Reserve batteries you will always have with you

There is a battery that doesn't get much press. It's the battery that sits in standby in a very narrow niche. It's called a dry reserve battery and they work by being activated at use. They can be used 1 time and then they are replaced. Sure, that's the same as a primary battery, but by 1 use I mean they can't be turned off and they are used up in one session.

In general, their construction is simple in that the two main components, anode and cathode, are kept completely separate (dry) from each other and an electrolyte is added when the battery is needed.

Why would anyone use such a battery? Because they are very reliable. One might even say they are the utmost in reliability for a mass produced battery. When 99% reliability isn't good enough, a reserve battery is what you need. They can have a high energy density because they can sacrifice the property of bad self discharge to gain capacity. And they can be very cheap, even cheaper than primary batteries, but have a better performance in a one time use situation.

For example, a missile needs power only when it is fired. And it will only ever be used 1 time. The energy requirement must work the first time 100% of the time and be light, too.

Or take some electrical equipment that is only used in case of a fire. It might sit for years before there is a fire and it will never be maintained. With a standby life of an undetermined very long time measured in decades, perhaps centuries, a reserve battery might very well last longer than the device it will power.

In mines carrying any extra weight is a negative. But running out of power in some situations can be deadly. A very light high density battery that can be activated just to get out, and absolutely will work even if carried/stored for years, can save lives. This idea isn't used in modern mining much anymore, but there is a market for reserve batteries in survival packs when civilization is very far off.

And when the very inexpensive reserve battery types are marketed they are bought to be used in the same place as primary batteries in remote areas.

There are 3 main types of reserve batteries: mechanically activated, heat activated, and water activated. The mechanically activated need something to be broken, perhaps by a person or an automatic trigger, to release the electrolyte into the battery and begin supplying power. The heat activated reserve batteries use a melting barrier that breaks down in the event of excessive heat. And a water activated reserve battery doesn't need the electrolyte carried with it as any brackish, dirty water is the preferred way to start getting power.

So reserve batteries have their place. And their continued use will come even after the "holy grail" of secondary batteries pushes a number of other types of batteries exclusively into history.