If you leave the key in ignition of a lawnmower will it kill battery?

Yes, leaving the key in the ignition of a lawnmower can kill the battery. Most lawnmowers have a magneto ignition system, which means that the spark plug is powered by a magnet that rotates when the engine is running. When the key is left in the ignition, the magneto continues to rotate, even though the engine is not running, and this can drain the battery. Additionally, some lawnmowers have a battery-powered fuel pump, which will also continue to run if the key is left in the ignition, further draining the battery. To avoid killing the battery, always remove the key from the ignition when you are finished mowing your lawn.