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 DS!

the GBA SP battery is a bit larger than the regular DS battery so maybe some small force will be needed to fit the battery inside the console and some tool to remove the battery are recommended. but everything should fit in and you should be able to close your console with no problems.

and now you have a new battery for your DS!