What was found
(CAN) IN IFR FLIGHT DURING DESCENT, SMOKE APPEARED IN THE COCKPIT. CREW SHUT THE HEATER AND THE MASTER SWITCH, OPENED SIDE WINDOWS TO CLEAR THE SMOKE. AFTER LANDING, THE AME NOTED THAT THE BATTERY RELAY HAD HEATED AND THAT TERMINAL 2 OF THE RELAY HAD MELTED. DETAILED INSPECTION OF THE BATTERY AND ELECTRICAL WIRING WAS DONE. THE RELAY WAS REPLACED.