Peukert's Law and discharge of lead acid batteries

In 1897, the German Physicist Peukert proposed that as the rate of a lead-acid battery's discharge increases its available capacity.This is known as Peukert's law.  It can be Mathematically represented as

C_p=I^kt

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.