My battery recently died when I left my '05 Prius parked for a few days, and had to get it jumped. The hybrid battery was still about half full, so it must be the auxiliary one. I can only come up with one candidate problem, so I'd love to know if I'm on track or not.

When the car is turned off and car is locked then the headlights turn off, and the inside light triggered to door opening & closing turns off. However, the reading light (the lights above the rear-view-mirrow to the left & right of the light triggered by door opening) apparently does not. At least, when I walked around for 5 minutes and came back it was still on.

Now, in my case, after the car was jumped I found that my wife had left the reading light on when she parked the car 3 days ago. Could it be that this is the only light in the car that does not automatically turn off, and hence that tiny light can actually drain the battery if left on for several days? Is this a "feature" or a bug? I'm guessing it's a feature to let me say read in the car when it's turned off, but it ended up being might inconvenient in this case.

Anyways, just want to make sure I diagnosed the problem correctly so that I don't have to wonder if the car will be kaput again tomorrow morning for instance. Thanks in advance.

Unless a hybrid's different in some way, then yes, interior lights left on for long periods can drain the under-hood 12v (traditional) car battery. I've never fully drained a battery due to this but it happens once in awhile in my '92 Corolla since there are no warnings or notices about lights (including headlights) left on.

- Shannon (Geeky, Wild Texan)

It's actually not under the hood, but yes this can happen, and it happened to my parents when they went on a ski trip this winter. They noticed the car had melted the snow on the roof around where the light is, and had to get the battery jumped.

Luckily it doesn't take much to jump a Prius as the actual starting of the engine happens with the HV battery, so the 12v battery just needs enough juice to fire up the HV relays and computers, then the built-in charging system takes care of the 12v.

A Prius does use a rather small size of 12v battery since it doesn't need high starting current, so they can drain a bit quicker than most cars, but should still be able to supply power to accessories for quite a while without a problem.

2001 Prius Photos       2005 Prius Photos
'97 Prius Simulation Prius inner workings

It has happened more than once in our Sienna. 3 year old kids LOVE to push buttons.

Sorry, I fell into Ford Escape Hybrid specific verbiage. The owner's manual, in an attempt to make it clear which battery it's referring to, refers to the traditional car battery as the "low voltage (under hood) battery" and the hybrid battery pack as the "high voltage (cargo area)" battery pack. Our 12 volt ("low voltage") battery is up in the engine compartment in just about exactly where one would expect it in a non-hybrid.

- Shannon (Geeky, Wild Texan)

Kuroyama, your diagnosis is almost certainly correct. The Prius high-voltage battery cannot be discharged in this way. My only concern is that typically, once lead acid batteries are fully discharged, they have lost some capacity. If the problem does recur you should consider replacing that battery. It is sometimes possible to convince a Toyota shop to replace a 12 v battery under warranty, if your diplomatic skills are good.

DAS

I left my key on ignition with headon light on overnight because the engine didn't come on. I didn't know until next afternoon. The auxillary battery was completely drained. I used my battery charger to charge it and it was fine. About a year later, the auxillary battery couldn't charge any more. I took it to the dealer's service dept. and they told me one of the cell was dead. Leaving lighton overnight might have killed the auxillary battery. If I didn't have any warranty, I could have pay $120.00 for bateyr with installed.

We just had to jump our prius this morning. I looked and my 3 year old son had turned the interior light to on. We were gone for a day and a half so it must have run the battery down. It started fine once we jumped it.

Crystal