How does a lawnmower charge a battery?

I suppose you're referring to a riding mower. As you didn't specify the age, make, etc, this will be an educated guess. If the mower is kept in a controlled environment, such as a garage with heat during the winter months, the battery should hold charge for years. If it is kept in a shed outside, then you'll probably need to charge the mower's battery every spring. All you need is a battery charger with jumper cables. Make sure it is capable of charging you're mower's battery.

The battery is charged by a generator or magneto. It is located under the flywheel.