Most dead lawn mower batteries will start with a boost from a standard 12-volt car battery. Before you attempt such a task, take proper safety precautions. Battery acid is dangerous. The mixture of sulfuric acid and water will burn skin and corrode clothing. Also, ensure that your mower battery is the same amperage as the automobile battery. Do not attempt to jump-start the mower battery if the two are different.
Set the baking soda within reach of your working area. Put on your gloves and eye protection.
Turn the ignition key. If the mower won't crank, open the battery compartment and make sure the terminals are connected securely. Clean any corrosive material from the terminals with a stiff brush.
Start the car and drive to within a few feet of the lawn mower. Turn the engine off. Open the hood of the car and remove the battery terminal covers, if present. Make sure that the lawn mower ignition is in the off position.
Connect one end of the positive (red) jumper cable to the car battery. Squeeze to open the clamp and attach it securely to the positive (+) terminal. Connect the other positive (red) end of the cable to the mower battery positive (+) terminal.
Connect one end of the negative (black) cable to the negative (-) car battery terminal. Connect the other end of the negative (black) cable to the mower engine block or other metal part of the mower frame, away from the battery.
Start the car. Allow the car engine to run for about 60 seconds. Turn the mower ignition key to start the engine. If the mower does not start, wait a few minutes and try again.
Remove the cables in the reverse order that you connected them. Take care not to let the cable clamps touch. Let the mower run for at least 30 minutes to charge the battery.