One of the tried and true methods of starting a motorized vehicle with a dead battery is with a jump from a vehicle that has a lot of juice. That typically begs the question of whether this can be done from a vehicle to a mower. While it is always important to understand what you are doing when you are dealing with live electrical currents, this can actually be achieved. The reason for this is most car batteries are 12 volts and as you have seen, so are most in mowers. But, double-check to be certain as they need to have matching voltages. From there, hook up the jumper cables as you would from two vehicles and you should be able to start it.