Mercury Mariner 25HP 4-Stroke Outboard Service Manual 1997, Page 75Get this manual
4Hold hydrometer vertically and draw in just enough liquid from battery cell so that float is freettoating Hold hydrometer at eye level so that float is vertical and free of outer tubethen take reading at surface of liquidDisregard curvature where liquid rises against float stem due to capillary action5Avoid dropping electrolyte on boat or clothing, as it is extremely corrosiveWash off immediately with baking soda solutionSpecific gravity of electrolyte vari es not only with percentage of acid in liquid but also with temperatureAs temperature drops, electrolyte contractsso that specific gravity increasesUnless these variations in specific gravity are taken into account, specific 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 800 (2]0 C)If electrolyte temperature is above or below 80 F, additions or subtractions must be made in order to obtain hydrometer reading corrected to 800 standardFor every 100 (3.30 C) above BO" F, add specific gravity points (.004) to hydrometer readingExample: hydrometer reading of 1.260 at 110" (43" C) would be 1.272 corrected to BO" F, indicating fully charged batteryFor every 100 below 80 F, subtract points (.004) from the readingExample: hydrometer reading of 1.272 at (-18 C) would be 1.240 corrected to 800 F, indicating partially charged batterySPECIFIC GRAVITY CELL COMPARISON TEST This test may be used when an instrumental tester is not availableTo perform this test, measure specific gravity of each cell, regardless of state of chargeand interpret results as follows: If specific gravity readings show difference between highest and lowest cell of .050 (50 points) or more, battery is defective and should be replaced Electrolyte Level