The "auxiliary" battery doesn't actually start the car. All it does is power up the ECMs and then engage the giant contactor (relay) in the back to connect the high voltage battery to the inverter. One of the two large electric motors, the one that's connected directly to the engine, actually starts it. The auxiliary battery does very little, but they seem to last a very short time. I bought a used 2008, and when viewing the vehicle's history found that the battery had been replaced in 2011, and then in 2014 after it was traded in. I had to replace ours last summer. They don't go bad like a regular battery, they usually still power the car when low and once the system is in a "ready" state, the inverter powers the 12V system, much like a running car's alternator normally does. However, when they start to go bad, they drag fuel economy down with them.