1997+ Mercury 35/40HP 2 Cylinder Outboards Service Manual PN 90-826148R2, Page 79Get this manual
Specific Gravity Readings Use hydrometer (a) to measure specific gravity of electrolyte in each cell
4Hold hydrometer vertically and draw in just enough liquid from battery cell so that float is freefloatingHold hydrometer at eye level so that float is vertical and free of outer tube, then 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 solution