An apparatus includes power blocks. Each power block includes converter modules. Each converter module includes a positive and a negative bidirectional converter and a battery module. The bidirectional converters are connected to the battery module and outputs are connected in parallel. Paralleled positive bidirectional converters are connected in series between a positive connection and a neutral connection and the paralleled negative bidirectional converters of each power block are connected in series between the neutral connection and a negative connection. A DC-link controller controls a positive output voltage between the positive and neutral connections to follow a positive voltage reference and controls a negative output voltage between the neutral and negative connections to follow a negative voltage reference. A state-of-charge controller controls a ratio of power supplied by each power block and a ratio of power supplied by each converter module based on a target state-of-charge for each battery module.