I was at a Recreation Center in a local community this week and looked at the AED they had hanging on their wall. The battery was Dead and so the device could not be used.
If you have an AED that does NOT give you an audible alarm when the battery capacity is low then the only way you can verify battery capacity is by looking at the battery gauge on the device (some devices dont even have those). If the battery is dead you can not save a life. While you are at it, please check your PAD/Electrode expiration date. It is marked on the outside packaging.