[Ptero]

Use a load tester. It's a metal box with a resistance circuit. You hook it up to your battery terminals and close the circuit. That enables a serious draw across a heater element. The needle on the gauge of the load tester will slowly drop as your battery charge falls off over ten seconds. If your battery cannot deliver a sufficient flow of electrons, the instrument will indicate that your battery is in need of replacement.

A tired battery will register 12.5 volts - the same as a strong battery - but it will fall off more quickly under load. A tired battery will also register 13-15 volts under charge from a healthy alternator but it will make the alternator work harder because as batteries age, sulfate flakes off the plates and creates a growing internal resistance. Older batteries do not stand up to the vibration of mobile applications as well as new batteries. I replace my mobile batteries after 3 years at most and use them for stationary applications if they are still strong.

BTW, when you coast engine off, if you are not on the accessory position but rather return to the ignition position to run accessory electrical systems after the engine quits, you are also energizing your alternator and coil, which is a boo-boo. A bigger boo-boo would be to accidently lock your steering column while futzing around.