The old PP3 (transistor) battery. I have a few in projects. On my freezer, I have the battery, a microswitch and a "SonAlert" along with a 555 timer. When the door is opened the timer starts a countdown. After 60 seconds (roughly) the SonAlert will start sounding, alerting us if the door is left open. It's quite effective and I haven't had to change that battery in two years. Of course, the more we open the freezer door the quicker it will deplete. But for now it works nicely. Smaller than a couple 18650's and safer to handle/replace.

 

The old PP3 (transistor) battery. I have a few in projects. On my freezer, I have the battery, a microswitch and a "SonAlert" along with a 555 timer. When the door is opened the timer starts a countdown. After 60 seconds (roughly) the SonAlert will start sounding, alerting us if the door is left open. It's quite effective and I haven't had to change that battery in two years. Of course, the more we open the freezer door the quicker it will deplete. But for now it works nicely. Smaller than a couple 18650's and safer to handle/replace.

Recent improvements in the PP3 designs from major manufacturers has greatly increased the shelf life of PP3 batteries. One of the previous objections to them was the unholy self-discharge rate. It meant you couldn't buy extra batteries if you weren't going to use them within a few months. They now advertise 5 year shelf life. This is probably due to the improvements in alkaline cells in general, which thankfully now make up the internals of any decent PP3 rather than the zinc carbon cells of the past.

"Heavy Duty" on a battery means "trash (Voltaic) pile" because it is marketing speak for carbon batteries.