Finding a Replacement For Mercury Batteries

At this point I have several different cameras that require the old mercury batteries, which are no longer available. Since alkaline batteries have a different voltage and discharge pattern, some cameras cannot accept them as replacements because it would render the meter wildly inaccurate, and in some cases risk damaging the electronics.

The good news is that there are a couple of alternatives that work pretty well.

Wein Cells are zinc-air replacements that come in several different common sizes.  They don't last as long as alkalines, but they register the correct voltage for most of the battery life (maybe two months or so) before slowly dropping off. Since zinc-air batteries don't start working until you pull the tab off of them (the tab blocks exposure to air, stopping the chemical reaction), you can buy several and have them on hand for when the current one goes dead. Once you pull that tab off, they will self-discharge whether you're using them or not.  You can replace the tab (or use a piece of tape or something) to cover the air holes when you know you won't be using them for a while, slowing or stopping the self discharge. The down side to Wein Cells is the relatively short lifespan and high price. $4.50 for a battery that lasts two months isn't terrible, but it's still a repeating cost.

If you have a way of adapting different sized batteries (some people use a washer of some sort), you can go even cheaper by purchasing different sized zinc-air batteries from the corner drugstore in packs of 8 or 10, and then adapting them to fit in the battery compartment with the washer. I have not tried this, but if you can get it to work, it would be significantly cheaper than buying the Wein Cells.

Finally, the most expensive (but probably the easiest and maybe the longest lasting) option is to buy a voltage-dropping adapter for about $40 that allows you to use a standard, readily-available silver oxide battery. Alkalines fit the adaptor as well and they'll register the correct voltage with the adaptor, but there's still the issue of the discharge curve.  Silver Oxide batteries are preferable for this reason.