An apparatus, comprising: an input circuit configured to generate a sinusoidal input current; transformers (1225, 1223) including primary windings (1224, 1235) configured to receive the input current, the primary windings (1224, 1235) comprising first and second primary windings coupled together in series such that the input current flows through the first and second primary windings (1124,1235) at the same time, and secondary windings (1226, 1236) configured to charge battery cells that are coupled together in series, each of the secondary windings being configured to simultaneously generate, in response to the input current, a respective charge-balancing current flowing through a respective at least one of the battery cells such that the respective charge-balancing current has a magnitude that is related to a voltage across the respective at least one of the battery cells, the secondary windings (1226, 1236) comprising first and second sets of secondary windings, each secondary winding of the first and second sets of secondary windings being configured to simultaneously generate the respective charge-balancing current in response to the input current flowing through the first and second primary windings, respectively, at the same time, characterized in that the input circuit includes a low-side switch in parallel with a power supply and includes an inductor and a capacitor in series with the primary windings (1224, 1235), the capacitor being in series between the inductor and the primary windings (1224, 1235) and the inductor being in series between the low-side switch and the capacitor, and wherein the input circuit is configured to generate the input current having a frequency that is approximately equal to a resonant frequency of a series combination of the inductor and capacitor. an input circuit configured to generate a sinusoidal input current; transformers (1225, 1223) including primary windings (1224, 1235) configured to receive the input current, the primary windings (1224, 1235) comprising first and second primary windings coupled together in series such that the input current flows through the first and second primary windings (1124,1235) at the same time, and secondary windings (1226, 1236) configured to charge battery cells that are coupled together in series, each of the secondary windings being configured to simultaneously generate, in response to the input current, a respective charge-balancing current flowing through a respective at least one of the battery cells such that the respective charge-balancing current has a magnitude that is related to a voltage across the respective at least one of the battery cells, the secondary windings (1226, 1236) comprising first and second sets of secondary windings, each secondary winding of the first and second sets of secondary windings being configured to simultaneously generate the respective charge-balancing current in response to the input current flowing through the first and second primary windings, respectively, at the same time, characterized in that the input circuit includes a low-side switch in parallel with a power supply and includes an inductor and a capacitor in series with the primary windings (1224, 1235), the capacitor being in series between the inductor and the primary windings (1224, 1235) and the inductor being in series between the low-side switch and the capacitor, and wherein the input circuit is configured to generate the input current having a frequency that is approximately equal to a resonant frequency of a series combination of the inductor and capacitor. primary windings (1224, 1235) configured to receive the input current, the primary windings (1224, 1235) comprising first and second primary windings coupled together in series such that the input current flows through the first and second primary windings (1124,1235) at the same time, and secondary windings (1226, 1236) configured to charge battery cells that are coupled together in series, each of the secondary windings being configured to simultaneously generate, in response to the input current, a respective charge-balancing current flowing through a respective at least one of the battery cells such that the respective charge-balancing current has a magnitude that is related to a voltage across the respective at least one of the battery cells, the secondary windings (1226, 1236) comprising first and second sets of secondary windings, each secondary winding of the first and second sets of secondary windings being configured to simultaneously generate the respective charge-balancing current in response to the input current flowing through the first and second primary windings, respectively, at the same time, characterized in that the input circuit includes a low-side switch in parallel with a power supply and includes an inductor and a capacitor in series with the primary windings (1224, 1235), the capacitor being in series between the inductor and the primary windings (1224, 1235) and the inductor being in series between the low-side switch and the capacitor, and wherein the input circuit is configured to generate the input current having a frequency that is approximately equal to a resonant frequency of a series combination of the inductor and capacitor.