A battery pack is a basic unit of a working battery; the standard voltage is 12V. There are many different capacities, such as 4.5Ah, 5Ah, 7Ah, 9 Ah and high-capacity models, to several hundred Ah. To connect the battery pack in series to get high voltage, and in parallel to get more capacity. Usually, each UPS has a fixed voltage to battery, then it can gain more capacity when connecting more batteries in parallel.