My friend and I put amps and a computer into his Prius, without any problems. The current to the 12v system is provided by a 100 amp DC-DC converter so that the engine does not need to run in order for the accessories to stay powered and the battery charged, like in a normal car. This eventually uses up power from the traction battery, but regen and charging puts power back into the traction battery.
See
http://www.greenhybrid.com/discuss/i...rius.2558.html