Mercury Optimax Models 135, 150, Direct Fuel Injection., Page 78Get this manual
CHARGINGSTARTING SYSTEM
2When adding distilled water to battery, be extremely careful not to fill more than 316 in(4.8 mm) above perforated baffles inside batteryBattery solution or electrolyte expands from heat caused by chargingOverfilling battery will cause electrolyte to overflow (if filled beyond 316 above baffles)3Grease terminal bolts well with 2-4-C Marine Lubricant and store battery in COOLDRY placeRemove battery from storage every 30-45 days, check water level and put on charge for or hours at amperesDO NOT FAST CHARGE4If specific gravity drops below 1.240, check battery for reason and 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 storage, for best possible maintenance during inactive periods to ensure good serviceable battery in springWhen 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 reinstall battery