The automated external defibrillator (AED) is one example of a medical product that's experiencing extremely fast adoption. These growth rates are due to improvements in the ease of use and to the battery technology that enables mobility and remote use. In the event of an emergency, the battery pack must be a reliable power source. Lithium-ion (Li-ion) cells have the highest energy density by both weight and volume. They offer the most attractive method of portable battery power for many medical devices.
Challenges to the power-system designer arise from the presence of high currents, dynamic loads, wide temperature variations, aggressive power management, and certain charging regimens. AEDs place demands on the lifetime and capability of batteries—performance criteria that push technology to its limits. The construction of a usage profile, which constitutes the unique set of power drain and temperature profiles, is the first step in battery-pack design.
AEDs typically have pulse currents, and they operate in two modes: a low-current mode while the AED monitors the heart rhythm, and a short period of time when the current is higher while the capacitors are charging. An AED must maintain its charge for significant times, such as during ambulance transport, and it may need to operate in a low-current mode for up to an hour and a half.
Li-ion chemistry is more reactive than the other battery chemistries. It requires several safety precautions in a battery pack (thermal sensors and overvoltage protection) and in the cells (the separator and vents) (see the figure). The host device also may display a warning message if failure is impending due to an overcurrent condition. In addition, the user should know the state of the charge from a gas gauge, so the battery needs to communicate with the host. These requirements led to the need for advanced electronics within the pack's plastic enclosure.