  A repeated interaction process assisted by auxiliary thermal systems charges
a quantum battery. The charging energy is supplied by switching on and off the
interaction between the battery and the thermal systems. The charged state is
an equilibrium state for the repeated interaction process, and the ergotropy
characterizes its charge. The working cycle consists in extracting the
ergotropy and charging the battery again. We discuss the fluctuating efficiency
of the process, among other fluctuating properties. These fluctuations are
dominated by the equilibrium distribution and depend weakly on other process
properties.
