A system for power balance monitoring in an energy storage battery comprising a plurality of energy modules connected in a series-parallel configuration. The energy storage module comprises a plurality of cells. Each module is a three-terminal module. The three terminals comprise a positive output terminal and a negative output terminal for connecting the modules in a series string to a load and an energy sharing terminal for sharing energy between modules is other battery strings. A module power management sub-system comprising a current monitoring circuit is connected to each of the energy sharing terminals. Each module power management sub-system is in communication with a battery power management sub-system so that a weak module can be detected based on module current output. Advantageously, the module power management sub-system is not connected across the main power pathway of the module and so does not appreciably increase module impedance.