1996-2005 Suzuki DF40, DF50 Four Stroke Outboard Service Manual, Page 46Get this manual
PERIODIC MAINTENANCE 2-20 BATTERY SOLUTION LEVEL CHECK Battery solution level should be between the UPPER level and LOWER levelIf the level is low, add distilled water only Once the battery has been initially serviced, NEVER add diluted sulfuric acid, or you will damage the batteryFollow the battery manufacture's instructions for specific maintenance procedures
BATTERY SOLUTION GRAVITY CHECK Measure the gravity of battery solution using hydrometerBattery solution gravity1.28 at 20C 09900-28403Hydrometer