Yamaha F6A Owner's Manual page 107

1) Check the electrolyte level at least once
a month. Fill to the manufacturer's rec-
ommended level when necessary. Top
up only with distilled water (or pure de-
ionized water suitable to use in batter-
ies).
2) Keep the battery always in a good state
of charge. Installing a voltmeter will help
you monitor your battery. If you will not
use the boat for a month or more,
remove the battery from the boat and
store it in a cool, dark place. Completely
recharge the battery before using it.
3) If the battery will be stored for longer
than a month, check the specific gravity
of the fluid at least once a month and
recharge the battery when it is low.
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