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Battery capacity

Definition

The battery capacity is the cumulated current that you can draw from the fully charged battery during a reference time. It is expressed in [Ah]. The nominal capacity is the main characteristics, along with the nominal voltage, which defines a battery. In PVsyst we always define the nominal capacity as C10, i.e. a discharge in 10 hours.

The capacity is not a constant, it is dependent on:

  • The charging/discharging rate (or current). This variation is very high for lead-acid batteries, less for modern Li-Ion batteries. The rate variability is the main reason why the State of Charge (SOC) is not always well defined, which leads to uncertainties in the charging/discharging balance.
  • The temperature, this is expressed as a profile in PVsyst.
  • The aging. The end of life of a battery is usually defined when the capacity is degraded to 80% of the initial capacity. Recently, some manufacturers specified 70% on their datasheets. The ageing diminution of the capacity is not yet explicitly implemented in the PVsyst simulation. This will be done in a next version.

Nominal capacity

As the capacity is highly varying with the discharge rate, it is necessary to specify the rate at which it is defined.

  • With lead-acid batteries, manufacturers used to define the nominal capacity for a discharge rate in 10 hours, noted C10. However usual stand-alone solar systems involve rates of the order of 20 to 100 hours (a full charge corresponding to 3-4 days of use). Therefore many manufacturers of solar batteries specify the nominal capacity of their batteries as C100 instead of C10, which is of course quite different (40% higher)!
  • With Li-ion batteries, most of the nominal definitions are for a discharge rate in 2 or 5 hours; moreover they changed the notation: a discharge in 10 hours is now noted 0.1C.

On most Li-Ion datasheets, you will find a capacity defined for:

  • 0.5C = C2, a discharge with a current of half the capacity, i.e. a discharge in 2 hours,
  • 0.2C = C5, a discharge with a current of 0.2 x the capacity, i.e. a discharge in 5 hours.

In PVsyst, the reference capacity is always defined at C10. In the Battery definition dialog, you have a tool for evaluating the C10 capacity from the specification of the manufacturer (e.g. 0.5C or 0.2C, or even C100).