An experimental analysis based on Li-ion discharge to evaluate the influence the discharge rate has on the battery capacity is presented. Measurements have been made on various capacity units at different discharge rates. As a result of the tests, a new mathematical model has been developed; this new model predicts the battery capacity for any discharge rate within excellent correlation to observed results. Measurements show the manufacturer value for the battery nominal capacity is currently deviated from its real value what forces to determine this real value before testing. The mathematical model presented can be used to determine realistic capacity expectations based on conditions that are shown to impact battery operation time and autonomy.