@hi_cial gba games, except for a *very* few early ones like Fire Emblem, use non-volatile flash memory for save data! So they don't need a battery any more than your usb flash drive needs a battery
except pokemon rse, which have a battery for timekeeping because they also have a real time clock (but if it dies they don't lose the save, time-based things like berries growing and espeon/umbreon evolutions just don't happen)
-F
@hi_cial flash memory (or at least the related technology called EEPROM) existed in the 90s and could have been used for gameboy and snes games, but it was just much too expensive for mass produced stuff like that, it would have made each game cost like $10-$20 more
-F