A battery energy storage arranged to be connected to a capacitor link, which is connected in parallel to a power converter. The battery energy storage includes a battery module and a direct current energy source connected in series with a voltage source converter. The voltage source converter is adapted to insert a positive voltage when the voltage of the battery module falls below a first threshold value, and to insert a negative voltage when the voltage of the battery module exceeds a second threshold value. The direct current energy source is adapted to be either charged or discharged during the voltage insertion by the voltage source converter. The disclosure also provides a battery energy storage system including such battery energy storage.