The typical heat pump models in TRNSYS take a control signal and the entering conditions and calculate the leaving conditions and the input power required. That power could be 'removed' from battery storage and the amount of additional power needed from the grid calculated. If you want to reduce the heat pump capacity based on the available power in the battery, then you will need to rewrite the heat pump model to take power as an input. This might be more difficult that it sounds since the data files used to model the heat pump performance are not set-up to provide coverage of the heat pump performance based on changing input power.