Also used to keep computer/radio/clock memory alive during a battery change.

I suppose if you had long enough time to wait you could recharge a car battery.

Let's see, at 10 amps, 12 volts minus the resistance of the longer wire and you need a minimum of 300 cranking amps at 11.0 volts to turn over the starter........ Yup! About 1.5 tanks of fuel in the charging car should just about do it!