Car battery: PV surplus use with constant charging

Step by step.

Feeding PV energy surplus into the grid is less lucrative for refinancing a PV system. In addition, the technical conditions for feed-in must be met. Therefore, optimising self-consumption is the most economical solution. One way to increase self-consumption is to use the PV surplus efficiently to charge a car battery. If there is no possibility to control the own wallbox, charging is done with constant power by simply switching it on via a relay contact.

Charging the car battery with constant power

The charging of the car battery with constant power takes place after preselection of the charging current at the wallbox. The duration of charging is not recorded and is carried out via an enable/disable object until the wallbox confirms "ACCU charged".

About the functioning of the logic group

The measured values of the service recording reach the statistics element every minute via the intermediate storage (buffer). In the statistics element, 10 values are recorded, from which the average value is formed and sent every 10 minutes. The averaging and the 10-minute interval serve to smooth the measured value for the switching threshold. The upper and lower switching thresholds are set by the user, whereby the lower switching threshold is offset for switching back. This prevents charging from being stopped too early, even if this means leaving the PV overshoot range. During charging, the power measurement value must be corrected by the power value of the charging so that the switching thresholds themselves are not shifted. Enabling "ON" activates the logic group and "OFF" stops the logic group, just as the switching command to the wallbox is set to "OFF". An integrated start value ensures that the logic group has a defined output value.

Input values (settings of the user):

  • Default Power Charging: This is the predefined charging power, e.g. as specified or preset by the wallbox. If the wallbox only has a default value, enter it here. Note that there is no feedback on the charging process, so the entered value is used for all calculations. (EIS9, DPT 4 byte float)
  • Release: Activation of the pulse generator to start the logic group. With "OFF", the pulse generator is stopped and the switching command to the wallbox is set to "OFF". (EIS1, DPT
  • Current performance: The power values of the house connection meter (bidirectional meter). Positive values represent consumption (power drawn) and negative values represent yield (power fed into the public grid). (EIS9, DPT 4 byte float)
  • FB Switching: This is the feedback e.g. from the wallbox when it is in charging mode. Without active feedback, it is the same group address that is used to switch the wallbox. (EIS1, DPT
  • Switching threshold: This is the limit value for switching to the operating mode "charging". Both negative and positive values should be possible. Negative values indicate that power is already being fed into the public grid. (EIS9, DPT 4 byte float)
  • Power OFF Offset: Only positive values are valid here and indicate the value at which charging is stopped. The limit value is the sum of the switching threshold and the power OFF offset. (EIS9, DPT 4 byte float)


Initial values:

  • Switching (charging): This is the group address to switch loading on or off. (EIS1, DPT