[AndyGon]

Yesterday I had to jump start my girlfriend's Honda because I left the door open overnight. I found the positive terminal jack underneath the hood of my Prius, but struggled in vain to find the negative terminal. Exasperated, I looked online and found that the 12V battery is hidden away underneath the trunk. I found the battery, connected the negative and positive terminals on my Prius and her Honda, and jump started the Honda.

Anybody know why the 12V battery is essentially in the trunk and relatively inaccessible?

Can I jump start somebody else's car by hooking up the positive terminal underneath the Prius's hood and an exposed metal part? I noticed that the negative terminal was grounded to the metal frame of the Prius, so that seems possible.

 

[yauman]

Yesterday I had to jump start my girlfriend's Honda because I left the door open overnight. I found the positive terminal jack underneath the hood of my Prius, but struggled in vain to find the negative terminal. Exasperated, I looked online and found that the 12V battery is hidden away underneath the trunk. I found the battery, connected the negative and positive terminals on my Prius and her Honda, and jump started the Honda.

Anybody know why the 12V battery is essentially in the trunk and relatively inaccessible?

Can I jump start somebody else's car by hooking up the positive terminal underneath the Prius's hood and an exposed metal part? I noticed that the negative terminal was grounded to the metal frame of the Prius, so that seems possible.

Just a couple of notes: You are not suppose to use your Prius to jump start another car - you should just hook up your battery to her battery, start the Prius, let it run for a few minutes to charge her battery and disconnect before she try to start her car.

Also, the proper procedure for jump starting DO NOT require locating the Neg terminal of the battery - for safety reason. You are suppose to connect the pos to pos. In all cars, the neg is the car. So, connect any metal part of the engine using the neg wire - preferably as far away from the battery as possible - the reason being that the finally connection always draw sparks and if you do it at the battery neg terminal, the spark may ignite hydrogen gas emitted. Exploding batteries are very dangerous.

 

[AndyGon]

You are not suppose to use your Prius to jump start another car - you should just hook up your battery to her battery, start the Prius, let it run for a few minutes to charge her battery and disconnect before she try to start her car.

Why is that? And is that just with the Prius, with hybrids in general, or with cars in general?

Thanks for the info!

 

[KTPhil]

Since it needn't drive a starter motor, the Prius 12v battery is very small. It won't crank another car very well. But the Prius can charge another battery as described above. If the other battery is dead or shorted, this won't work.

 

[yauman]

You are not suppose to use your Prius to jump start another car - you should just hook up your battery to her battery, start the Prius, let it run for a few minutes to charge her battery and disconnect before she try to start her car.

Why is that? And is that just with the Prius, with hybrids in general, or with cars in general?

Thanks for the info!

It's just with hybrids. Other cars have starters and the batteries in cars with starters have the capacity to supply enough current to turn the engine over - cranking power. Most starters draw 80-100amps when they crank - much more when it's really cold outside. So jump starting car to car is ok when both cars have "cranking" batteries - provided the connections are made safely - ie last connection away from battery on car body/frame/engine. Hybrids don't have starters so the 12v battery is only for powering up accessories like dome lights and computer chips. It does not have the cranking power to run a starter so if you hook it up to another cars to start the car, it won't have enough amps to crank. Worst, if you have your hybrid running, when the other car cranks and draws 100 amps of current, the juice will have to come from the HV electrical systems of your hybrid - I think there is a main fuse which protect it from delivering such a surge of power - so you may blow the main fuse.

I'm not sure, but I remember reading in the manual about how to be a good samaritan with the Hybrid - and they mention not letting another car crank start with your little itsy-bitsy modped size 12V battery.

 

[hdrygas]

I once had a guy with a big F350 ask me to jump him in the parking lot of a grocery store. I knew it would not work so I looked him in the eye and asked "how would your truck handle 500 volts?" After the shock passed I 'fessed up and explained the problem to him. He waited for the person on the other side of him.

 

[AndyGon]

...So jump starting car to car is ok when both cars have "cranking" batteries - provided the connections are made safely - ie last connection away from battery on car body/frame/engine. Hybrids don't have starters so the 12v battery is only for powering up accessories like dome lights and computer chips. It does not have the cranking power to run a starter so if you hook it up to another cars to start the car, it won't have enough amps to crank. Worst, if you have your hybrid running, when the other car cranks and draws 100 amps of current, the juice will have to come from the HV electrical systems of your hybrid - I think there is a main fuse which protect it from delivering such a surge of power - so you may blow the main fuse.

So don't start the other car, but just charge up its battery. Do you know how much time it would take to recharge the dead battery enough that it can crank?

Anyway, even though I'm not supposed to crank the other car using my Prius's battery, I did it anyway not realizing that I wasn't supposed to. It turned out just fine. Perhaps I lucked out.

 

[yauman]

So don't start the other car, but just charge up its battery. Do you know how much time it would take to recharge the dead battery enough that it can crank?

Anyway, even though I'm not supposed to crank the other car using my Prius's battery, I did it anyway not realizing that I wasn't supposed to. It turned out just fine. Perhaps I lucked out.

Yes - the warning about how to be a good samaritan with your Prius is actually in the manual - page 239 on mine. Usually about 5-10 minutes should be enough if the other battery is good and is discharged only because the lights were left on or something like that. If the battery is dead (old) then no amount of time will recharge it. If it's not a "maintainence free" battery, the first thing to do before trying to charge it is to open up the little caps and peak in to make sure there's liquid and that the cells are not dry. If it is dry, fill it up with clean water first (de-ionize or distill water if you have any handy - and please please do NOT overfill) before trying to recharge it. You see, when you hook up the other car's battery in paralley with your's and you run the Prius engine, you are charging both batteries - but since your little battery is already quite fully charges, most of the charging current will go into charging the other car's battery.

Ok.. so you lucked out this time - chalk it up to dumb luck.. but please don't do it again.. be nice to your baby and it's little itsy-bitsy baby 12V battery!!!