Mercury Mariner V-250 V-275 Outboard Service Shop Manual 1990, Page 40Get this manual
Specific gravity of electrolyte varies not only with percentage of acid in liquid but also with temperatureAs temperature increases, electrolyte expands, so that specific gravity is reduced As temperature drops, electrolyte contracts, so that specific gravity increasesUnless these variations in specific gravity are taken into account, speci fic gravity obtained by hydrometer may not give true indication of concentration of acid in electrolyteA fully charged battery will have specific gravity reading of approximately 1.270 at an electrolyte temperature of BO (27 C)If electrolyte temperature is above or below BO F, additions or subtractions must be made in order to obtain hydrometer reading corrected to 80 standardFor every 10 (3.3 C) above BO F, add specific gravity points (.004) to hydrometer readingExample: hydrometer reading of 1.260 at 110 (43 C) wou ld be 1.272 corrected to BOo F, indicating fully charged battery For every 10 below BOo F, subtract paints (.004) from the reading Exampl e: hydrometer reding of 1.272 at (-lBO C) would be 1.240 corrected to BO F, ind icating partially charged batterySPECIFIC GRAVITY CELL COMPARISON TEST This test may be used when an instrumental tester is not avai lableTo perform this test, measure specific gravity of each cellregardless of state of charge, and interp ret results as follows: If specific gravity readings show difference between highest and lowest ce ll of .050 (50 points) or more, battery is defective and should be replaced During service, only distilled water should be added to the batterynol electrolyteLiquid level in cells shou ld never be allowed to drop below top of plates, as portion of plates exposed to air may be permanently damaged with resulting loss of performance