Dead Prius 12V battery

any accidents or aftermarket electronics?

 

Was thinking that was a lot of amps for just lights.

 

when you park on the incline, are you facing up or down?

 

its amost lik

Its almost like several systems are running on the 12 volt battery.

 

was thinking the brake pedal was resting on the switch if downhill...
























































Merged.





































when you say major, whats the biggest draw the 12v supplies, hybrid battery relays?

 

Wonder what all the 12volt supplies with the car turned off? Maybe it is not powering completely off.

Seems I remember a vacation switch on some of my hybrids under the steering column. Can't remember if that was the Honda or Prius. It stopped all power from the 12 volt battery to computer systems for long periods parked.
lights are 16 watts or less

 

yes, it's constantly seeking your fob, unless you switch it off. and of course, all the memory, dado presets, clock, windows and etc. all a very small draw unless away for several months, dependent on battery health.
this is definitely out of the ordinary.

 

maybe there is a FOB in the car somewhere.

 

You may want to question whether your neighbor saw brake lights or tail lights that night. I'm not certain I'd know the difference (he did say "red", not "bright") and you could spend forever checking out something that wasn't the problem. The tail lights switch off after 30 seconds, but that can seem like forever, if you are waiting for them to go off.

The brakes are not electric, but the booster pump is. This pump runs in the morning when you open the driver's door. It is also the first thing to (try to) run when you first try to charge or jump a dead battery. It's unfortunate, because if you could just get the traction battery on line, the inverter would take care of all the accessories, and charge the 12v battery as well. But if you are seeing a high current drain while trying to resuscitate the 12v, that's probably it. It will go away once the brake vacuum is achieved. It is very audible in my situation. I suppose cars and ears may vary.

 

Mendel, thanks for this post as I was unaware and will now use amy old 2-amp motorcycle battery charger if I have to charge the Prius battery rather than the higher-amp automobile chargers.

Hesitate to put in $.02, but could leaving the key fob near the car for the whole night cause that much of a parasitic draw and drain the battery? I know I was surprised when I just walked past my car at night and saw the Scangauge light up like a Christmas tree--it apparently was communicating with the car's computers, which implies to me that they were powered up and consuming at least some decent amount of current. But on further thought, that would not activate the brake lights. Sigh.

 

My ScanGauge only wakes up when the car is on. That's odd.

 

The Scanguage will turn on if there is a "spike" on the 12V system. As in something switches. If the FOB is in your pocket the interior light will turn on if you stand by the door. That is enough to trigger the Scangauge.

Back to the 12V battery.

The 4.2 Amp number Mendal posted is the max. recommended charge rate, probably due to the small vent tube. I have charged a Prius 12V battery at much higher current for short durations. Without any problem. Out of the car.
If the brake boost pump is running or trying to run you will hear it. It is very noisy. It's not a "vacuum pump", but rather a hydraulic pressure pump. It pressurizes the brake system directly (which is why when working on the brakes you -must- disconnect the 12V - if the pump fires it will pump out the caliper pistons - not a "good thing" (tm) ).

Now the interesting thing is the brake booster pump WILL fire up with the car off when you open the drivers door if the pressure has bleed down overnight. It's the car "getting ready for you" system. This fact alone could be confusing you. It will very quickly fully drain a weak or failing battery.

The 12V system with the car off will draw in the order of 10-20 mA. If you see zero there is something wrong, either with your meter or the way you're using it. If you open the rear hatch interior lights WILL turn on. You can't turn off the door lights. That's over an amp draw. Remember the lights have a "on all the time" position. The hatch one will get you if you're not careful as it isn't very noticeable.

So, how to proceed?

FIRST, check the bolt on the negative lead on the 12V battery where the cable attaches to the car body. It may be loose and the 12V battery may not be charging properly.

Note that in a Prius, once in "ready", you can't tell if the 12V battery is ok. All power comes from the traction battery via the inverter/charger.

Next, car turned off, put your meter on the floor of the hatch, connected in series with the 12V from the battery, on a current scale - preferably a digital one set to a 5-10 Amp range. Close the hatch and wait for the interior lights to go off (a few seconds). The current reading should drop to -almost- zero (10-20 mA if it can read that low on a high current range - you need the higher current range so as not to damage the meter when the interior lights go on or the brake booster pump fires).

This should give you a clue. The brake light switch is used for other functions as well and if the adjustment is off it will not function properly. The adjustment is the pedal arm as I recall. By watching the reflection on a wall you can quickly determine if it is ok. The brake lights should come on only when you press the pedal slightly or lightly. It they flicker when you touch the pedal it's out and if they only come on when you press hard it's out the other way.

Good luck with your diagnosis.

 

You can run leads out of the hatch for the above amperage check, the hatch lid gasket is flexible enough. Makes it easier to work with your meter. Definitely don't do anything else during this test, say open any doors.