Ultra-long shelf life battery
A battery using magnesium and silver plates, which is activated on contact with seawater, has an almost-infinite shelf life provided the plates are kept isolated from moisture and circulating air. Such batteries are used in emergency equipment (for example, submarine escape systems), where typically they are activated by removing sealing plugs and immersing the equipment. Naturally, they are extremely expensive.

When I worked for a manufacturer of rescue equipment, such batteries were produced as one-offs by a single specialist craftsman; each took nearly a week to construct. The design was proven by previous experience, as new batteries could not be tested without destroying them.