2003+ Suzuki DF9.9/DF15 four stroke outboard motors service manual, Page 44Get this manual
PERIODIC MAINTENANCE
BATTERY SOLUTION LEVEL CHECK The battery solution level should be between the UPPER 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