The owners' manual says don't do it.
On Sun, 30 Oct 2005 08:36:35 -0700, Chris Shearer Cooper wrote:
You can sometimes use the Prius to jump-start another vehicle, but you
should do it somewhat differently than the normal way. The Prius 12-volt
system is protected by a 100-ampere fuse. Starter current through
heavy-gauge jumper cables can exceed this, blowing the fuse and rendering
the Prius 12-volt system inoperative. You can safely use the Prius
12-volt battery and inverter system to charge the other car's battery but
not to crank the starter motor.
The recommended way is to connect the cables with the Prius in READY mode,
but do NOT crank the starter of the other car. Allow the Prius to charge
the dead battery for a few minutes, then disconnect the cables and crank
the other vehicle with the cables disconnected. This may not always work
if the battery of the other car is in very poor condition or deeply
Going the other way, if you drain the 12-volt Prius battery, you can
jump-start it with very little current. Even a 12-volt lantern battery or
a pack of eight D-cells will do it. You don't need to "crank" the Prius
to get it up and running. You just need eneough electricity to boot the
computers and energize the high voltage relay (the double-click you hear
when the car goes READY).
No problem, but the phone won't charge when the car is off.