The battery in your gasoline-powered lawnmower serves the same purposes as your car’s battery. It supplies the initial current to operate the starter motor that starts the engine. After three to six years of use, the battery in your lawnmower will fail and must be changed, just like the battery in your car. The process for changing the battery in all types of riding or walk-behind mowers is similar to replacing the battery in a car.
Locate the battery compartment of your lawnmower, which normally is under the hood or under the seat of a riding mower, or at the rear of a walk-behind mower, near the handle. Don safety goggles and rubber gloves. Wear old clothing. Remove the cover of the battery compartment, if applicable.
Remove the battery hold-down bracket, if your mower has one, by undoing the screw or nut securing the bracket with a screwdriver or wrench. Push the bracket aside or lift it up and out.
Disconnect the negative battery terminal first. It is the one with the black cable, but the battery itself also is marked to indicate the polarity of each post. Put the adjustable, socket or open-end wrench on the nut or bolt securing the negative battery cable terminal to the negative battery post, and turn it counterclockwise to loosen the terminal. Pull the black cable up and off the battery, and move it aside so the terminal end doesn’t contact the battery or any metal part of the mower. If the terminal is stubborn about disconnecting, carefully pry it off with the flat-bladed screwdriver. Loosen and remove the positive terminal in the same manner. The cable to the positive terminal is red.
Remove the battery from the battery compartment by lifting it up and out. Make a note of how the battery was oriented in the battery compartment. Wrap the old battery in paper or plastic and take it to a store that sells mower batteries. Ask for an exact replacement battery with the terminal posts oriented in the same way. Ask the store to dispose of the old battery; most will do so. Bring the new battery to your mower worksite.
Dissolve a teaspoon of baking soda in a cup of water, using a disposable cup. Dip an old toothbrush in the solution and scrub acidic corrosion from the battery terminals. Acidic corrosion is a grayish-white deposit on the terminal. The baking soda solution may fizz; this is normal. Scrape stubborn corrosion from the outside and inside of the terminals with the pocket knife or wire brush. If you have a battery terminal cleaner, insert the pointed end into the hole of the cable terminal and turn the cleaner in a circle until the sides of the hole are shiny.
Dip an old towel in baking soda solution and wipe out the inside of the battery compartment to remove dirt and acid traces. Dip an old towel in clear water and wipe the battery compartment again. Allow to dry.
Place the replacement battery in the battery compartment, checking that it is oriented the same way as the old battery was. Dip a finger into petroleum jelly, and wipe it liberally on the new battery’s terminal posts to help prevent corrosion.
Attach the positive terminal at the end of the red cable to the positive terminal post of the new battery, and tighten the nut or bolt that secures the terminal to the battery by turning it clockwise. The battery’s positive terminal post will be marked with the word “pos” or a plus sign.
Attach the negative terminal at the end of the black cable to the negative terminal post of the battery, and tighten in the same manner. The negative terminal post on the battery will be marked with the word “neg” or a minus sign.
Reinstall any battery hold-down bracket and/or battery compartment cover.