Your cars engine recharges the battery when the engine is running.
A battery can store only so much power. Many batteries (like in your phone) use relatively little power at one time, because your phone doesn’t have moving parts (except maybe a vibration function).
A car battery needs to deliver a huge burst of power all at once- enough to turn your car engine over until the engine fires up and then uses fuel to continue running.
The battery can power your engine itself if you wanted … for about 30 seconds. Then it would be flat.
Once the motor starts, the energy comes from the petrol burning. That energy then partly flows back into the battery to recharge it, ready for next time.
This uses something called an alternator
If your alternator fails, the battery will eventually go flat and you won’t be able to start your car.
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