AEDS are powered by means of a battery. Usually this is a lithium battery. No external power is required to the AED, this means that you don't have to plug it in the outlet.
The reasons for choosing the lithium batteries are:
The capacity of the lithium batteries exceeds that of the alkaline batteries;
The voltage of the lithium battery is higher;
Lithium batteries perform better in extreme temperatures;
Lithium batteries are lighter, which is useful for portable devices;
Lithium batteries have limited internal discharge. They retain their energy better when they are not in use.
Lithium batteries are however more expensive than alkaline batteries. In addition, the batteries for AEDs meet the heavy demands of medical equipment, which means that their price is higher.
Batteries last, depending on the brand and type 2 to 7 years when the AED remains in standby mode. You can find the estimated standby time of each battery in the additional information in the product description. This standby time is not guaranteed. The battery may, depending on circumstances, discharge sooner. The AED will raise an alarm when the battery power is low. Try to prevent this by a timely replacement of the battery in order to ensure that the battery retains enough power for the AED to do its job.