A power system within a battery-powered node includes a primary cell, a secondary cell, and a battery controller. The battery controller includes a constant current source that draws power from the primary cell to charge the secondary cell. The battery-powered node draws power from the secondary cell across a wide range of current levels. When the voltage of the secondary cell drops beneath a minimum voltage level, the constant current source charges the secondary cell and a charging signal is sent to the battery-powered node. When the voltage of the second cell exceeds a maximum voltage level, the constant current source stops charging the secondary cell and the charging signal is terminated. The battery-powered node records the amount of time the charging signal is active and then determines a battery depletion level based on that amount of time. Battery replacement may then be efficiently scheduled based on the depletion level.