For Symmetra PX 100, it is recommended to replace all four batteries in a row at the same time to ensure optimal runtime. If you are ordering this battery unit for a Symmetra PX 100, you must order battery units in multiples of 2 and you must replace string positions A+B or C+D, as the batteries are connected in pairs. The batteries in each half string must be from the same manufacturer. For Symmetra PX 250/500, it is recommended to replace all six batteries in a row at the same time to ensure optimal runtime. If you are ordering this battery unit for a Symmetra PX 250/500, you must order battery units in multiples of 3 and you must replace the batteries in string position A+B+C or D+E+F, as they are connected in pairs. The batteries in each half string must be from the same manufacturer.