1987-1993 Mercury Mariner Outboards 70/75/80/90/100/115HP 3 and 4-cylinder Factory Service Manual, Page 33Get this manual
Specific Gravity Readings Use hydrometer to measure specific gravity of electrolyte in each cell
5Avoid dropping electrolyte on boat or clothing, as it is extremely corrosiveWash off immediately with baking soda solutionSpecific gravity of electrolyte varies not only with percentage of acid in liquid but also with temperatureAs temperature drops, electrolyte contracts, so 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 80_ (27_ C)If electrolyte temperature is above or below 80_ F, additions or subtractions must be made in order to obtain hydrometer reading corrected to 80_ standardFor every 10_ (3.3_ C) above 80_ F, add specific gravity points (.004) to hydrometer readingExample: hydrometer reading of 1.260 at 110_ (43_ C) would be 1.272 corrected to 80_ F, indicating fully charged batteryFor every 10_ below 80_ F, subtract points (.004) from the readingExample: hydrometer reading of 1.272 at 0_ (18_ C) would be 1.240 corrected to 80_ F, indicating partially charged battery