I found out recently that the original Nintendo DS (Phat) battery fits into the GBA SP with just a small adjustment to the battery. All you need to do is clip off the two little tabs and the battery will fit perfectly into the GBA SP.
Here’s a third party GBA SP battery on top of the Nintendo DS Phat battery. Third party GBA SP batteries seem to be terrible quality. This one only lasted a year or so before it wouldn’t hold a charge.
I’ve already clipped off one of the tabs in this image, but left it at its original location so you can see where it was. The tabs on the SP battery and the DS battery are in slightly different locations.
Lining up the DS battery inside the GBA SP battery compartment I noticed that the other tab was almost in the correct place. So rather than clipping it off completely, I decided to cut a chunk out of the tab so it would still fit in.
With the left tab clipped in half, it now fits into the retaining hole in the GBA SP battery compartment.
The idea of leaving half a tab was that it would hold the battery more securely in place, but it actually fits perfectly anyway, so you should be safe to clip both tabs completely off.
The GBA SP charges fine with the DS battery. The GBA SP and the Nintendo DS Phat actually use the exact same charger.
The console powers up and works perfectly. I did a battery test on a full charge and got almost 9 hours. That’s a lot better than the crappy third-party SP batteries from eBay.
Also, just for the record. The DS Lite battery will not fit.
The DS Lite battery connectors are on the side of the battery and not on the end like the SP. However, I did find a forum post from someone who modded a DS Lite battery by re-locating the battery contacts, so that might be a future project.