A battery pack topology wherein the battery pack has multiple battery sub-stacks electrically connected in parallel such that the capacity of each battery sub-stack may be utilized but one is reduced unequally as to the others. As a result, one battery sub-stack will reach a point of failure before the other, which causes a drastic, observable change in the output voltage of the battery pack, but provides sufficient reserve capacity to permit a user of a device, such as an AED, having the battery pack to be notified in a timely fashion of the need to replace the battery pack.
