An Automated External Defibrillator (AED) is a portable, life-saving device designed to treat sudden cardiac arrest by delivering an electrical shock to restore a normal heart rhythm. Understanding the power capacity of the device is essential for preparedness and maintenance. This rapid intervention is the only definitive treatment for certain life-threatening heart rhythms outside of a hospital setting.
The Determinant of Shock Capacity
The shock capacity of an AED battery is not universal; it is determined by specific technical factors detailed in the manufacturer’s specifications. Most public-access AEDs use specialized, non-rechargeable lithium batteries, such as lithium manganese dioxide, known for their high energy density and long standby life. Capacity is typically measured by the number of maximum-energy shocks the battery can provide when new.
A new, fully charged AED battery commonly provides between 100 and 400 maximum-energy shocks, though this varies significantly by model. Capacity is directly linked to the energy level, or Joules, delivered per shock, with modern AEDs typically providing biphasic shocks between 120 and 200 Joules. The manufacturer’s rating is based on providing a specific number of shocks at that maximum energy level.
The actual energy delivered to the patient is accumulated in a capacitor, which draws power from the battery for each charge cycle. As the battery ages or is used, its internal capacity diminishes, meaning the number of shocks it can deliver decreases from the original rating. This reduction necessitates managing AED batteries according to strict replacement schedules.
Operational Capacity Versus Monitoring Time
The number of maximum-energy shocks is only one aspect of capacity, separate from the battery’s ability to power the device for continuous monitoring. An AED battery is constantly used, even in standby mode, consuming energy for regular functions like running diagnostic self-tests, maintaining internal circuitry, and powering indicator lights. These self-tests are performed daily or weekly to ensure readiness, and the cumulative drain is significant over years.
When an AED is actively deployed, battery drain accelerates dramatically due to functions other than shock delivery. The battery must power the sophisticated rhythm analysis, run voice prompts, and continuously charge the high-voltage capacitor to prepare for a shock. While a battery may be rated for hundreds of shocks in a laboratory setting, its continuous operational time in an emergency is often rated for only four to six hours. This distinction highlights that the battery is a time-limited resource in a real-world rescue scenario.
Low Battery Alerts and Replacement Cycle
AEDs proactively manage their power supply and alert owners when capacity is low using automated internal diagnostics. Most modern devices perform regular self-checks and indicate readiness status using a visual indicator, such as a flashing green light, or an auditory signal. When capacity diminishes to a predefined threshold, the device activates a low-battery alert, which may include a chirping sound or a specific voice prompt.
The battery replacement cycle is governed by the device’s use and the expiration date printed on the pack. AED batteries have a strict “install by” date and a specified standby life, typically between two and five years, once installed. The battery must be replaced immediately after any use that involves delivering a shock, regardless of remaining capacity, and always before the printed expiration date.