The battery in your car is there to serve one purpose: to provide enough amperage to start the engine. After the engine is running, and the alternator is spinning, the battery acts as a load. If you add a second battery, it’s going to act as a second load when the engine is running because the alternator keeps both batteries charged.
One battery is good, so two must be better, right? There are a few situations where that’s the case. When the engine isn’t running, any accessories you turn on pull current directly from the battery. That’s why you’ll come back to a dead battery if you accidentally leave the headlights on overnight. If you add a bigger battery or a second battery, you end up with extra reserve power.
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