It will not do any damage to the mower to jumpstart it with a car battery. The lawnmower will only draw enough energy from the car battery to start it. There is no need to have the car running as its battery is far more powerful than a lawnmower battery. Dead batteries do produce larger quantities of hydrogen gas than a charged battery, therefore a greater chance of an explosion does exist than with a fully charge battery. The proper procedure for jumpstarting is as follows - 1. Ensure the mower is not touching the car in any way. - 2. Connect the positive clamp to the positive terminal of the dead battery. - 3. Connect the other positive clamp to the positive terminal of the live battery. - 4. Connect the negative clamp to the negative terminal of the live battery. - 5. Finally, connect the other negative clamp to a good solid grounding point of the mower. This could be a lifting eye of the engine or a convenient spot on the frame, anywhere a good ground can be established without being damaged by any moving parts once the engine cranks. The reason why you make this last connection away from the battery is due to the hydrogen gas being released by the dead battery and the spark that will most likely occur when you make that last connection.
