Mercury Optimax Diagnostic Guide., Page 12Get this manual
OPTIMAX DIAGNOSTIC GUIDE
The following basic rules apply to any battery charging situation: 1Any battery may be charged at any rate (in amperes) or as long as spewing of electrolyte (from violent gassing) does not occur and for as long as electrolyte temperature does not exceed 52 (125 F)If spewing of electrolyte occurs, or if electrolyte temperature exceeds 52 (125 F), charging rate (in amperes) must be reduced or temporarily halted to avoid damage to the battery2A battery 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 hydrometer reading is 1.260 S.G 1.275 S.G corrected for electrolyte temperature with electrolyte level at mm (316 in over the plates, unless electrolyte loss has occurred (from age or over-filling) in which case the specific gravity reading will be lowerFor most satisfactory charging, lower charging rates in amperes are recommended3If, after prolonged charging, hydrometer reading of at least 1.230 S.Gon all cells cannot be reached, the battery is not in optimum condition and will not provide optimum performanceHowever, it may continue to provide additional service if it has performed satisfactorily in the past4To check battery voltage while cranking the engine with the electric starting motor, place the RED (+) lead of the tester on the POSITIVE (+) battery terminal and the BLACK () lead of the tester on the NEGATIVE () battery terminalIf the voltage drops below 9.5 volts while cranking, the battery is weak and should be recharged or replacedIf standard (original) battery cables are replaced with longer cables, the wire gauge size must increaseSee the chart below for the correct wire gauge size