12 volt battery charge?

The 12v battery charges whenever the car is 'Ready', the engine may be running or not.

The computers, to get it started, and all accessories when not 'Ready'.

 

Including the lights, radio, etc. Everything electrical in the car runs on 12V -except- the hybrid synergy drive and A/C compressor.

The 12V battery is charged by an electronic charger powered from the traction battery (the 201V one). The engine will run to charge the 201V battery when needed and to power the car down the road when needed. It will shut off when not needed.

 

So, the 12v battery doesn't start the ICE?

 

Correct, it does not. The engine is started by MG1 (motor-generator 1, the smaller of the two motor-generators in the Prius) which is powered by the traction battery. There is no conventional starter in the Prius at all.

 

Good question. I have a similar question. Would it be safe to say that it is easier to start the Prius with a low battery since all it needs the 12v to do is to get the electronics going? (No starter, which draws a lot of amps)

 

Yes, it is easier compared to a regular vehicle. However this does not mean that you can ignore the Prius 12V battery - if you desire reliable operation. The car needs to be driven regularly so that the battery is kept in a charged state.

 

I had to jump start mine last night, and it was a very small load on my wifes car, just enough to power up to the electronics.

 

The gasoline engine will run to spin MG1, which will charge the high voltage traction battery as needed.

The 12V battery is being charged whenever the Prius is READY.

 

For a "startup" battery to attach up front, one could use a small 12-volt battery, right?

How much current does it need to supply for the few seconds that the electronics need to run before the DC-to-DC converter starts supplying the 12-volt power?

Maybe a "small" hand-held "pack" of 8 Alkaline D-cells (or even smaller) would work?

Is the "dead-start" contact under the hood connected directly to the 12-volt battery in the back?
Hopefully not.

 

Philmore and others have "8 cell holder", just search.

C-cell or even the smaller AA cells might work for an emergency-startup.

If one measures the voltage across the front "startup" conection points,
what do you get with the car ON, and OFF?

Presumably one needs around 13 to 14 volts to charge a 12 volt lead-acid battery?

Or, probably the 12-volt charge just a "trickle" (or "slow") charge?

 

Prius needs two (usually) 50 Amp 1/2 second pulses of power to go to "ready". It draws just a few amps before the 50 Amp pulses. One charges up the brake pressure, the other puts the car in "ready". If you're going to try to build a small "boost battery" you'll have to make sure the connections in the battery are really good and able to handle that kind of current without too much voltage drop. It won't heat up because it's so short, but you may get too much loss in voltage. There are some nice aluminum (aluminium in some places) AA cell holders that are up to this. Most of the plastic ones won't cut it. You'll also need good quality AA cells. Or perhaps a small gel cell. You can keep that charged by plugging it into the accessory plug in the car. It will slowly charge when the car is in "ready". I'd be tempted to put a diode in there to limit where the power goes and how much goes there. Prius runs the "12V system" at 13.8V, so you'd need a "hot carrier" or "schottky" diode to keep the voltage drop low.

Once in "ready", the relay inside the "traction battery" has been closed, so 201V is available for the inverter to produce 500+V to turn MG1. MG1 turning also turns the "engine" (gasoline engine), starting it when fuel is injected (there is a slight delay with fuel injection, to allow the engine to build up oil pressure etc).

Note that, unlike other ancient designs, Prius will start instantly in -40C or F temps. I have never experienced Pearl having any problems starting the engine. If you think about it for a minute, you wouldn't expect any problem, with a 201V battery boosted to over 500V, and a 20 HP "starter motor". If fact, this week, while it wasn't really cold (just below 0F or -22C), I didn't even hear the engine start, after sitting for 5 days! Maybe I was just wearing too many cloths.

DO keep the correct oil in the engine. Otherwise the start might be rough (some shaking of the parts). I'm using 5W30 Mobil 1 in my GII. You'll want 0W20 in your GIII.

 

The 12 volt battery is at the passenger side of the cargo compartment, at the very rear.

The special "start-up" connection point is under the front hood, inside the fuse-box that is on the driver side. The latch is on the front of the box lid, and the "connection" (+12 only) is under a flip-up cover marked "+" at the very rear of the fuse box.

 

Wow, 50 amps, that is a lot more than I guessed.
There are not many fuses in the fuse box that will take that much current.

But, Thanks for the info.

I got 12.33 volts on the "start-up" tab when the Prius was OFF.
With it ON, I looked at the "hidden" service screen and the battery was 11.8 (or so).

 

Small tractor battery?

 

Or buy a small jump box. They also come with a small air compressor, which is handy for keeping your tires inflated.

Tom

 

Tom has the best solution, if you really think you need boost capability. Most small batteries will vent H2 into the cabin, if you carry them. Not a "good thing". (tm) Note that in the 2 1/2 years I've had Pearl I've never needed a boost (knocking on wood). I've also got a "battery tender" mounted on the garage wall beside Pearl, but I've never used it either. It's there "just in case" on -30C days "something bad" happens. I can recharge without having to mess around in the cold!!

The portable boost units use a gel cell battery. It -can- vent in extreme situations, but is usually sealed. DO be careful how you carry it. You don't want it in the back of your head in an accident!