This is from the APU section of 747 manual. Apu and main batteries are the same. Obviously the main battery will not be subjected to the enormous current drain of a starter motor but this should give you some idea of what the battery is capable of.
"The Starter Motor is powered from an APU battery. It is rated at 7
HP at 4,500 rpm. Current drain is approximately 500 amps (1,000
amps momentarily) tapering to approximately 200 amps at starter
cutout."