Depth of Discharge explained.

Depth of Discharge is something that's relevant to all rechargeable batteries. This type of battery is specifically designed to perform and discharge before being recharged and is commonly referred to as a deep cycle battery.

Deep cycle batteries quite often don't advertise a CCA (cold cranking) rating as this is more typical with starting batteries. Dual-purpose batteries like some AGM batteries, however, have good deep cycle ability and a large surface area of active material which gives them high cranking output when compared to equivalent-sized starting batteries. Remember, the purpose of the battery is the most important consideration here.

Battery spec sheets of deep cycle batteries will show a cycle life that is expressed by a graph showing different depths of discharge with a corresponding life expectancy. When the depth of discharge is deeper the number of times that battery can cycle reduces. So general-purpose batteries might only be 50-70 cycles at 30% depth of discharge. These are sometimes referred to as Leisure batteries but in fact, are just standard typical starting batteries. 400 cycles at 50% D.O.D. is really where a purpose-built battery might be advertised as a deep cycle. This will extend to 80% D.O.D at 700-800 cycles before you get to the internationally recognised quality brands that might be as much as 1000-1200 cycles at 50% which is similar on the graph to the 80% at 700 cycles but shows the manufacturers intent of the battery to be longer-lasting over 1200 cycles than say once a day for 2 years until end of life.