Cycle life is a specification that attempts to define how many charge and discharge cycles a battery can undergo before it needs to be replaced.
A single cycle occurs when the battery is charged and then discharged.
A key point to understand when interpreting a cycle life specifications is that it valid for a specific level of discharge.
Generally speaking, the lower the depth of discharge, the greater the cycle life. Consider the following specifications for a typical deep cycle lead acid battery.
If we were to use the battery in a manner that caused us to discharge it completely (100% Depth of Discharge), we may get 200 cycles of life out of it.
On the other hand, if the application dictated that we only needed to 30 percent of the batteries stored energy before charging again, we could get more than 1000 cycles.
Clearly there is a benefit to managing batteries to a lower depth of discharge. Assume the deep cycle battery being discussed stores 1 kilowatt of energy.
When used to a 100 percent depth of discharge, it will store and release 200 kilowatts of energy throughout its life. (1000 watts x 200 cycles).
On the other hand, the same battery will store and release 300 kilowatts if managed to a 30 percent depth of discharge. (300 watts x 1000 cycles).
In other words, the battery performance improves by 50 percent by simply managing limiting the depth of discharge. Put another way, the money spent on batteries is reduced by 33 percent by simply managing the depth of discharge.
Obviously there are cases where a full 1 kilowatt of energy between charges is required. Rather than simply surrendering to that need and living with a higher battery cost, it may make sense to evaluate the design of the battery installation. For example, it may be possible to lower the energy costs simply by increasing battery size or putting three batteries in parallel.