Deep cycle batteries are those with a discharge depth of more than 80%.

Discharge depth (DOD) refers to the percentage of the charge taken out of the battery as a percentage of the rated capacity. The discharge depth of shallow cycle batteries should not exceed 25%, while deep cycle batteries can release 80% of the charge (Ball& Risser, 1988). Since the battery life is affected by the average charge state of the battery, we must coordinate the relationship between the cycle depth and capacity of the battery when designing a system.

Additional information:
The discharge depth of a single battery or battery measures the amount of charge released. It's written in the form A dot H. DOD can also be expressed as a percentage, which is how lead-acid batteries usually express DOD. It is more useful to express DOD in terms of A· H, so that the combination of state of charge (SOC) (expressed as A percentage) and DOD (expressed as A· H) conveys more information than if both indicators were expressed as percentages.
This is obvious for a battery whose actual capacity is greater than its nominal capacity (e.g., 100A·h, but 105A·h). When a battery with a rated capacity of 100A·h releases a charge of 100A·h, the SOC will become 0. In this case, the DOD of the battery can be expressed as 100% or 100A·h.
However, if you want to release all the charge of the battery, the SOC of the battery is still 0 (because SOC cannot be negative), and the DOD of the battery marked by percentage can only be 100% (because DOD marked by percentage cannot be higher than 100%). However, if the DOD is A dot H then the DOD is going to be the correct 105A dot h.

It is more useful to know that the DOD of a battery is 105A·h than to know that it is 100%, because even if the DOD of a battery is 100%, you can still release energy from it. Another important reason to express DOD in A· H is that the discharge depth of the battery is independent of its discharge rate.

http://http://huyi.tigerwing.net/AdminArticle/AddArticle