One of the big drawbacks to a lot of technology is that it requires batteries. I think that there is a certain rule about batteries which states that they are always full when you check them but always empty when you go to actually use them.
One of the more often used battery powered devices in our home is the Gyration mouse in the kitchen. It is on and off of its charging base dozens of times every day. Over the course of a couple of years the batteries cease to function and we are left trying to remember all of the keyboard shortcuts to open and close windows and navigate around the screen (is it enter or space to click a link in a web browser?).
Last time this happened we were fortunate to have a spare handy since a family member had already been down the road of having their mouse die and needing to get a replacement. After swapping out the old one I noticed that the battery pack said it was an NiMH battery. This piqued my interest and I decided to take it apart to see what was inside. As it turns out the battery pack is a multi-purpose plastic case with ordinary rechargeable AAA cells inside soldered together. Lifting the cells out you could see the markings telling you how to install your own AAA cells in the case. There were even plastic clips and such to hold springs in place so that they could use the same casing for non-rechargeable battery packs as well. After seeing all of this I decided that next time the battery pack needed replacing instead of trying to track down a new one I would try my hand at building one from the old pack.
Well after a couple of years here we are with a dead battery pack. Mary fluttered her eyelashes at me and off I went to the store to pickup some AAA NiMH rechargeable batteries. I had originally intended to just solder the new batteries together and call it a day but upon seeing the little slots for springs again I decided that if I spent a little longer I could probably add the spring in and have a user-serviceable version which wouldn't require soldering next time it needed new batteries. I proceeded to dig through the endless drawers of random small parts that I have in the basement and found some small springs and flat metal plates which I had carefully removed from some previously defunct battery powered device. I soldered them together and slid them into the battery pack (noticed that I had done it all backward since the markings on the case don't match the polarity of the cells which had been in the case before - and redid it all). I then popped the new cells in, installed the new battery pack in the mouse and slid it onto its charging base. It worked!
Now next time the batteries need replacing it will be a simple matter of popping out the old ones and popping in some new ones. In principle the same sort of thing ought to work for lots of "custom" battery packs.