Reese, with the battery disconnected look under machine to see if anything is wrapped around the blade and shaft that could be working the motor hard, electric mowers normally can only cut half the height of grass as a gas mower. Make sure wheels are not set too low and overworking motor.
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It may be overheating from the sun? Are outdoors in the heat? I've had battery powered hand tools burn up in direct sunlight. Lithium batteries can get pretty hot.