Some recommend testing battery condition by using a hydrometer to measure the specific gravity of battery electrolyte, which is heaver when the battery is fully charged. This only works if you know the specific gravity of the battery when it was new and fully charged, and you let the battery rest with no load for at least a day, and you correct for temperature, and you are confident that your hydrometer is accurate. That said, typical wet cell electrolyte has a specific gravity of 1.265. Differences between cells are more important than actual values. Differences up to 0.030 can sometimes be corrected by equalization but larger differences indicate a bad cell.
Add only distilled water to just cover the plates of a non-VRLA battery. Do not add acid or water that is not distilled. Do not overcharge the battery by applying charging voltage (13.8 v) for a long time. Do not undercharge the battery by failing to apply charging voltage occasionally. Recharge as quickly as possible, preferably the same day. Do not let the battery get hot. Do not let the battery freeze while discharged. Do not let a starting battery fall below 80% of capacity or a deep-cycle battery fall below 20%.
Batteries must be totally immobilized and should have caps that cover the terminals. Keep terminals and connectors clean and shiny to prevent voltage drops. Never connect batteries of
different types together within a bank. Attach as few wires as possible directly to the battery and never more than four. Attach cables with stainless steel hex nuts, rather than the wing nuts that battery makers provide; however, use only copper washers, never steel washers that can get hot and start a fire. Corrosion appears as green or white deposits around battery terminals and is often a sign of overcharging, which is harmful to the battery. Remove corrosion from terminals and from the ends of battery cables by gentle use of a wire brush chucked into an electric drill. Terminal corrosion.