Battery Cycles vs. Battery Lifetime?
Often the lifespan you can expect to get out of your battery is referred to in terms of "cycles". A battery cycle is one complete discharge and recharge cycle. The discharge state of a battery is often measured in Depth of Discharge (DOD). This refers to how far down the battery has been taken, for instance a battery that has 25% of its capacity remaining would be said to be at 75% DOD.
The lifetime of a battery is directly related to the depth of the discharge that it regularly experiences. Lead acid batteries are fickle things. If you subject a deep cycle battery to 80% DOD on a regular basis you will get roughly half the life out of your battery than if you were to cycle it to 50% DOD.
While this doesn't mean that you can't go down to 80% DOD you should generally try to design your battery banks to allow for cycling at around 50%.
Conversely there is also an upper limit on the DOD of a battery, usually a battery that is only regularly cycled down to 5% or less will not last as long as a battery cycled to 10% or more. This is because on smaller cycles the Lead Dioxide can clump up around the positive plates. On heavier discharges this would be more of an even film.