The battery inside of your vehicle will lower in its charge when it's being used for things like your radio, air conditioning or the startup of your vehicle. As long as the engine is running at some point in the near future, your battery will be recharged thanks to your alternator. As you drive faster, the battery will charge faster.
There are situations where people draw power from their battery without charging it. Once the battery is dead, you're not going to be able to rely on your alternator for a charge. You'll have to use an…