Most Solar Feed-in-Tariff's require that you do not expand your existing solar panel setup, that you do not upgrade your solar inverter, and that you don't feed energy back into the grid that has come out of anything except (directly) from your solar panels.
So, here's the thing - most battery inverter/chargers can be configured to never send electrical energy back to the grid. Instead, the stored battery energy will only be delivered to your house (as 'self consumption' of locally generated energy).
One way to help prove to a grid operator (via your installer) that your battery system is genuinely incapable of exporting energy to the grid (and hence that it is not classed as a 'generator' in their view) is to install an additional device in the circuit, called a 'zero export control' box. This device independently measures the energy flowing to and from your home and will disconnect your battery inverter if it ever tries to export energy to the grid.
If you are seeking to install a battery and to avoid losing an existing Solar Feed-in-Tariff, make sure you raise this request with your installer, and ask them to draw up a suitable proposed installation diagram (the industry calls this an SLD, or Single-Line Diagram), that demonstrates the scenario suggested here.
This diagram is then submitted by your installer to your grid power operator, seeking their prior approval to create your ZCell system in a manner that will not result in you losing your Feed-in-Tariff.
Should your grid operator reject your request to install ZCell and yet retain your Feed-in-Tariff (as submitted by your installer), you can then consider whether to proceed with the battery installation, or whether to wait until your Solar Feed-in-Tarriff has expired before adding a battery to your home.
In the future, grid operators are expected to shift their stance and to actively seek to purchase power on demand from home battery owners. At that point, the 'zero export control' box can be removed (if present) and/or the configuration of your battery inverter/charger changed in order to allow energy to be exported back to the grid from your battery at times that the grid operator is prepared to pay you to do so.