On the Prius, the traction battery is protected by three safety relays that are operated by the battery ECU. All of the ECUs require 12 VDC to run. However, once enabled, ICE operation is completely handled by the traction battery. The 12 VDC battery has only about 36 AH since all it has to handle are the ECUs and some 12 V. auxillaries.

Bob Wilson

Operation Iraqi Oil Freedom:

Automatic, stock, project car.

My other 1500 cc car:

Automatic, stock, backup car.
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