When your lawn mower engine won't even turn over when you turn the ignition key, you can most likely blame it on a dead battery. A lawn mower battery will go dead after sitting for a long period of time, after the battery is shorted out, or after the battery is drained by turning the starter on the engine for too long. You must charge the lawn mower battery with the proper sized battery charger in order to get it correctly charged again. Twelve-volt and 6-volt batteries are the two most common types used on lawn mowers.
Locate the battery on the lawn mower. On push lawn mowers, the battery is usually at the base of the handlebars under a case. On riding lawn mowers, the battery is usually under the rider seat or somewhere centrally located on the riding mower frame.
Determine the dimensions of the battery. The smaller 6-volt batteries are a small rectangle approximately 4 by 6 inches. The bigger 12-volt batteries are a larger rectangle, approximately 8 by 10 inches.
Look at the top of the battery. The label will state if the mower is a 12-volt or 6-volt battery. Over time the label will wear off, though, so this is not always an ideal indicator.
Locate the cell cap on the top of the battery. There will be one cap on 6-volt batteries and 2 caps on 12-volt batteries. Pry them off with a flathead screwdriver to expose the cells filled with distilled water. 6-volt batteries have three cell holes under the cap, while 12-volt batteries have three cell holes under each cap, for a total of six cells.
Charge the battery with the correct voltage level corresponding to the charger, to prevent ruining the battery.