Mercury Mariner Outboard 225 3 Litre Service Manual 1994, Page 41Get this manual
SPECIFIC GRAVITY CELL COMPARISON TEST This test may be used when an instrumental tester is not availableTo perform this test, measure specific gravity of
each cellregardless of state of charge, and interpret results as follows: If specific gravity readings show difference between highest and towest cell of .050 (50 points) or more, battery is defective and should be replaced
To check battery VOltage while cranking engine with electric starting motor, place RED (+) lead of tester on POS ITIVE (+) battery terminal and BLACK (-) lead of tester on NEGATIVE (-) battery terminalIf the voltage drops below 9-1 2 volls while crankingthe battery is weak and should be recharged or replaced
Winter Storage of Batteries Electrolyte level
Check electrolyte level in battery regularlyA battery in use