C_{p}=I^{k}t
where Cp is the capacity of the battery in ampere hours
k is the Peukert's constant
I is the current and
t is the time
The Peukert's Constant k is specified by the manufacturer of the battery and is usually in the range of 1.2 to 2.
Thus the time a battery can provide sustain a certain current without any appreciable drop in voltage would be given by
t=C_{p}/I^{k}
Thus a 100Ah capacity with a Peukert's constant of 1.2 will be able to supply a current of 5A for 14.5 hours.
