Gas-powered lawn mowers with an electric starter rely on rechargeable storage batteries to supply the electrical energy to start the mower. Over time, these batteries lose their ability to take and hold a charge and must be replaced. The replacement process involves removal of the old battery and installation of the new one.
Park your mower in a level, clear area. Check that any mower starting key is in the off position, then remove the key. Disconnect the spark plug wire. For riding mowers, make sure blades are disengaged, the transmission is in neutral and any parking brake is engaged. Locate the battery compartment of your mower, which normally is under the seat or the hood of a riding mower or at the rear or top of an electric push mower.
Lift the hood or seat on riding mowers to gain access to the battery. Remove any covers, panels or shrouds on riding or push mowers that block access to the battery. On most mower models the covers, panels or shrouds are held in place by clips or by screws that you must undo with a flat-bladed or Phillips screwdriver. On some models the covers, panels or shrouds are held by hex-headed bolts that you undo with the appropriately sized wrench.
Disconnect the black or negative cable from the old battery's negative terminal by undoing the screw or bolt holding the cable to the battery terminal. The negative terminal will be marked with a dash or the words "neg" or "minus." Pull the cable off and away from the terminal. Then disconnect the red or positive cable in the same manner. The positive terminal will be marked with a plus sign or the words "pos" or "plus." If a connector is corroded, put on work gloves and scrub off the corrosion with a stiff wire brush or coarse steel wool pad. If the connector is tight, carefully pry it loose with a flat-blade screwdriver.
Remove any hold-down rod, bar, frame or clamp that secures the battery to the mower. Be sure you wear your work gloves during the battery removal step. On most mower models, battery hold-downs are secured by one or two hex bolts that you undo with a wrench. Move the hold-down out of the way, lift the battery up and out of the battery compartment and set it aside. Many mower batteries have a strap for pulling them out of the mower. Wipe out the battery compartment or tray with an old rag dipped in a 50-50 solution of baking soda and water. Dry the compartment with another old rag.
Put the old battery in a cardboard box or paper bag and take it to a store that sells mower batteries. Ask for a new battery that exactly matches the old one. Double-check the replacement battery to make sure it matches the dimensions, capacity, voltage and terminal placement of the old battery. Lower the new battery into the battery compartment, making sure it is oriented the same way as the old battery. Reinstall the battery hold-down by performing in reverse the steps you took to remove it.
Reconnect the battery cables to the new battery. Connect the red or positive cable to the battery's positive terminal and tighten the nut or bolt securing the cable to the terminal. Then connect the black or negative cable to the negative terminal. Tighten the nut or bolt to secure the cable to the terminal. Start the mower. If it starts in the normal fashion, the battery was installed correctly.