Riding lawn mowers have a construction similar to that of automobiles: a gas-powered motor that has a battery-powered electric ignition. Like a car, if this battery dies, you cannot start the engine. However, you can charge or "jump-start" a lawn mower battery just as you can a car battery.
Locate the negative terminal on the lawnmower battery. Terminals are the places on the batteries where the cables connect. The negative terminal is black and usually has a "minus" (-) sign by it, while the positive terminal is red and has a "plus" (+) sign by it. Disconnect the negative contact on the lawn mower battery. This may require a screwdriver or pliers, depending on the model. Do not let the cable touch anything.
Connect the lawn mower battery and car battery with your jumper cables. Connect a positive (red) clamp to the positive lawn mower battery terminal. To open a jumper cable clamp, squeeze it as you would with scissors or tongs. Springs in them will make them latch on to the battery terminals. Connect a positive clamp to the positive car battery terminal. Connect a negative clamp to the negative (black) lawn mower battery terminal. Connect a negative clamp to the frame of the car.
Turn the car on, but keep it in neutral. Press the accelerator pedal. Let it run at around 2000 RPM for about two minutes.
Disconnect the jumper cables. Replace the negative battery contact on the lawn mower. Start the lawn mower.