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Battery capacity

Definition

The battery capacity is the cumulated current that you can draw from a fully charged battery over a reference time. It is expressed in [Ah].

The nominal capacity is the main characteristic, along with the nominal voltage, that defines a battery. In PVsyst, we always define the nominal capacity as C10, i.e. a discharge over 10 hours. The capacity is not a constant; it depends on:

  • The charging/discharging rate (or current). This variation is very high for lead-acid batteries and lower 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 battery operating temperature.
  • The battery degradation (aging). The end of life of a battery is usually defined when the capacity has degraded to 80% of the initial capacity.

Nominal capacity

As the capacity varies strongly 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 over 10 hours, noted C10. However, typical stand-alone solar systems involve rates on 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 (about 40% higher)!
  • With Li-ion batteries, most nominal definitions are for a discharge rate over 2 or 5 hours; moreover, the notation has changed: a discharge over 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 equal to half the capacity, i.e. a discharge over 2 hours,
  • 0.2C = C5, a discharge with a current equal to 0.2 × the capacity, i.e. a discharge over 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 manufacturer’s specifications (e.g. 0.5C or 0.2C, or even C100).

Nominal Energy

The nominal battery energy is the amount of energy that the battery can theoretically deliver at its nominal conditions. It is derived from the nominal capacity and the nominal voltage and expresse in kWh or MWh.

Useable Energy

When designing a battery pack for a system, relying on the nominal battery energy can be misleading, as the energy that can actually be stored and delivered in practice is lower.

PVsyst defines the usable energy as the nominal battery energy multiplied by the allowed State of Charge (SOC) range. This reflects the fact that batteries are not operated over their full SOC range, in order to preserve performance and lifetime.

For example, consider a battery with a nominal energy of 10 kWh. If the SOC limits are set to 20% (minimum) and 95% (maximum), the usable SOC range is 75%, and the usable energy is therefore 7.5 kWh.