BatGBA is a now out dated GBA Emulator. It was once promising but was eventually surpassed by the many other GBA emulators. it was one of the first emulators to start really tackling the GBA, however like a few other emulators of its time requires the GBA Bios.
The emulation abilities of BatGBA was never fully fleshed out, as newer versions of the emulator was moving to eventually remove the requirement to need a GBA bios alongside the emulator, with work being done to emulate the BIOS within BatGBA itself.
Sadly this is another promising emulator that never survived long enough to become fully fleshed out, and was surpassed by emulators such as Visual Boy Advance. If your after a stable and fast GBA emulator your better off looking at newer emulators such as mGBA or Visual Boy Advance.