Comment by zero_iq

One possibility is to replace part of the battery. The smaller battery can be designed to lie about its charge, or you can replace with a higher energy-density battery and use the space saved for a detonation system (perhaps even incorporating the battery itself into this) and a small quantity of high explosive, which is pretty stable and safe until detonated. Contrary to popular belief, high explosives are actually relatively safe, and usually even burn safely or are hard to ignite at all in some cases. Package it up into something that looks identical to an unmodified battery. Modify device firmware and battery control circuitry to detonate it on receipt of a specific signal and... boom.