The depth of discharge of the battery measures how deeply depleted the battery is, compared to a state of full discharge. A battery in a state of full discharge has discharged all of its energy capacity.

When a battery has discharged its full energy capacity, the Depth of Discharge, or DoD, is 100%. When it is anything less than that, the depth of discharge, is the ratio of the energy that would have been discharged by the battery to reach its present state of discharge from a state of full charge, divided by its energy capacity. When the battery is fully charged, its DoD is 0%.

The life span of many types of batteries (lithium-ion batteries (Li-Ion) or nickel-iron batteries) depends heavilyupon the number of charge/discharge cycles they undergo. The deeper the DoD per cycle, the shorter the lifespan.

The lifespan of a battery is typically specified by two numbers:

• the number of years the battery can function fully charged
• the number of charges/discharges (cycles) it can absorb while still providing nominal performance.

The energy capacity of the battery depends upon the discharge current (in Amperes), and that the number of cycles in the lifespan depends upon how deep the discharge is.
reference: energymag.net