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Battery operating limits

Lead-Acid batteries

Lead-acid batteries have no specific limitations in PVsyst parameters or datasheets.

However when using a battery pack in a system, PVsyst will check that the charging / discharging rates are reasonable. The limits, producing errors at the design time, are specified in the advanced parameters, and are modifiable: maximum charging rate in 2.5 hours, and maximum discharge in 2 hours.

The usual "starting" batteries for cars are designed for delivering high currents during a short time (typical rate of C1). However they are not well suited for solar systems.

"Solar" batteries are intended for discharge rates corresponding to C10 or higher (i.e., current less than 0.1 times capacity). While peak discharge currents may occur (such as for motor starting), normal operation in standalone systems involves charging the battery over at least one day of solar generation. Load profiles should avoid prolonged high-discharge periods.

NB: In practice, lead acid batteries are never used in grid storage systems, where the problematics is different.

Li-Ion batteries

Most battery blocks are managed by a BMS (battery management system)—an electronic circuit that monitors voltage and current. These typically include built-in protection systems.

The datasheets usually mention the following limitation:

  • Minimum and Maximum voltages: These are absolute limits that should never be exceeded. In PVsyst simulation, the voltage model never reaches these values. In practice, however, applying excessive voltage increases current to prohibitive levels (due to internal resistance voltage drop). High battery terminal voltages can only occur when the battery is fully charged, creating a risk of battery destruction and explosion.
  • Maximum charging and discharging current: For Li-ion batteries, specified values typically approach the capacity (i.e., discharge in one hour). For solar systems, this is not recommended, as high discharge rates significantly impact battery longevity. NB: It is not always clear on the datasheets whether the values are "nominal operating" or "maximum" values, and during which time. Some datasheets specify a value close to C2, which probably correspond better to "nominal" currents. In the PVsyst database, the "Maximum Current" parameter is the datasheet specification, whatever its interpretation. By the way this value is not used in the simulation, only for warnings at design time.
  • Temperature range: Most batteries specify a minimum charging temperature of 0°C, sometimes -10°C. Discharging is often allowed at lower temperatures than charging. In PVsyst, we do not store maximum operating temperature as a battery parameter, so it is not considered in simulations.