1998 Mercury 9.9/15HP 4-stroke outboards factory service manual, Page 82Get this manual
CHARGINGSTARTING SYSTEM
10 AMP BATTERY CHARGING COIL 1Check battery voltage at battery with engine running2If battery voltage is above 14.5-15.0 volts, replace voltage rectifierregulatorCheck condition of battery as overcharging may have damaged battery3If battery voltage is below 14.5 volts, charge battery; refer to "CHARGING DISCHARGED BATTERY"If battery can NOT be satisfactorily charged, replace battery4If battery accepts satisfactory charge, check battery voltage while cranking engine; refer to "CHARGING DISCHARGED BATTERY"If cranking voltage is not acceptable, replace battery5If cranking voltage is acceptable, disconnect RED (fused) lead bullet connector between the positive (+) starter solenoid terminal and Red lead from rectifierregulator6Connect RED (+) ammeter lead to RED lead from fuse, and the BLACK () ammeter lead to the bullet from POSITIVE terminal from rectifierregulator