If you give a jump to someone's car, or receive one, here's a useful tip. If the car doesn't start, turn on the headlights. If they don't come on, you don't have a good connection. Adjust the cables and it should work. (Assumes it is indeed the battery...)
It was also helpful with a car with a weak battery. The lights came on, but dimly. When the connection was corrected, the lights were bright.
I apologize if this is common knowledge - I only learned of it recently. (Too soon old; too late wise)