Jumper Cables

I use a 7AH Gel Battery with housewire and alligator clips.

 

Indeed, large jumper cables can be hard to attach to the rather small connection points provided in the engine bay. The thinnest jumper cable you can find will be sufficient.

As to your other point, actually the original Japanese Prius had the opposite - an on-board emergency charger that would boost the HV battery from a 12V battery...

 

Or ready to kill someone if the safety features failed. A means of charging the traction battery from the 12V battery would require operating or bypassing the high-voltage isolation relays which are otherwise open whenever the car is OFF. Toyota must have decided that it was not worth the added complexity, cost, and risk to provide a backup charging system that would only benefit a few drivers a relatively small number of times.

A manual switch to turn on one of the accessory outlets (that is, connect it to the 12V battery so that it could be used for charging) would have been useful, cheap, and not too dangerous (though undoubtedly a number of people would have left it ON with some device attached and drained their 12V battery).

 

How to use a Prius to jump start another car

I couldn't find much information on this, but went ahead and did it anyway--successfully.
The main thing to realize is that the Prius 12v battery is smaller and doesn't deliver as much current as a regular car battery, so you should DISCONNECT IT BEFORE TRYING TO START THE OTHER CAR.
So I offer this how-to with the usual disclaimer: you mess up, it's your fault.

I hadn't been using my old car much since I got my Prius 6 months ago. When my son went to start it up, it had just barely died--not enough juice to turn it over or even use the electric windows.

Pull your Prius into position, cars not touching.
Turn off your head- and interior lights (I actually forgot to do this).
Turn off and remove your prius key.
Open the hood and remove the plastic 12v battery cover.
I hooked up the cables as explained in the owner's manual (2006, p. 358).
I had borrowed mega-cables, and couldn't get them onto the tiny Prius bolt, so I clipped the positive to the silver clamp that rises behind that small silver positive bolt holding down the copper strip. Worked fine.
The second end of the negative cable is grounded to the "live" battery car, in this case the prius.
(The owner's manual says to do it to the dead battery car--but the Prius has no negative terminal.
I bet you don't even need to hook up the negatives at all--??)
Anyway, for the negative ground I used a bolt on the chassis just behind the 12v prius battery.
I turned on the Prius, and let it sit for 7 minutes to charge the other battery.

The Prius did its little thing with the gas motor going on for a minute or so, and then just sat silently.
(I listened to a CD--I say this and my headlight mistake to indicate that it didn't seem to matter for starting the Prius up again later.)
I turned off the Prius, DISCONNECTED THE JUMPER CABLE, and the other car started up just fine.
And so did my Prius.
If your dead battery is very dead, you'll probably need to let it sit charging for longer, but if you remember to turn off your prius headlights, you'll probably be ok.

Any additional comments or feedback would be welcome.

 

Quote: I bet you don't even need to hook up the negatives at all--??)

If you don't complete the circuit you'll wait forever and nothing will happen other than you'll grow very very old. Yes both positive and negative need to be connected. Since both cars are negative ground any part of either car is a good choice. Generally I'll choose an unpainted bolt somewhere away from the battery.

 

Could I replace the 12v battery with, say, one from a 1996 Geo Prism?
(The battery itself is relatively new, about one month old -- I only mention the year in case someone happens to know how much juice the battery in the Geo of that particular model/year has.)

I guess what my question boils down to is even though the 12v battery in the Prius is used just to start up the computer, could it be replaced with a heavy-duty 12v battery?
And if so, could you then use the beefier 12v battery to jump-start other dead cars instead of having to wait the 7-15 minutes in order to go the "recharge the other battery" route?

 

<div class='quotetop'>QUOTE(Manksgloob @ Jun 9 2006, 11:11 PM) [snapback]269002[/snapback]</div>

If it would fit, I suppose you could replace it with one, assuming the terminals were correct. Be aware however that the Prius battery is an AGM battery. The reason I mention this is, AGM batteries produce next to nothing for outgassing when they are charged. Flooded lead/acid batteries produce huge amounts of H2 gas which is highly explosive. The Prius battery is in the cabin, do you need a better picture? Like the Hindenburg? So yes, it might fit etc., but I wouldn't do it.

FWIW, in photovoltaic systems that have an indoor battery bank and flooded cells, a sealed and vented box is used to make sure all the H2 is vented outside. Hydrogen is nothing to play with.

OK, all of that having been said, if you got the battery installed and properly vented, you use it to jump start an ICE powered car. Now you need to recharge the battery. The charger in the Prius is fairly pitiful and without a fairly prompt recharge battery life will suffer.

If it's for jump starting vehicles at home it'd be much easier/safer to get a hand truck and a starter battery and an inexpensive charger and wheel the battery to where you need it then bring it back and recharge it. If the charger is small enough you can leave it on the battery forever. Just check the electrolyte level every few months.