A power supply system can include a plurality of battery backup units and a control unit. The battery backup units may be utilized to supply backup energy to a power supply unit, for example, in case of a power outage event. The control unit can gauge a respective capacity, temperature, age, and/or other characteristic of each respective battery backup unit of the plurality of battery backup units. The control unit can designate a particular battery backup unit of the plurality of battery backup units as a spared unit, e.g., to be spared in the power outage event. The designation may be based at least in part on the respective capacity or other characteristic of each respective battery backup unit gauged.