For a long time, I was on a mission to eradicate all but AA batteries from my life. It was a quixotic mission, at best, as it seemed that every time a "must-have" bit of electronic kit came out, it would have some exotic battery (Coast Flashlights & "N" cells, various handy gizmos with "AAAA" cells).

In the last year, the amount of kit I have with various button cells - those flat disk batteries - seems to have proliferated. Add to that my kids getting various toys of all sorts, and I had to concede defeat and start stocking a level of button batteries in the same way I stocked AA cells (and, I have to admin, "D" cells, which are exceedingly wonderful for their power to weight ratios).

But what to stock? There's approximately 3,456,344,023 slight variations on the button cell theme, but after a review of what we have and use the most, the list quickly trims down to the following:

CR2032 - This flat battery is found in everything from your desktop computer (it backs up the BIOS settings) to wrist watches. It's used in remote controls for electronics and in some garage door opener transmitters.
Pricing for CR2032's are insane. If you buy at a local store, they can be as much as $7.00 each, or you can buy online at a place like atbatt.com for about $1.34 each, and if you buy bulk packs (be careful, a lot of scammy operators out there) you can get them for as little as $0.35 each. I keep about 20 of them on hand.

CR2016. It looks like a 2032, it isn't. Used in much lower-drain applications, like clocks and remotes, the 2016 is the same diameter and half the thickness of a 2032. I also keep about 20 on hand.

LR44 (also called A76) is a relatively common small button cell. I've found this one in toys and devices of all kinds. I keep 10 on hand.

LR41 This is one of the most confusing of cells but I find it in a LOT of small devices and lights, and it's the battery used in the new eGear PICO light.
The LR 41 spec is for a 7.9mm Diameter, 3.6mm high, 1.5v alkaline cell with a 25 MaH capacity. But there are other 7.9 mm cells that are ALMOST the same height - the SR 726 at 2.6mm height and the SR721 at 2.1mm in height and the SR731 at 3.1mm height. They all will fit an LR41 opening - in the diameter, but the the device may not quite close right, and the voltage is 1.55 volts, which is not really a big deal, but it's not spec for the LR41. (Get my battery spex from this chart PDF file http://www.maxellcanada.com/pdfs/batteries/watch_micro.pdf) I keep 20 to 30 of these things around because they tend to be used in clusters of 3 in the devices I use.

Buy online, it's a lot cheaper. These batteries keep a long time, so you don't need to worry too much about them. There's a lot of stuff (like GPS devices) that use button cells for memory and settings backup and you tend to not notice them until they fail - after years and years of use.