In some examples, a controller may determine, from among a plurality of energy storage units, a first energy storage unit having a higher than median charge and a second energy storage unit having a lower than median charge. The controller may connect the first energy storage unit to a resonant circuit to transfer energy from the first energy storage unit to the resonant circuit. The controller may then disconnect the first energy storage unit from the resonant circuit, and connect the second energy storage unit to the resonant circuit to transfer energy from the resonant circuit to the second energy storage unit to at least partially balance a charge difference between the first energy storage unit and the second energy storage unit.