1986+ Mercury 6/8/9.9/10/15HP 2-stroke Factory Service Manual, Page 70Get this manual
CHARGING AND STARTING SYSTEM
Battery Charging System Troubleshooting
A fault in the battery charging system usually will cause the battery to become underchargedCheck battery electrolyte level, and charge batterySee Electrolyte Level, and Charging Discharged BatteryIf battery will NOT accept satisfactory charge, replace batteryIf battery accepts satisfactory charge, determine the cause of the charging system problem as follows1Check for correct battery polarity [RED cable to POSITIVE (+) battery terminal]If polarity was incorrect, check for damaged rectifierRefer to RECTIFIER, RECTIFIERREGULATOR TEST2Check for loose or corroded battery connections3Visually inspect wiring between stator and battery for cuts, chafing; and disconnected, loose or corroded connection4Excessive electrical load (too many accessories) will cause battery to run downIf visual inspection determines that battery connections and wiring are OK, perform the following stator and rectifier tests