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.

An accurate operating voltage determination is essential when the controller decisions are based on voltage thresholds (as involved in most practical systems). SOC-based controls are less used, as the SOC is not directly accessible to measurement.

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 for some technologies the voltage variations are very low, and the begin/end of charge are not always well defined before attaining a dangerous region, we have only allowed using the SOC, wich is well-defined in the simulation; the SOC is usually calculated in the BMS, which make possible to use it in the real 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 big problem during the simulation, is that the capacity is dependent on the ]charging/discharging rate](./capacity-vs-discharge-rate.md), the temperature, the wearing state. This prevents having a quite accurate energy balance during the simulation. NB: In PVsyst, the nominal capacity is always defined as C10. i.e. a discharge in 10 hours.

Aging, wear and tear

During the simulation, the wear and tear are depending on the operating conditions. PVsyst makes a distinction between the "static" longevity (named SOWStatic), i.e. when the battery is not in use (depending namely on the temperature), and the deterioration due to the use (charge/discharge cycles and depth of discharge), named SOWCycles. The worst value of SOWstatic and SOWcycles is chosen at each simulation step. This is useful for the determination of the replacement time of the battery. The battery life is usually admitted when the wear state attains 80% of the initial capacity. However recently several manufacturers specify their maximum number of cycles for a wearing state of 70 %.