When your power lawn mower will no longer start on its own or hold a charge, the voltage regulator may be damaged. Every power lawn mower runs off a 12-volt charging system. In order for the battery to stay charged, a voltage regulator must create a higher voltage to send back to the battery to make it operational. Testing your mower's charging system should take you about 10 minutes to complete.
Park the lawn mower on a hard, level surface and set the parking brake.
Open the hood of the lawn mower and locate the battery behind the engine. Turn the key clockwise until the mower headlights come on for 15 seconds, which will partially drain the battery.
Start the engine. Turn the voltmeter on and set it to DC current. Place the positive (red) lead from the voltmeter on the positive battery post, and place the negative (black) lead on the negative battery post.
Check the reading on the voltmeter; it should be between 13.0 and 15.0 volts. Turn the engine off and check the reading; it should be between 11.2 and 12.0 volts.