How a drive for donuts helped with a depleted car battery

This morning, rather than drive kids to school, I found myself concentrating on not swearing in front of them. It was apparent that the battery in my car was dead (ok, mostly dead,) and that the car was only going to move when I pushed it out of the parking space to better allow for a jump. After what was really only minor issues in retrospect, one neighbor took the kids to school and another helped jump my car. I was then left with the three-year-old, a running engine and no need to go anywhere, so our next stop turned out to be a donut shop. This baffled my first-grader when she heard about it later in the day. Most things run batteries down when you use them, so why did I think I was helping the battery by driving the car?

Battery needed at the beginning

While moments like this make batteries seem central to car engines, they’re really used that much. A battery’s primary duty is to provide electricity to your starter engine, which then gets your internal combustion engine fired up. Once the combustion engine is running, the starter and battery aren’t really needed to drive anywhere. In some cases, people have found themselves forced to drive relying on their battery, and it usually doesn’t go so well. As big as a car battery may seem, it can be drained surprisingly quickly if you’re running multiple systems on your car at once, like fans, radios and lights. This kind of drain also happens with someone, someone who I still took to a donut shop for some reason, fiddles with dome lights in the car, leaving them on all night.

Continued current

So once a battery has gotten a car started, what’s providing power to the car? The combustion causes pistons to move, which in turn spins your crankshaft. One of the things indirectly spun by the crankshaft is the alternator, which importantly contains a set of three magnets and a mesh of copper wire. As the magnets get spun along the copper, they produce electric current, basically converting the mechanical energy from the moving parts into electricity (and heat). That electricity then powers electrical systems in the car, including topping off the charge in your battery.

Depending on the circumstances  of your battery, ambient temperatures, etc., you can help pull a weak battery back to a state where it will be ready for the next start if you let the alternator have some time to work. A common recommendation is go for a 10 to 20 minute drive, preferably with things like the radio, lights and heater turned off to avoid other demands for electricity. Which finally brings me back to why I was feeding my three-year-old a donut this morning, instead of driving his sister to school.

Source: How Does An Alternator Work? by Matthew Wright, About Autos