What causes a 12 volt battery to be always drained? It will be drained if sitting over 1 hr

I feel your pain. However my car can no longer go more than 10 days without drawing down a brand new auxiliary battery. I don’t drive the car much and I do not actually set the alarm but rather the red light does remain on when I hand lock the vehicle with the key. I can’t figure it out and would surely like to know.

You could have a bad 12 volt battery altogether. There must be some electronics in the car that simply drain on the car while parked. I can’t imagine the inverter is your issue since you’re not actually driving the car. Let’s see what the folks here have to say.

I would be interested in knowing what electronic is likely draining the battery and if that device could somehow be disabled by pulling a fuse or if it is the alarm if a kill switch could somehow be installed for those who don’t drive the cars much.

I have a trickle charge on hand but access to using that is limited and not a very functional device with the Gen 1 Prius. It cannot charge off the cigarette lighter and appears to have to charge off the 12 volt battery.

 

Hmm I am not sure what your are doing here? A trickle charger ie Battery Maintaining need's to plumed into to an AC source to work?

AFAIK the 12 volt sources on a Prius don't function if the car is off?

 

Don't overlook the obvious first. make sure the interior lights the cabin or the trunk are not being left on? And yeah the headlights.

One hour is pretty quick to drain a 12 volt battery? But back to basics, have you had the 12 volt battery load tested? Just about any Auto Parts store can do that for you. You need to start with that.

 

If you can get to a quiet place, you might listen for any motors running after shutoff. An hour is real fast for a battery to run down without something mechanical pulling it down. You could always pull a fuse or rig up a way to easily disconnect the battery. For example, I use the cheapo $22 riding lawn motor batteries from Walmart in my car. They don't last long but you can buy a bunch of them for the cost of a yellow-top. I keep a regular car battery in my trunk with jumper cables attached to the lawn mower battery. At this point, the lawn mower battery is no good. If I disconnect the jumper cables, it won't start. That's pretty easy to do - pop the trunk and connect one jumper cable.

 

Jumper cable must be attached in front, no need to open trunk...

 

I'll bow to the greater knowledge of others regarding how to jump start. Just saying, my way has worked for over a year now. There's no room under the front hood for another battery, so having to lug one up there to jump every time I start the car would be no good. But popping the trunk and attaching the black jumper to one battery is pretty easy.

 

What exactly makes you think that the battery itself is "perfectly fine" ?
ONE incident of accidentally draining it down will often cause it to STAY low and that tends to make it fail completely in a fairly short time.
The car takes a LONG time to fully recharge a drained battery.

How old is the 12 V battery ?

Testing the inverter is a simple matter if you have a voltmeter.

 

I just had to get a new 12V. I could get it charged up fully (by car or charger) and it would come out of it at about 13V (which is should have been), but within a short period of time - like a matter of minutes - it dove to 11V. It was only two years old (my replacement was free), and it wasn't the car doing it.

As has been suggested, take it to a battery place and have it checked.

 

I may have misstated above.
I’ll have to get back on this post. Something about the trickle charger and electronics meant that it would have to hook directly to the 12 volt and not charge through the cigarette lighter which is problematic in my situation. As far as the OP, I addressed it as well.

 

The 2002 Gen 1 does not have a place to jump from the front of the vehicle. You must jump from the rear of the vehicle. There are no posts as in later models. I know this and am a 2002 owner. I’ve jumped started the vehicle so many times. Be careful you do not reverse the polarity on the cables because it’s a PITA replacing the burned fuse and hopefully you don’t fry the inverter.

 

There are solar trickle chargers.

The 12V sources on all cars have to do stuff while the car is off. (e.g. the antitheft system). But it should be doing very little. However, various kinds of problems can end up creating too much 12V current draw - generally called a parasitic draw.

My guess in this case is just a bad 12V battery.

 

I've been having the same issue with 12v battery drain. I am wondering if perhaps a voltage regulator, wherever it is, could be faulty. Does this make sense to anyone? Does anyone have an idea where that voltage regulator might be located? Best to all, Gleno

 

The thing that most often "drains" an old battery is.......the battery itself.

You can test the "voltage regulator" by measuring the charging voltage at the battery while the car is running (READY).
It needs to be above 13.6 volts or no charging will occur.

 

The car doesn't have any "voltage regulator" in the sense of the old-school box that would vary the field current to your alternator to get the right voltage out. And even those old-school circuits generally had the field current downstream of the ignition switch, so it wouldn't be draining anything with the ignition off. The Prius, of course, doesn't need a voltage regulator for its alternator because it hasn't got an alternator.

On the other hand, in these days of microelectronics, you can count on about two dozen different devices in the car having their own voltage regulators inside them. But those are just internal details of how those devices are built; you generally wouldn't think at the level of "has the on-board regulator in device x gone bad", you would just wonder if device x has gone bad.

From Electronic Specialties Inc., you can get a handy thing called the Fuse Buddy, which makes it super easy to measure for unexpected current drains in different circuits of the car. The 301M is the mini-fuse size. You just pop one fuse out of the fusebox at a time, put the fuse into the Fuse Buddy, plug the Fuse Buddy in where the fuse came out, and run the leads to the amp inputs of your multimeter.

They also sell versions with a meter built in, if you don't have a multimeter to use.



Here's mine in action showing the charging current after I rigged up a solar panel charging circuit:



Not quite obvious in the photo is that I modded mine a bit: I cracked the plastic case open to see how far I could chamfer the corners in front without exposing the metal inside, then cut the corners down and stuck it back together with a couple coats of Plasti Dip. The chamfered corners let it in to some of the car's fuse positions where it otherwise wouldn't go.