Lawn mower batteries, like all other batteries, last between three and five years. If your battery is older than that and your lawn mower doesn't start, the battery is the first place you'll want to look to as the cause of the problem. Most lawn mowers run on 12-volt batteries that are specifically designed for lawn mowers. You can find the battery under the hood of the lawn mower or, in some models, under the seat. Hooking up a new lawn mower battery is easy but will vary from model to model.
Locate the battery. It is usually near the top of the engine under the hood or under the seat.
Loosen the negative battery cable clamp with your adjustable wrench. The negative cable will be marked with a "minus" sign and may have a black cable. Set aside the cable so that it can't accidentally fall back onto the battery.
Loosen the positive battery cable clamp with your adjustable wrench. It will be marked with a "plus" sign and may have a red cable. Set this cable aside.
Loosen the straps or clamps that hold the battery in place. Remove the battery by pulling it straight up and out from its compartment. If there is corrosion on the battery terminals, wear gloves to handle the old battery.
Place the new battery into the tray. Ensure it is oriented in the same way that the old one came out so that the positive and negative terminals are in the same position. Clamp down the battery.
Place the positive battery cable on the positive terminal and tighten it with your adjustable wrench. Repeat this process for the negative cable. Terminal grease can be added to the terminal ends before attaching the clamps.