C Rating and battery capacity.

The capacity of a battery is indicated in Ampere-hours (Ah).  For instance, 200Ah, 80 Ah, etc.  The rate at which the battery can be discharged is indicated by the C rating.  

Batteries are usually rated as C/10, c/8, etc.  a battery with a 200 Ah capacity and a C/10 rating indicates that the battery can supply 20 A for 10 hours. 

The capacity of a battery varies inversely with the discharge rate. Batteries usually have different discharge rates for a fixed capacity.