1995 Mariner Mercury Outboards Service Manual 50HP 4-Stroke, Page 40Get this manual
Charging Discharged Battery The following basic rules apply to any battery charging situation: 1Any battery may be charged at any rate (in amperes) as long as spilling of electrolyte (from violent gassing) does not occur and as long as electrolyte temperature does not exceed 1250 (52 C)If spewing of electrolyte occurs, or if electrolyte temperature exceeds 1250 F, charging rate (in amperes) must be reduced or temporarily halted to avoid damage to the battery2Battery is fully charged when, over 2-hour period at low charging rate (in amperes), all cells are gassing freely (not spewing liquid electrolyte)and no change in specific gravity occursFull charge specific gravity is 1.260-1.275, corrected for electrolyte temperature with electrolyte level at 316" (4.8mm) over plateFor most satisfactory charging, lower charging rates in amperes are recommended3If, after prolonged charging, specific gravity of at least 1.230 on all cells cannot be reached, battery is not in optimum condition and will not provide optimum performance; however, it may continue to provide additional service, if it has performed satisfactorily in the past4To check battery voltage while cranking engine with electric starter motor, place RED (+) lead of tester on POSITIVE (+) battery terminal and BLACK (-) lead of tester on NEGATIVE (-) battery terminalIf the voltage drops below 9-12 volts while cranking, the battery is weak and should be recharged or replaced