12v battery dead- related to transaxle?

[tmorrowus]

About 6 or so times, I've found my 12V battery mysteriously dead when I return to my prius after a few hours or overnight. Strangely this only happens when I am out of town on road trips... it has happened in the Sierras, Calistoga, and Sunnyvale, but never in my home town of San Francisco. I've left the car unused for weeks and the 12V battery still started it up, so I know the 12V is okay. I can't figure out what is causing these dead batteries though. I used to have a problem with accidentally leaving the headlights on, running down the 12V battery, but I'm sure that's not the cause of these cases.

Sometimes when I would jump start the vehicle, it still wouldn't run... I would get a message on the MFD that the hybrid transaxle was in an abnormal position and that I should move it to a level position. Eventually it would usually start if I waited and tried jumpstarting again in 30 minutes.

Does anyone know what this message means? I looked in the owner's manual and in the hybrid transaxle section it says (p. 138)

When the auxiliary battery is disconnected
Before you disconnect the auxiliary battery, confirm the shift position is "P" and apply the parking brake completely.
After you reconnect the auxiliary battery shift the lever to "N" and confirm the shift position is in "N" when the "IG-ON" mode is enabled.

What is this all about? I am wondering if this applies to dead-battery situations when I jumpstart... should I apply the parking brake before jumping, and should I be shifting to "N" after reconnecting? Actually I just reread the jumpstart section of the manual and it does say to apply the parking brake, but doesn't say anything about shifting to "N",

And I'm wondering if anything about the position of the transaxle could somehow be the source of my dead batteries? Perhaps only when it's hot from lots of driving?

[qbee42]

The manual is telling you to engage Park before removing power from the system. The parking pawl, if you want to call it that, is powered by the 12V system. If you pull the 12V battery in another gear the car may be free to roll. That's all it's about.

The error message you see is a red herring caused by a low battery. Low voltage on the 12V system will cause all sorts of weird errors.

Tom

[David Beale]

Pearl hasn't lost 12V yet (knocking on wood). I suspect many of those who fully drain the 12V battery do so by leaving the headlamps on. If you power off the car and don't open the drivers door, the headlamps will stay on (halogens will draw about 10A on low beam, HIDs will draw about 6 Amps). If you enter the car from the passenger side and power it on then off (for whatever reason) the headlamps could come on and not go off if -
1. You have a US car and leave the headlamp switch on.
2. You have a Canadian car and don't apply the parking brake - it stops the DRLs from lighting.

Otherwise perhaps you have not fully closed a door or the hatch, or have accidentally switched an interior lamp to always on. While this will cause a drain, it does take a long time to fully drain a healthy 12V battery (about 35 A-hr) with a few interior lamps (say 1 Amp each).

[Bob64]

Uhh.. I'd go and get your 12v battery checked... Each time you "drain" your battery, it'll lose a bit of its life... and it might be on its last legs.

[Lord_Towers]

i actually had the same problem it turned out to be a bad 12v battery on my 08 - took about 90 min at the dealer to let them test and then replace the battery (all covered)

[tmorrowus]

I've had the battery tested by the toyota dealer in the past twice and they said it was fine, but I never really trusted their test... I doubt they do a full discharge test to determine the capacity; they are probably just doing a simple load test which only shows how the battery reacts with a full charge, not how quickly it discharges.

As I said these dead batterys always happen when I'm out of town, leaving the car for as little as a few hours, but they never happen when I am at home, leaving the car unused for days or even weeks at a time. I just had an idea... perhaps the very act of driving long highway distances is the issue... This is obviously counterintuitive since one would think long drives would top the 12V up, but perhaps there is some kind of issue with charging the battery.

That brought me to thinking about my stereo system... I have a honking car stereo system rated for at least 400 watts output. I do tend to listen to the stereo loud more on long highway trips. I don't think I listen loud enough to use too much power (given an understanding of crest factors and the fact that I don't hear distortion), but I suppose it's possible that my stereo is using more 12V power than that inverter can supply, leading the battery to get discharged rather than charged over time. Often the dead batteries happened after mountain twisty-road driving with lots of acceleration demands; perhaps in that situation the car throttles off the 12V DC/DC to devote power to the hybrid system instead.

I had no problems with dead batteries when I drove all day for weeks driving across the USA, listening to the stereo loud, but that was mostly flat driving.

[richard schumacher]

I gather that this is indeed the same 12V battery you drained multiple times? If so, replace it. End of problem.