Hybrid & Electric Vehicle "double feeds" of 12-volt electricity
The most important issue for responders right now to understand hybrid vehicles and safely deal with them at an incident scene is realizing that hybrids and electric plug-in vehicles actually are designed with two electrical sources that feed their 12-volt electrical system. There is a lot of confusion about this reality. We know they have two batteries and we know they have a high voltage as well as a 12-volt electrical system. "Double 12-volt feeds" has always been the design; it's that this fact is just now showing up on our radar screen.
Hybrids, plug-in hybrids, and electric plug-in vehicles all have two electrical systems; a 12-volt system and a high-voltage system. That is pretty much common knowledge among responders. You have all read the recommended procedures for dealing with a hybrid or electric at an incident; including shut off the ignition and shutting down 12-volt power at the battery. Maybe it was not emphasized to you how important it is that the ignition be turned off FIRST and then the 12-volt battery be worked with.
What is not so well known is that the vehicle's 12-volt accessories such as lights, horn, radio, etc may not shut down at a crash scene when you disconnect or cut the 12-volt battery cables. Several years ago, responders in Philadelphia found this out first-hand. They were unable to access the interior so by forcing the rear hatchback, they cut the 12-volt battery cables. To their surprise, the 12-volt electrical system on the Prius remained fully energized!
Hybrids and electric vehicles also feed power from their high voltage battery through a DC to DC converter to actually supply 12-volt power to the vehicle as well. This is how the 12-volt battery keeps it's charge for example.
What this design means to responders is that at a crash scene, you need to make every effort to turn OFF the ignition BEFORE you disconnect or cut the 12-volt battery cables. If you cut the cables and have not shut off the ignition first, you'll be surprised to find that all 12-volt accessories throughout the vehicle will still be fully energized.
Fire & rescue personnel from Germany have documented this situation in a short video clip. The Lexus RX vehicle is shown energized initially and then cables are removed from the 12-volt battery in the rear area. As the camera pans back to the vehicle, you see that all 12-volt electrical accessories on the hybrid remain fully energized.
Page 15 of the Pruis V Emergency Response Guide shows two diagrams that clearly depict this 'double feed' of 12-volt power. Study this issue and know what to do next time you respond to a hybrid or EV incident.
Screen capture (below) from Prius V ERG used courtesy of Toyota Motors.