| Just because the battery says 12 volts does not mean that it is charged well enough to turn the motor over.
One way to check the battery, while it is on the mower is to have someone help you, they sit on the mower, and make sure all of the safety switches are like they should be, the seat switch, the clutch, the engaged blade switch. After making sure about these make sure the mower is out of gear, now use a battery jumper cable and go from the hot post on the battery to the large post on the starter, if the battery is good the starter should engage and that would mean that you problem is somewhere else. It could be a loose wire in the safety system or a bad solenoid |