I guess I was way more tired than I realized last night. I parked the car, unloaded my luggage, then went back out to write down the mpg for the trip. Well, I left the keys in the car all night... with the ignition "ON". Woke up this morning to 4V on my 12V battery, and a car that would not start.
So I rolled by bicycle over, and hooked its tiny little 12V illumination system battery to the Insight's battery cables with 16 gauge wire. Turned the key in the ignition and it fired right up. Then I hooked my Optima Yellow Top back up and took off. I got predictably terrible mileage as I used gasoline to charge my two batteries.
The DC/DC converter runs until the high voltage battery is almost empty, then it thankfully shuts off and allows the 12V battery to deplete itself. To recover from two flat batteries, you need the engine running to recharge the HV battery. But to start the engine, you need a few amps of 12V to power up various computers. But to turn on the DC/DC converter, you need a tiny amount of current to kick over a few relays and whatnot. So that's why a Honda hybrid can be jump started with a 9V battery, or a bicycle.
