Power transfer system with multiple energy storage modules

A power transfer system includes a series of energy storage modules (ESMs) or energy storage devices (ESDs) that are coupled together to be able to transfer power between one another, as well as receive power from a power source, such as an onshore power generator. The energy storage modules may be hybrid energy storage modules, each including an electrical-machine-inertial energy store and an electro-chemical energy store. The energy storage modules are configured to receive constant-current DC or AC input from the power source, and are able to provide constant-current and constant-voltage output, either sequentially or simultaneously. The power transfer system allows the modules to operate independently or in conjunction with one another, should some of the connections of the system be broken. The energy storage modules may be used to provide power to underwater systems, for example sonar systems, weapons systems, or underwater vehicles.