The car is always charging when you are running the car. Otherwise you can't start the car the following time. It will only discharge if (1) you have too many accessories that draws too much current and the alternator can't keep up with charging the battery, (2) you charge or use the battery while the engine is not running, (3) the alternator (which charges the battery) is busted,, (4) the battery reached its end of life and will not charge despite your alternator working.

Hth

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