and not surprising that it must have just been hanging on by a thread all these years and we doomed it by ~20 minutes usage without the DC/DC charger kicking in. The voltage was down to like 5v. For kicks, I put it on the battery tender, but actually the model we have charges both 6v and 12v automatically, so the thing just said full charge at 6v thinking it was hooked up to a 6v battery (not that it would have mattered anyways as I am pretty sure anything below 11v is nonviable). We replaced the battery and all was well for a few weeks. I did not check the new battery voltage, nor did I check periodically the voltage after it was installed.

Fast forward a couple of weeks, and after having left the car undriven for about a week, somehow the new battery has also died. The car was locked, out on the driveway. Totally unresponsive. A check of the new battery revealed it was down to around 6v as well. I'll be darned. No explanation for this one. No one left anything on, nothing was plugged in. Haven't noticed any electrical gremlins. Yet somehow this new battery is also a goner. Unfortunately I never measured the new one, I just assumed it was a good 12.5v+ fresh battery. It's entirely possible it was a dud, but it just seems so improbable. So we hooked up a spare 12v battery and started troubleshooting.

With the car on, the voltage at the rear terminals increases to around 14V, leading me to believe the DC/DC charger is fine. I used my multimeter as an ammeter between negative on the spare 12v battery and the ground of the car. When we muck around with it, like have a door open or the like, it draws a few amps as I would expect. With me tucked in the rear, all doors closed, and car locked from outside via key, it sits between 1-2A for about a minute or two before finally dying down to 10-20mA. I would assume this is an acceptable value for parasitic draw while the car is closed and locked.

Anyone know where to go from here before I lose another new battery?

TLDR: original battery died fair enough, new one died not long after, DC/DC charger puts out ~14V and parasitic draw with car closed and locked around 10-20mA - what killed my new battery and will it kill my replacement new one?

Thanks!

 

Just to add some more info prompted from your great replies so far:

Yes the battery charger is definitely a 6v/12v automatic sensing. Made by Battery Tender. It was one of the cheapest/only options they had at Pep Boys a while back. Wish I still had my CTEK but I don't. I was only using on the failed batteries in the faint hope I could resurrect them from the dead to help troubleshoot some more, I realistically new I had no chance. Anyways that's besides the point.

I have some confidence that nothing was left on. The couple of light switches are always on the off position, not even the door position, and everyone walks by this car on their way in so someone would have noticed during the week the "new" battery died. The "new" battery had been on the shelf since Feb, and silly me didn't think to check it's voltage when I took it home. This 2nd "new" battery had a Feb 2019 sticker on it as well, so we will see how it goes. As per your suggestion, I am leaving it on the battery tender for a while to get it's initial charge properly after 5 months of just sitting I suppose.

I'm not intentionally trying to leave the car alone for weeks at a time, but it seems like it should be able to be left alone with no problem? The original battery lasted 12 years and was left alone for many an extended period - it was only my foolishness listening to the radio with the car off that doomed it in the end. I don't think a week at 0.02A should kill a new battery within a month...

I guess I can't rule out a parasitic draw that is occurring at some point in time other than when I am measuring it... such as in the middle of the night or something - has anyone heard or know of any system on the Prius that kicks in long after closing and locking the car? I only checked for parasitic draw for about 10 minutes after locking myself inside, although I repeated the test more than once.

Another note of weirdness that just happened was the old dead dash display. Got through that one by unplugging/plugging the white connector on the battery, and understand this could be due to bad combination meter - but could this possibly related to killing my "new" battery? I wouldn't think the combination meter would be trying to draw any amps while the car is locked and sitting silently in the driveway but who knows.

Thanks for all the excellent replies and discussion.

 

Happened to notice my brake actuator buzzes a lot more frequently than I remember it doing nowadays. So I searched the forum and found this:

Brake actuator "warranty extension" from Toyota | PriusChat

In Post #3, he says his brake actuator was triggering quite often, even when the car was off. Do you guys think this might be my problem? I haven't noticing it triggering with the car off, besides when you first open the door after unlocking, but then again I don't sit there and listen to the car while it's off for extended periods. I will try to do next time I wash it, unless anyone has any other ingenious ideas to monitor what goes on with the car while it is off and parked in the driveway?

Many thanks!

 

Yeah that one. Seems to be buzzing everytime I push the brake pedal nowadays.