While the engine supplies the horsepower that propels an ATV, most of the accessories that you might want to use with an ATV run off the battery. Whether you're using the headlights, a winch or some other kind of attachment, you need a battery that can supply the necessary power. The battery also powers the electric starter, so you need it just to get going.
ATVs use lead acid batteries; you can use the conventional (the kind you have to top off with distilled water) or AGM (absorbed glass mat) varieties. AGM batteries generally cost more, but provide more power and require less maintenance. ATV batteries are designed to be good at providing a brief shot of power, such as when the ATV starts or when using the winch for a few minutes. Long-term use, like leaving the headlights on without the engine running, can drain the battery.
The battery is usually charged by the ATV's charging system. Whenever the engine is running, the battery is being charged -- as long as you're not doing something that drains the battery faster than it can be charged. It's important to make sure the battery is charged up before you shut the ATV down and put it away. Without a full charge, sulfates can build up inside the battery and reduce its efficiency.
ATV batteries can also be charged by an external charger. It's recommended that you use a charger designed for "powersport" batteries. There are crucial differences between ATV batteries and automotive batteries, so using a car battery charger isn't the best choice.
Up next: ATVs that run on battery power alone.