It's my understanding that the big battery starts the Clarity, but the 12 v is big enough to jump start another car, heck one person connected an inverter to the Clarity 12 v battery and powered some of his appliances during one of the big storm that hit the east coast. Technically the 12 v battery in the clarity doesn't do anything other than close contacts to "wake up" the car computers. The big battery powers all accessories via the DC-DC converter which converts 300+v to 12v. Honda missed a big opportunity to save weight by not eliminating the 12v battery, or at least replace it with a motorcycle battery. If you're really concerned about hurting your Clarity but want to help out a fellow motorist simply disconnect one of the terminals to isolate the battery from the car and give a jumpstart. Again, that is just my understanding of the Clarity system so if you read it on the internet that you can't do that then it must be true......it's on the internet.