Battery model

The model should describe all the behaviors necessary for the simulation, namely the operating voltage, capacity, stored energy, ageing.

Voltage model

The model should evaluate the battery voltage at any time, as a function of the State of charge (SOC), the current, the temperature.

Accurate operating voltage determination is essential when controller decisions are based on voltage thresholds (as used in most practical systems). SOC-based controls are less common, as SOC is not directly measurable.

For Lead-acid batteries PVsyst lets the choice of the control mode to the user, either working with the voltages, or on the SOC calculation (see Controller operating thresholds) . And therefore the voltage is essential

With Li-ion batteries, where some technologies show very low voltage variations and where charge start/stop points are not always well defined before entering dangerous regions, we allow only SOC-based control, which is well-defined in the simulation. The BMS typically calculates SOC, making it suitable for real-world controllers.

The voltage is defined in 2 steps: a basic open-circuit voltage (i.e. without current) and the full voltage in operation.

Capacity and State of Charge

The main parameters of a battery pack are its capacity and nominal voltage. The stored energy is closely related to the capacity and the state of charge. A significant challenge during simulation is that capacity depends on charging/discharging rate, temperature, and aging state. This prevents achieving precise energy balance during simulation. NB: In PVsyst, the nominal capacity is always defined as C10. i.e. a discharge in 10 hours.

Aging, wear and tear

During simulation, aging depends on operating conditions. PVsyst distinguishes between "static" longevity (named SOWStatic)—aging when the battery is not in use (primarily dependent on temperature)—and deterioration due to use (charge/discharge cycles and depth of discharge), named SOWCycles. The worse of the two values is used at each simulation step. This is useful for determining battery replacement timing. Battery end-of-life is typically defined when wear state reaches 80% of initial capacity. However, some manufacturers now specify their maximum number of cycles for a wear state of 70%.