Abstract: A method and apparatus for indicating a low battery condition and for dynamically controlling the load on a battery in order to optimize battery usage. A dynamic load controller for a battery includes detection circuitry for measuring at least one condition related to battery capacity, and power control circuitry for adjusting a power load on the battery based upon the condition. The dynamic load controller may be employed to control the power load on a battery that powers an electrotherapy device, such as a defibrillator. The battery condition may include the slope of a capacity curve, which may be the product of the battery voltage and the power delivered from the battery as a function of the delivered power. Based upon this condition, the power control circuitry adjusts the power load to optimize power delivery from the battery.