An Automated External Defibrillator (AED) is a portable medical device designed to treat sudden cardiac arrest, a condition where the heart abruptly stops beating effectively. The device analyzes a person’s heart rhythm and, if a life-threatening electrical problem is detected, it delivers an electric shock to reset the heart’s rhythm. Understanding the device’s operational limits is necessary for ensuring it remains rescue-ready. A primary limitation for any battery-powered device that must deliver high-energy pulses is the capacity of its power source.
Operational Capacity of AED Batteries
The power source of an AED is engineered to store and deliver the electrical energy required for defibrillation. Manufacturers typically express battery capacity in two ways. The first is the maximum number of full-energy shocks the device can deliver before depletion, often ranging between 100 and 400 shocks depending on the model. The second measurement relates to continuous operational time, which is the duration the AED can monitor a patient and run its internal systems. This is often stated as being between four and 16 hours of continuous use. These figures represent the theoretical maximum under controlled testing conditions, and the actual number of shocks available in a real-world scenario can be lower.
Factors That Determine Available Shocks
The stated maximum shock capacity serves as a guideline, but several factors influence the practical number of shocks an AED can deliver during an emergency. The most significant variable is the energy level, measured in Joules (J). A higher energy shock requires the AED to draw more power from its battery to charge the internal capacitor. Some AEDs use escalating protocols, meaning subsequent shocks are delivered at a higher Joule setting, such as 200 J, then 300 J, and finally 360 J, which rapidly drains the battery.
Power is also consumed by the device’s non-shock functions, which are essential for the rescue process. These functions include analyzing the patient’s heart rhythm, running diagnostic tests, and providing voice prompts and display screen usage. Charging the internal capacitor, which stores the energy before a shock is delivered, is a major power draw even if the shock button is not ultimately pressed.
The chemical composition of the battery also plays a role in the available shock capacity and lifespan. Most public-access AEDs utilize non-rechargeable lithium batteries, designed to hold their charge for years in standby mode. Rechargeable batteries are typically used in professional settings. Non-rechargeable batteries have a specific standby life, often around four to seven years, after which internal chemical reactions reduce their ability to deliver the necessary high-voltage pulse.
Ensuring AED Readiness and Power Maintenance
Maintaining the AED’s power source is necessary to ensure the device performs at its maximum capability when an emergency occurs. Nearly all AEDs feature an internal self-test cycle that automatically checks the battery level and circuitry on a daily, weekly, or monthly basis. The status of these internal diagnostics is communicated through a readiness indicator light, which is usually green to signal the device is ready or red/yellow to indicate a fault or low battery that requires attention.
Owners must monitor and track the expiration dates for both the battery and the electrode pads. Battery expiration dates are safety limits determined by rigorous testing and are essential to preventing a malfunction during a rescue. Some manufacturers label the battery with an “Install Before” date, meaning the standby life only begins when it is placed inside the AED.
Electrode pads also have an integrated expiration date, typically between two and five years, because the conductive gel and adhesive can dry out over time. Environmental conditions also affect performance, as extreme cold temperatures can temporarily reduce the battery’s ability to deliver a full charge, potentially rendering the device ineffective.