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Battery operating limits

Lead-Acid batteries

There are no specific limitations for lead-acid batteries, neither in the PVsyst parameters, nor in the 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.

The "solar" batteries are supposed to be used with rates corresponding to C10 or more (i.e. current < 0.1 * capacity). There may be some peak discharging currents (e.g. electrical motor starting), but the normal operation in standalone systems is to charge the battery in at least one day of solar yield. The load profile should not involve hours with too high discharges.

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

Li-Ion batteries

Most of battery blocks are managed by a BMS, i.e. an electronic circuit which handles the voltage and current of the block. These have usually built-in protection systems.

The datasheets usually mention the following limitation:

  • Minimum and Maximum voltages: these are absolute limits which should never be exceeded. In the PVsyst simulation, the voltage model never reaches these values. However in practice, if you try to apply a high voltage, the current will increase up to a prohibitive value (voltage drop in the internal resistance). The only possibility to reach high voltages on the battery terminals is when the battery is fully charged. In this case there may be a risk of destruction of the battery, and explosion.
  • Maximum charging and discharging current: with Li-Ion batteries, the specified values are usually close to the capacity, i.e you can discharge it in one hour. In batteries used for solar systems, this is not recommended, as high rates have a big impact on the 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 are specified for a minimum temperature of 0°C, sometimes -10°C, during the charge. The discharge is often allowed by lower temperatures than the charge. In PVsyst, we don't store the maximum operating temperature as a battery parameter. This is not taken into account.