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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, along with nominal voltage, is the main characteristic that defines a battery. In PVsyst, we always define nominal capacity as C10, that is, a discharge over 10 hours. Capacity is not 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.

  • Lead-acid battery manufacturers typically define nominal capacity as a discharge over 10 hours, denoted C10. However, stand-alone solar systems typically involve discharge rates of 20 to 100 hours (a full discharge over 3–4 days of use). Therefore, many solar battery manufacturers specify the nominal capacity as C100 instead of C10, which is significantly different (about 40% higher).
  • Li-ion battery nominal definitions are typically for discharge rates over 2 or 5 hours. Additionally, the notation has changed: a discharge over 10 hours is now denoted 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, reference capacity is always defined as 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 expressed in kWh or MWh.

Useable Energy

When sizing a battery pack for a system, relying solely on nominal battery energy can be misleading, as the energy that can actually be stored and delivered is typically 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.