Q: Why does a car battery stay full and ready even when the car is not in use and the ignition switch is off?

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Already have.

My car has 93000 miles.
My car has an automatic transmission.

A battery uses basically lead and lead acid to cause a chemical reaction that produces the required voltage that a vehicle uses to run the vehicle electrical systems. The amount of material put into the battery when it was made will allow a vehicle battery to store and produce the required voltage needed to sustain a vehicles electrical system for about 3 to 5 years before a replacement battery is needed. Here is a very good explanation of how a battery works with a car’s electrical system.

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