Cheap ones promise about 650 to 850mAh but they are less then 250mAh if they even work at all. Its best to build your own. Search makho on YouTube if you want to know more about gba sp batteries. MrPudding28 • 3 yr. ago. An eBay seller jeeper5150 sells pre-made makho batteries for around $14.
So i dont see the usefullness of this (GBA sp battery would last half of the time of a pack of AA on a regular gba). If you want a AA rechargeable mod one user from here has already made one. If you just want a battery solution nintendo did release a battery and charger pack for the gba ( search agb 008 on google and check the images).
Got my son a GBA SP pack (Black GBA SP, Case, and Super Mario 4 w/guide) for Xmas. I'm tempted to spend the 90-100 minutes to break open the hermetically welded plastic to charge it up so that he can immediately play it.
【User Tips】Battery need to be charged before first use.Fits right in the battery slot, and you cover it with the GBA SP battery cover. Recharges your GBA SP battery quickly, Best replacement for original PSP travel charger 【Perfect Kits Gadgets】Package included 1Pcs Replacement battery, 5.2V 450mA wall charger.

Buy Step 1 Battery Use a standard Phillips #00 screwdriver to loosen the 4 mm screw that keeps the battery cover attached to the device. 2 comments Step 2 Lift the battery cover up and remove it from the device. The screw will remain attached to the battery cover because of Nintendo's design. Add a comment Step 3

Gameboy Advance SP Battery Pack, 850mAh Replacement Rechargeable Lithium-ion Battery + Tool Pack Kit Compatible with Nintendo GBA SP 4.6 out of 5 stars 342 $10.99 $ 10 . 99 aqknMO. 303 345 54 105 235 364 91 429 209

gameboy advance sp battery cover