Anyway, I had replaced the batteries once before. I picked up a replacement battery at a Ham Fest or some such place. It worked for a while, but I got to thinking: why am I bothering with these odd-sized custom batteries when I've got plenty of rechargeables lying around?
Step 1: Take It Apart
Unfortunately, the people who made this shaver decided that it was important to use Torx-like screws rather than some kind of standard Phililps-like one. Fortunately, the trick to these is to use a small flat-bladed driver that can seat between two points. In this kind of application, the torque on the screw is very small. Obviously, you're not going to get a rusted Torx off a car with a flat-blade, but for this, it works great.
Step 2: Remove the Battery and Charging Electronics
I also checked and the battery is 2.4V -- 2 1.2V rechargeable cells. The capacity is rated at 600mAH, so by using some meager 1,800 mAH AA batteries, I can triple the run-time ... as if that was ever an issue.
Step 3: Install a Suitable Battery Holder
To mount it, I just drilled a couple holes in the holder and then used them as a template to drill holes into the front of the shaver. I decided to use the front so I could still use the beard-trimmer feature on the back and so the shaver would sit in its charging cradle (even though it won't be plugged in anymore).
Don't forget to find screws that are short enough that they don't hit the batteries when they're installed.
Step 4: Add a Switch
Step 5: Wire It Up
Step 6: Put It Back Together, Add Batteries, and Start Shaving
To recharge the batteries, I just use my existing AA charger. I could have kept the charging circuit in the shaver, but I think it's kind of a waste to let the shaver charge all day just to use it for a few minutes in the morning. Admittedly, it's "only" 2 watts, but now I have some neat parts to play with too.
I'll take the existing NiCD rechargeable pack to Radio Shack to be recycled.