What exactly is the 12V battery used for?

Are you thinking of when it fills the coolant thermos? Because that's gone on the 2010, in favor of running the coolant through the exhaust to heat it up faster.

 

No, I'm thinking about after you kill your 12V battery by leaving the lights on. It's only a small water bottle after that.

(Technical explanation: As you use a lead-acid battery, the acid concentration drops. If you run it all the way down to flat, the acid is so weak that it's mostly water.)



Tom

 

The manual seems misleading about "power modes", mentioning "Accessory" running some things, and "ON" running all accessories, but omitting "Ready" except in a different section and topic. The accessory modes appear not to engage the high voltage traction battery or the gas engine and will drain the small 12v battery if the high draw ventilation fan is running (available only in "ON"). I'd skip "ON" and use Ready in Park for anything with substantial 12v load; the engine will come on to keep the batteries charged.

 

OK, I was pretty sure you were aware, but that was a rather out-there use for a dead battery.

However, now that you mention it, there are plenty of other uses for it too... For example, you could use it to chock your wheels, or to help weigh down a wheel that's slipping on sand or snow when stuck, or...

 

Now you are talking. The uses are nearly endless. We could collect a lot of them and build a garage for your Prius. They would have to work at least as well as fruit cakes.

Tom

 

Is this right? A healthy Prius 12V battery, with the car up and running, should be able to jump-start another car using the + terminal in the fuse box and a - ground elsewhere under the hood. Eh?

 

No, the manual specifically forbids jumping another car using the Prius.

 

Mostly this is because of concern over voltage spikes hurting the electronics. You could jump another car if you attached the jumper cable to the 12V battery, but I wouldn't risk it. It's a small battery, and the Prius contains a lot of expensive electronics.

Connecting to the jump terminal in the fuse box would likely blow a fuse. It's not designed for the sort of current necessary to start a normal car engine.

Tom

 

Regular car starter motors take a few hundred amps, to break loose all those engine parts. Prius has essentially a lawn tractor battery, and the 12v supply from the high voltage battery is only 100 amps max. The Prius battery is not needed for a starter motor because the traction motors do it. 12v for accessories and a safety gimmick where the Nickel battery has a relay inside the case requiring an external 12v input to make the high voltage terminals live -- fail safe. Jump starting the Prius itself is just tripping that relay. Space, money, weight put into the main battery instead of unused 12v capacity.

Portable jump start batteries good for only a couple of starts on regular cars are fairly small and inexpensive, and are used by road service people instead of booster cables from a truck. 4ah and up. In all the stores. Don't fall for the ones which try to recharge a battery through the cig lighter socket, however, doesn't really work when a battery very low. You need the high current from the alternator to bring the battery up. Can't use on Prius anyway, sockets cut off.

 

Just from my limited experience with my Prius, I have a dead 12v battery but I can easily start the car with one of those external jumper batteries. For a gas car, it's said this can do about 3 jumps (maybe).... seems the Prius barely makes a dent in it to where I've used it 3 times so far and the charge is still not under 75%. There's an appointment to get a replacement battery but in a pinch, this portable jumper battery will do to get me to my appointment this weekend.... and it's compact enough to keep charged in the car for just this very emergency to jump the 12v....