Rated capacity of the Lead-Acid Battery, based on 20-hour rate, is indicated by the capacity value when the battery of fully charged state is discharged to specified end of voltage.
The 20-hour rate is equal to 0.05C of Lead-Acid current rate and it is very low current rate condition. (Rated Current of Lead-Acid : 0.5C)
Generally, Lead-acid battery has poor capability under high discharge current. So it is easy to sacrifice its containing capacity at this higher discharge rate (In other word, Lead-acid battery can not used all rated capacity at higher discharge rate). Because of slow electrolyte diffusion into the electrode, the H2SO4 content is reduced in the micro pore that is occur IR drop rapidly.