That might seem counterintuitive, but the reasoning is simple.
Thebattery in your caris 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.
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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.
If you take your vehicle camping, that’s a good example.
If you add a second battery, the time will go up.
In many cases, a battery will show a reserve capacity in terms of minutes rather than amp hours.
In reality, the real number will differ depending on the ambient temperature and the condition of the battery.
In other terms, an electrical load is anything that draws current.
Your accessoriesfrom the headlights to your car stereoare loads, and so is the battery.
If the second battery is discharged to any great degree, it may overtax the alternator.
