I looked into this for racing , running without an alternator and using mechanical alky injection, so there is no fuel pump draw as well as no cooling fan draw. With what I need to run the ignition, water pump and restart to move up in line, a small Optima is as small as I can get away with and make two rounds. After that the percent of charge is too low for another run. Those two runs are consistent, by the way. Figure you need twice the reserve for actual good function in an auto. Below about 50 percent charge and the car may/probably will go below minimum ignition voltage requirements just to get it started. I decided it wasn't worth considering after looking at the numbers for these small batteries and knowing what I need.