NOVELTY - The system (3) has an electronic device (32) e.g. microcontroller, of a type connected with another electronic device (31) of another type via an electrical connection (30) i.e. line bus, with two electric conductors i.e. wires. The former electronic device is supplied with electrical energy from the latter electronic device via the connection. Messages e.g. charging states of energy storages (21), are simultaneously transmitted from the latter electronic device to the former electronic device and/or from the former electronic device to the latter electronic device via the connection.    USE - Communication system for use in an energy storage device such as an electrochemical energy storage device i.e. battery, with lithium ion cells and nickel-metal-hydride cells, for a motor vehicle (all claimed) e.g. hybrid vehicle and electrical vehicle. Can also be used for an electrostatic energy storage i.e. capacitor.    ADVANTAGE - The electronic device of the type is supplied with electrical energy from another electronic device of another type via the electrical connection, and the messages are simultaneously reliably and robustly transmitted from the latter electronic device to the former electronic device and/or from the former electronic device to the latter electronic device via the electrical connection, thus reducing a number of electrical lines, and hence saving weight of the storage device, and reducing complexity of cabling and cost of the storage device. The system simplifies mounting of the storage device in the motor car, and avoids limitations relative to a spatial arrangement of the electronic devices. The system increases a number of messages transmitted between the electronic devices, improves transmission security between the electronic devices, and reduces probability of communication bottleneck.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (1) an energy storage device    (2) a method for operating a communication system.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic block diagram of an energy storage device with an electrical connection.    Energy storage device (1)    Communication system (3)    Control device (10)    Energy storages (21)    Electrical connection (30)    Electronic devices of types (31, 32)