Have a spare DS battery? and want longer play time on your GBA sp?
Here a small tutorial on how to use a DS (Phat) battery on a GBA SP!
Step 1: Replacing the Battery.
As you can see the GBA SP battery is about the same size as the DS battery.
the biggest difference is mostly the ds battery has 3 pegs and the GBA SP battery has only 2 pegs in a different place. well just cut those pegs with a knife or some scissor and the battery will now fit in the GBA SP! you will gain longer play time and everything should fit in and you should be able to close your console with no problems.
caution : without pegs the battery can be inserted upside down and this is not good for the console.