HA11 is an energy transfer battery equalizer with voltage display on mobile phone, which can be used for 12V batteries in series to keep battery bank voltage balance. When battery banks are in charging/discharging/free status, batteries voltage will have difference, which may cause one of batteries fail prematurely. HA11 will start to work when it detect there are 20mV voltage difference between two batteries. It will transfer energy from the higher voltage battery to the lower one until same