There can be several causes. Need the model and type number of the engine and the model number of the equipment. Simply too many ways to setup equipment out there.
Batteries can self drain due to internal shorts. Even the battery case can become conductive and drain the battery. Auto part stores do not test for conductive battery cases as many don't how to test for this condition.
Depends on the equipment setup draining can occur defective components but we know how the equipment is setup electrically. Even a partially short electric PTO can drain the battery even though the charging system is fine as the PTO over tax it.
StarTech is talking about shorted battery cells and dirt slash acid on top of the battery draining itself down. Top of the battery needs to be clean and dry. Just like you bought it at the store.
