Storage batteries must be handled with care. They contain sulfuric acid (the electrolyte) and fragile lead-antimony or lead-calcium plates. Rough handling can damage the case, made of plastic or hard rubber.
The electrolyte in a battery is sulfuric acid. It is strong when the battery is charged and weak when the battery is discharged. The acid can cause severe tissue damage since it is approximately 36 percent sulfuric acid and 64 percent water. It will eat holes in clothing, burn skin, and cause blindness. Even in the discharged state, at 12 percent acid, it can cause burns. Tipping the battery should be avoided since it may cause the acid to spill, with possible injury or damage. Acid must be respected.
Lead-acid storage batteries produce hydrogen and oxygen gases when they are charging and discharging. Hydrogen mixed with oxygen is very explosive and can be ignited by a spark or a flame. This may explode the battery case. Always use an electric light to check the electrolyte level, never a match; it could ignite any gases present.
Remove and attach battery cables in the right order. The ground cable should be disconnected first and connected last. If a wrench were to slip while you are working on an ungrounded connection it could complete a circuit with part of the vehicle, produce a spark and ignite any hydrogen gas around the battery. Do not work on a battery while the vehicle's engine is running. Current may be flowing in or out of the battery, increasing the chance of a spark.
Use a battery tester to determine the level of charge. Never short across the posts of a battery to determine its level of charge. Even a heavily discharged battery can produce a spark that could ignite hydrogen gas around the battery and cause an explosion. Connecting the two posts of a battery with a short wire may also cause burns to the hands.