I acquired a while back a working GameBoy Advance SP. Unfortunately, I didn't have a charger for it at the time. So, when the battery finally died after a week of use, I started investigating to see if there were any charger mods I could copy, but the closest I could find to what I was looking for was modding the GBA SP charger cable to so that it was USB on one end, and GBA SP charger on the other. I finally had to resort to taking the metal part off of a USB cable and plugging the plastic part into the GBA SP charging port, as it sort of fit. Some time later, a friend gave me a GBA SP charger connector, and with that I made USB cable to charge the Gameboy. Later on, I found it to be quite a hassle to have to bring 2 separate chargers every time I traveled with my GBA: A micro-USB cable for basically everything I own, and the GameBoy Advance SP charger. Another problem I had with this charger was that each time I lost the charger, the GBA SP was basically useless, as I couldn't charge it. I finally decided that I had enough of the useless charger, and decided to implement the newer Micro-USB.
Step 1: Micro-USB Port
To be able to do this whole project, you will need a Micro-USB port. You can buy one, or you can just rip it off of any electronic device that you don't need anymore. I took mine from an old Nokia C2-05, as old Nokias don't use the Micro-USB port for charging. I actually had to sacrifice another Micro-USB port, as I ruined the first one by moving it around too much. In addition to this, you will need to have some medium soldering skills, as the Micro-USB port has a minuscule pitch between contacts. If you're not too confident with soldering, take a Micro-USB extension cable, and chop off the part that you don't need.
Step 2: Wire It and Place It
I chose to place my Micro-USB port between the two ports on the GBA SP, because I couldn't find anywhere else to put it. I wired Pin 6 from the GBA SP port to Pin 5 on the Micro-USB port, and Pin 3 to Pin 1. To make a hole for the Micro-USB port, you need to use a dremel and make a thin and tall hole on each side of the casing.
Step 3: Final Result
Test the port before closing it up. Once closed, test it out again. This mod leaves both charging ports functional, so you can charge with either the original charger, or the Micro-USB charger, although you musn't put both chargers at once as you'll create a short circuit.