https://smedley.id.au/tmp/org.openhab.binding.sonnen-5.0.0-SNAPSHOT.jar contains (untested) support for force discharging a battery. Code is at GitHub - psmedley/openhab-addons at sonnenforcecharge
The code is based off off @Christian_Feininger 's work to add support for charging. The code in communication/SonnenJSONCommunication.java could be made much simpler - right now the code differences between the functions to charge and discharge amount to about 2 lines - the URL, the config parameter and the error message.
If anyone can confirm this works, I’ll look to simplify the code and raise a PR.
Note that while the filename says snapshot 5.0, it’s built with Java 17 and OH 4.3 compatability, however, I’m not sure if any of the new tags added in other PRs as part of OH5 will break OH 4.3 compatability.