The problem is, some of my possessions don't take too kindly to being cold, like electric drill batteries, for instance. If you look online, you will find all kinds of opinions on the subject - take your pick. But, it is my opinion that cold NiCd batteries don't last as long in use as warmer ones, that they don't accept a charge as well, and will have a shorter life if they are kept in a cold environment. I have no opinion on NiMH batteries; I don't have any tools that use them.
Step 1: Framing Up
In my shop, I have shelves mounted between the studs, including a couple for holding my chargers and batteries. In order to hold them, as they are wider than the studs are deep; I made the shelves extend past the studs. As it turned out, this made building the compartment very simple. Most likely, your application will be different than mine. No problem, as with most Instructables, it is the original idea that counts. Just modify my idea to fit your need.
In order to make my existing shelves into a cabinet, I simply cut plywood pieces for each end. After that, all I had to do to finish the construction was to build a pair of doors.
Step 2: Doors
I made the door by cutting a piece of plywood to fit the cabinet, attached hinges to both sides, and marked the center. When I removed them and cut them into two pieces, I simply reattached them. That caused the two doors to line up perfectly; it is so much simpler that way than making two doors separately, and then trying to get them to match.
I attached a hook and eye inside the left door, and turn button on the outside of it. This pulls the two doors together; just one hook and eye on the pair of doors would allow them to hang loosely.
Step 3: The Idea Behind the Cabinet
Step 4: Finishing Up
I electrified the cabinet by using a candleabra base light socket; you can suit yourself about how you do this, but I added a 1N4004 diode in series with the bulb. This diode divides the voltage to the bulb in half, and so the power consumption. It has the effect of making the bulb last a teriffically long time. Using a 25 watt bulb, I am actually only consuming 12 1/2 watts. The glass envelope temperature is lower than with full voltage, but it is enough to keep the cabinet and everything in it warm enough for my purposes.
I have a used baseboard heat thermostat that I plan to add to the circuit; I am not burning much current as it is, but there is no need keeping the cabinet warm if the outside temperature isn't too chilly.
All in all, I am pleased with the results. My batteries stay reasonably warm and I feel like they last longer in use than if I allowed them to stay cold after charging them.
[Added: I have a wireless indoor - outdoor thermometer sensor in the cabinet, and have recently found that during a pretty cold spell, the temperature was at least 20 degrees warmer than outside.']