In this guide, I will show you how to mod your Game Boy Advance to use rechargeable LiFePO4 batteries and a USB port for charging. We're specifically using LiFePO4 batteries and not Li-Ion batteries because they are 3.2v as opposed to 3.7v of Li-Ion. This mean we only need a charging circuit for this mod, and a voltage regulator/converter is not required. It's easier and makes the mod alot more simple. That's all you need to know :)
Step 1: Items You Will Need for This Mod
Step 2: Disassembly and Preparation
Go ahead remove the back from the Game Boy Advance.
You won't need to remove the circuit board.
Now you will need to desolder the negative battery terminal from the circuit board. This is where it helps greatly to have a large "knife" tip on your soldering iron as you will need to transfer a vigorous amount of heat to the terminal to get the solder to melt. Apply a generous amount of solder to your iron, and place it on the terminal right where it's soldered to the board, give it a good 10 seconds or so, and it should lift out with bit of a tug.
Be careful not to burn yourself as the terminal will get extremely hot during this process. I used a pair of pliers to hold the terminal.
Once it's desoldered, you will need to connect it to the positive terminal. I simply bent over the part that was soldered into the board, and then connected it to the positive terminal with a generous amount of solder. You are free to connect these together however you see fit, but it must be relatively sturdy and the negative terminal must be positioned as it was before, so the case can fit back together as normal. See the pic for an example of how I did this (ignore the wires for the time being)
The Game Boy Advance uses 2 AA batteries in series as standard, so what we're doing here is changing it to a parallel setup since AA batteries are 1.5v and LiFePO4 batteries are 3.2v. If we were to put 2 LiFePO4 batteries in series, like AA's, the Game Boy would not switch on, since we'd be feeding it 6.4v. So this step is 100% necessary.
Step 3: Cutting a Slot for the USB Port
Use whatever tools you feel necessary to cut a notch in the front housing for the USB port. Remember, don't cut too much because you can't reverse it. Cut a bit, then test fit the USB port. If it doesn't fit, cut a bit more and test again. I find an Xacto knife helps wonders for getting a nice clean looking slot, but flush cutters will do a good enough job as long as you take your time.
I cut it right next to the center screw post, as seen in the pic.
Use extremely strong glue to hold it in place. I highly recommend 2 part epoxy as it needs to be very durable since you'll be connecting and disconnecting a USB cable on a regular basis. The epoxy I used needed about an hour to harden.
As for the USB port itself. It has 5 pins. You will only need the outer most 2 pins. 1 for 5v and 1 for Ground. Go ahead and cut the inner 3 pins with your side/flush cutters.
Step 4: Preparing the TP5000 Charging Circuit
You'll need to do a little trimming to the charging circuit to get it to fit comfortably in the Game Boy.
About 25% of the board is simply for the USB port that is attached to it. We don't need this since we're using our own. So cut the board just behind the Micro USB port, but don't cut too far. The pic should give you a good idea of where you need to cut it. You need to be able to solder to the 5v and Ground pins.
Now you need to secure the charging circuit to the Game Boy. Before you do this, make sure you make a note of the solder points on the back, as once you've secured the circuit down, you won't be able to check it again.
In the pic you can see where I have secured it. Under the circuit is a chip, which I glued the circuit to. You can use a drop of epoxy for this if you want to, but it's not necessary to use glue that strong. You'll be better off using a bit of super glue if you have it.
Make sure it's secured exactly like the pic, as the housing has posts that press against the the Game Boy motherboard, and you don't want the charging circuit getting in the way of those.
Step 5: Wiring
Now that you have your battery terminals and TP5000 charging circuit prepared, we can move on to the wiring.
Wiring is pretty self explanatory. The back of the TP5000 circuit is labeled so you know where to solder the wires.
You have B+ and B- which are the connections for the battery terminals. B+ is soldered to the positive terminal. In the pic in step 2, you can see the red wires (the ones I told you to ignore before) The left one is coming from B+ on the charging circuit, and you can see it connected to the positive terminal.
B- connects to the battery terminals in the back housing. There's a small gap where you can solder the wire. In the pic, you can see a grey wire going to this terminal in the back housing.
Out+ and Out- are connected to the Game Boy motherboard. Out+ need's to connect to the Game Boy's fuse. This is where the positive battery terminal would normally connect to in an unmodified Game Boy Advance. If you take a close look at the pic in step 2 again, you will see the right wire connects to the left side of the fuse.
This is the trickiest part of the mod. You need to desolder the left side of the fuse, but keep the right side connected. Then your Out+ wire needs to connect to the left side of the fuse. If this is a little too difficult for you, then you could try cutting the trace on the board that leads to the fuse and then just solder your wire to the left side of the fuse.
Out- is simple. Just connect it to any Ground point on the Game Boy motherboard. I used the trigger as it was conveniently located right above where we glued the charging circuit.
All that's left now is to wire up the USB port. If you have mounted your USB port exactly like mine, then the right pin is 5v and the left pin is Ground. These are wired to the right side of the TP5000 (right where you cut the board earlier) See the pic in step 4.
Step 6: Finishing Up
Now you can plug in your USB cable and you should see the TP5000 light up. You should also be able switch the Game Boy on.
If all is well, go ahead and secure your wiring and put the back housing on. It will be tight because of the wiring, but it should fit.
If you bought the batteries I linked at the start, you will notice they are flat on both ends. As is, these will not make contact with the flat terminals in the Game Boy. The solution is simple. Apply solder to these terminals so they "extend" and can make contact with the batteries. You could apply the solder to the batteries themselves, but you risk damaging them with the heat from the soldering iron. So I highly recommend doing this to the terminals instead. The pics show my terminals with the solder applied to them.
Now remember, the Game Boy is now setup for parallel batteries, not series. So do not insert the batteries the way you would normally insert AA batteries. You WILL damage the batteries and possibly the Game Boy if you do this. Both batteries must be inserted the same way. Positive to the right, negative to the left (just like in the pic above.)
If everything went well, you should be able to switch your Game Boy on now, and when you insert the USB cable, the LED on the TP5000 will be RED when charging and GREEN when fully charged!
With the batteries I linked, you can expect around 12 hours of battery life. Charging the batteries take roughly 2-3 hours. The only disadvantage to LiFePO4 batteries, is that they hold their voltage pretty much until they are empty. What this means is that your Gameboy won't display a red "low battery" light, until the batteries are literally dead. So if your battery light turns red, save your game and find a USB cable quickly. You'll have about 5 minutes of battery life left, tops.
This general method can be applied to any device that uses 3v. I use the same mod on my Wavebird controller.