In spinning reserve mode, the battery is connected to the power grid and will act as a reserve power source. Spinning reserve mode is by default operating in idle mode with no power delivered or consumed by the battery. If other power sources on the power grid fail, e.g. a generator stops working, the batteries will instantly provide energy to meet the power demand. Spinning reserve is useful for DP vessels where closed bus operations are allowed. A battery bank as a spinning reserve, instead of a running generator on low load, reduces both fuel usage and machinery wear.