A battery operated vacuum cleaner is provided with one or more principal batteries and one or more supplemental batteries. The batteries and a controller are configured such that as the power provided by the principal batteries drops, one or more of the supplemental batteries is operative connected to provide power to the appliance. A method for providing a substantially constant level of power to an appliance, such as a vacuum cleaner, using a plurality of power sources comprises providing power from a principal power source connected to the appliance; monitoring an operating voltage supplied to the appliance to detect if the operating voltage is below a predetermined threshold voltage level; and upon detecting that the operating voltage is below the predetermined threshold voltage level, providing power from k of n supplemental power sources connected to the appliance, where k and n are positive integers, and k is less than or equal to n. Optionally, upon detecting that the operating voltage is below the predetermined threshold voltage level and where k is equal to n, the principal and supplemental power sources are disengaged from the appliance.