EMPLOYEE STATED THAT HE WAS AT THE NO.1 BATTERY CHARGING STATION CHANGING THE BATTERY ON THE NO.1 COALHAULER. AS HE STEPPED OVER THE BATTERY LIFT ARM TO DISCONNECT THE JUMPER CABLE AND CONNECT THE COALHAULER CABLE TO PLUG TO THE FRESH BATTERY, HIS FOOT SLIPPED AND HE FELT PAIN IN HIS LEFT KNEE.