@NiallDarwin We (at FENECON) did something like this before. See this post in German:
Controller-Implementation is here: openems/io.openems.edge.controller.ess.fastfrequencyreserve at develop · OpenEMS/openems · GitHub
From experience it’s always best if the Battery Inverter can do this directly; but if not possible or sufficiently advanced for the application, EMS can do it as well.
If I remember correctly we achieved 1.2 seconds from Frequency Drop till Full Power Discharge of the battery, which was the requirement for FFR (Fast Frequency Response) here.
Regards,
Stefan