1997+ Mercury 35/40HP 2 Cylinder Outboards Service Manual PN 90-826148R2, Page 80Get this manual
Electrolyte Level Check electrolyte level in battery regularlyA battery in use in hot weather should be checked more frequently because of more rapid loss of waterIf electrolyte level is found to be low, then distilled water should be added to each cell until liquid level rises approximately 316" (4.8mm) over plateDO NOT OVERFILL, because this will cause loss of electrolyte and result in poor performance, short life and excessive corrosion
Winter Storage of Batteries
Battery companies are not responsible for battery damage, either in winter storage or in dealer stock, if the following instructions are not observed: 1Remove battery from its installation as soon as possible and remove all grease, sulfate and dirt from top surface by running water over top of batteryBe sure, however, that vent caps are tight beforehand, and blow off all excess water thoroughly with compressed airCheck water level, making sure that plates are covered2When adding distilled water to battery, be extremely careful not to fill more than 316" (4.8mm) over plate inside batteryBattery solution or electrolyte expands from heat caused by chargingOverfilling battery will cause electrolyte to overflow (if filled beyond 316" over plate)3Grease terminal bolts well with Quicksilver 2-4-C Marine Lubricant, and store battery in COOLDRY placeRemove battery from storage every 30-45 days, check water level (add water if necessary), and put on charge for or hours at amperesDO NOT FAST CHARGE4If specific gravity drops below 1.240, check battery for reason, and then rechargeWhen gravity reaches 1.260, discontinue chargingTo check specific gravity, use hydrometer, which can be purchased locally5Repeat preceding charging procedure every 30-45 days, as long as battery is in storageWhen ready to place battery back in service, remove excess grease from terminals (a small amount is desirable on terminals at all times), recharge again as necessary and re-install battery