Always use caution when working on or near batteries. There are explosive gasses produced by batteries such as hydrogen, be careful not to expose any sparks while working on or around batteries
Never allow a tool to contact both the negative and positive terminals at the same time. The negative terminal should be the first terminal to be removed, because if you slip or ground out the wrench it will not spark or cause injury. However if you removed the positive terminal and touch ground it may cause a spark, shock or burn to occur.
Battery electrolyte contains sulfuric acid. If you should splash any on your skin or in your eyes, flush the affected area with plenty of clear water. If it lands in your eyes get medical help immediately