JeromeP, I call BS on the fuse blowing comment. If the prius is off you cannot blow ANY fuses jumping another car because there are no fuses between the prius batt and the dead batt. and that is the only place current is flowing.

The only part I don't know is if you jump with the prius on or ready will the 12v converter between the big batt and the 12v batt current limit to protect the wiring to the 12v batt in the prius or is there a fuse?

 

Although technically correct, the auxillary battery doesn't have a whole lot of Ah.

This is the circuit in my NHW11 and we've had one report from an NHW20 owner who blew their fusible link:

I suspect the NHW20 has a simular fusing arrangement.

CLARIFICATION AND CORRECTION:

1) If the other vehicle has a weak, partially dead battery, recharging from the Prius is safe. Use this time to turn off all lights, fans and other auxillary loads in the target vehicle.
2) If you can, avoid having them try to 'turn over' while the Prius is connected and running because if their car draws over 100 A from the Prius, the voltage drop risks the fusable link.
3) It is OK, if the Prius is OFF, to have them try to start with auxillary battery connected. As a general rule, lead-acid batteries are fairly forgiving of abuse.

There is likly to be some safety margin for the 100A fusible link. I have put a fairly substantial load, 89A, on the 12 VDC system but the voltage drop, 11.48 V, means you're pretty much at the limit:


The senario I would follow:

1) Make sure target vehicle has some charge, cabin lights at a minimum. If it is completely dead, the risks increase. A dead short target vehicle or fully discharged battery is a different problem that should be handled by heavy duty or specialty equipment.
2) Connect jumper cables - CHECKING AND DOUBLE CHECKING POLARITY - with Prius running to recharge the target vehicle battery and say, "We need to give it a couple of minutes to put a charge on your battery."
3) Survey target vehicle to get all 12 V auxillary loads turned off "to help the battery recharge."
4) If you can, open the battery caps of the target vehicle. If one cell has a substantially different level, that cell is probably bad. They'll be able to get to a parts store for a replacement or otherwise off the street.
5) Say, "Let me get the engine ready, wait for me to call out TRY IT" and then turn OFF your Prius and yell, "TRY IT!"

Their vehicle starts - disconnect cables and have a happy. Don't be afraid to jump start another vehicle, just be deliberate and cautious and everyone will be happy. BTW, simular risks exists for alternator equipped vehicles. If the charging vehicle is idling, the alternator can't generate enough current to risk the alternator electronics but if the charging car reves the engine, it might.

Bob Wilson

 

Lets say that you jump a car with a conventional host vehicle, that being that car A is running and connected car B. You run car A for a while to generate power for the battery in car B, but it takes time to charge a full sized 12v battery. So, you leave the two connected and start car B while car A is on and the alternator is generating excess current. Car B starts and its electrical system takes over the charging function of the battery in car B. You disconnect vehicles.

I am not an electrical engineer, however I understand that if you leave the Prius connected to car B when it starts there is potential for a power spike to travel back to the Prius. That would be where damage could occur to the Prius. Now, I ws searching for the terminology which bwilson provided, but what he described is what I was thinking of when I wrote my first post on this.

But also, trying to start a vehicle with the aux battery of the Prius is a point of concern also. As bwilson points out, there aren't that many amphrs in the aux battery of the Prius. To dry start another car off the aux battery in the Prius may cause significant damage to the Prius battery. And frankly, that is one expensive little battery to be replacing.

However, a 12v car starting pack costs about $50. That is a whole lot cheeper than trying to use the Prius to jump another vehicle and not coming out of the operation unscathed.