If more energy is required by the consumers than can be provided by the PV system, the batteries are discharged. The conditions specified in the energy management section apply.
Battery charging can be divided into the current-controlled charge (C-charge), which is the standard case, and voltage-controlled charging processes, which essentially serve to maintain the batteries and increase their service life. A distinction is made between time-limited charging (V0 charging) and time-unlimited trickle charging (V charging).
In PV*SOL® an CV0V charging strategy is implemented as it is usual for most batteries based on lead acid or gel. Here a C-charge is followed by a short V0-charge from a certain state of charge in order to prevent various aging effects. If the battery is not needed in the meantime to cover the load, this is followed by a V-charge for an unlimited period of time, also known as maintenance or float charging.
In addition, two additional U0 charges are carried out in a fixed rhythm, which can increase the service life of the batteries:
If the battery is at a high state of charge and is not discharged, its self-discharge can be compensated by a U-charge. This charging method is also called maintaining charge.
The C-charge, also called current-controlled charging or bulk charging, is the normal case in which the batteries are charged with all available current. The battery voltage depends on the level (SOC) of the battery and the charging current. The more charged the battery, the higher the battery voltage rises. Therefore, it is not efficient to fully charge the batteries with maximum current. Usually the C-charge ends at about 85% to 90% of its total capacity. The energy for the C-charge is taken exclusively from the PV system, never from the grid.
The subsequent V0 charge, also called boost charge, is voltage-guided and has a predefined duration (the 0 in V0 stands for the time component of the charge). With constant voltage, the battery is charged a little full, but not quite full, over a specified period of time (usually 2 hours). The usual charge here is 90 to 95%. This is an important charging mode for all batteries, as the cathodes and anodes are cleaned here, which increases their service life. The V0 charge runs with high priority and is therefore driven out of the grid with energy if insufficient PV energy is available.
The float charge is a voltage-driven charging mode that is used when the batteries are full (after V0 charging) and are not currently needed. Only as much is recharged into the batteries as is lost through self-discharge. In this mode, the (very low) energy can be covered by both the PV system and the grid.
These two charging modes for battery care are, like the boost charge, V0 charges. They are therefore voltage-controlled and only for a predefined period of time. In contrast to the normal V0 charge, these two care charges only take place from time to time. The full charge should ensure that the batteries are fully charged from time to time to prevent irreversible deposits on the anode or cathode. The equalisation charge in a battery bank should ensure that differences in the electrical behaviour of the individual batteries, which inevitably occur over time, are compensated. These loading methods are highly prioritized. If no PV energy is available, the required energy is taken from the grid.
Lithium-based batteries do not require a full charge or equalisation charge.