1993 Johnson Evinrude "ET" 90 degrees CV Service Manual, P/N 508285, Page 230Get this manual
Battery Charging
Boost charge 12-volt batteries at 50 amperes for 20 minutes (1000 ampere minutes)If the charger will not give this rate, charge for an equal number of ampere minutes at best rate availableDO NOT boost battery more than this amount for theIight load test" If batteries are to be fully charged by means of quick charger, the charge rate must be "tapered" (reduced to safe limit) when the electrolyte temperature reaches 125F (52C), or when gassing becomes excessiveFailure to do so may harm the batteryIf the battery is to be slow charged, adjust electrolyte to proper level by adding water, then charge the battery at amperes until fully chargedFull charge of the battery is indicated when all cell gravities do not increase when checked at three intervals of one hour and all cells are gassing freelyPlenty of time must be allowed for slow charging Charge periods of 24 hours or more might be requiredBattery Care