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23.10.2014
Generally the prediction of the battery lifespan is done by engineers in a very simplified way, using the number of equivalent full cycles (method using by HOMER software).
The equivalent number of cycles is data supplied by the manufacturer, although generally not given directly, but in some cases they provide the number of cycles to failure at 80% depth of discharge (Cycles_80), thus equivalent full cycles can be calculated as Ciclos_eq = 0.8*Ciclos_80. Another more complex way of estimating the useful lifespan of the batteries is to use the cycles to failure vs. The two methods described before DO NOT give good prediction of batteries lifespan in the cases where the batteries are long periods at low state of charge (SOC), for days or even weeks in winter, which is quite common in PV systems without diesel or gasoline genset. More advanced methods for estimating the battery lifetime consider the time that the batteries are at low SOC, the value of the current, the effects of corrosion, gassing acid stratification, sulfation and the effect of temperature.

In other cases, the manufacturer supplies the number of cycles to failure for each depth of discharge (see figure below), then the number of equivalent full cycles can be calculated as the average of the equivalent full cycles for each DOD. Nor give good estimates in cases where the discharge current is relatively low, which is also common in PV systems.
These models are much more accurate, but also much more complex and difficult to program in a simulation and optimization software. In this case , the calculation is more complex , and we must estimate the number of cycles that the batteries performed annually for each depth of discharge (for example, by the " Rainflow " method used for calculating fatigue cycles on materials), calculating then the useful lifespan.
For example, there are photovoltaic systems in which simple calculation models can estimate a duration of 12 years or more (taking into account only the charge-discharge cycles), but in reality, due to many factors that simple models do not take into account, the batteries can last much less, say 5 years or less.

This method is more precise than the full equivalent cycles method, and is one of the methods modeled in iHOGA software. The results obtained by the advanced model are much similar to real lifespans, as seen in the article published in the Elsevier journal "Applied Energy", paper " Comparison of different lead- acid battery lifetime prediction models for use in simulation of stand -alone photovoltaic systems " (R.