Things You'll Need
12-volt car battery
Lawnmowers of years gone by relied on a pull-string to start the engine. This, of course, required some arm strength and usually a willingness to pull the cord multiple times before the engine fired. Today, riding lawnmowers generally rely on a battery to start the engine. While this is much simpler and easier on your arm, the battery will occasionally lose its charge, most often during winter storage. When this happens, you don't need to purchase a new battery for your mower. By boosting/jump-starting the battery, you can be out mowing the lawn in no time.
Turn off the lawnmower ignition switch and locate its battery. For most riding mowers, the battery will be found under the seat. The exact location will depend on the make and model of your mower. Remove any guards or terminal covers from the mower's battery.
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Check to ensure that the mower's battery is a 12-volt battery. Most riding mowers will use a 12V battery. Some, however, may utilize a 6-volt battery. If the mower's battery is 6 volts, you cannot boost/jump-start it with a car battery. You will need to connect it to a portable charger and recharge the battery slowly. If the mower has a 12-volt battery, you may proceed with the jump-start.
Drive your car near the riding mower -- just close enough for the jumper cables to reach from the vehicle to the mower. Turn off the car's engine and open its hood. If your car's battery has a cover and/or terminal guards, remove them.
Attach the positive/red clamp on the jumper cable to the positive terminal on the car battery. Connect the other positive/red clamp to the positive terminal on the riding lawnmower.
Attach the negative/black clamp on the jumper cable to the car battery's negative terminal. Connect the remaining negative/black clamp to a metal portion of the lawnmower's frame. This will ensure the cables are properly grounded without the risk of sparks flying.
Turn the ignition switch on the riding lawnmower to start its engine. You should not start the car engine. The car's alternator will produce too many amps for the lawnmower battery. Starting the car may result in irreversible damage to the lawnmower battery. After the lawnmower is running, disconnect the jumper cables in the opposite order you attached them that is, remove the negative clamp from the frame of the lawnmower first, followed by the negative clamp attached to the car battery.