question

Bluetooth is a poor plan, it isnt stable enough if it worked at all over that distance.

RS485 or an analouge signal, current preferably, is the way to go, Ethernet is also an option but will cost more.

What you need to do is control the charge base on the voltage of the small bank.

If the charger can be comunicated with you could do this dynamically, limiting the AC load.

That said you could also manage the charge as a duty cycle, simply turning it on or off.

A simple external PLC or DSP could easily handle both tasks with dynamic control being the best option whilst duty cycle control being by far the easiest to impliment.

If you went the latter route you wouldnt actually need and comunication as the inverter on the small bank could simply be turned on and off, based on battery voltage.

I am not suggesting you cycle the battery... If you measure the load when the inverter is on that will tell you what the charger at the other end is doing, but more importantly it will tell you what the 100% duty cycle load is. You will have a target load to aim for based on battery voltage so all you have to do then is calculate the duty cycle yiu require to achieve that as an average.

Depending on the size of your bank you would probably want a base frequency for the duty cycle of several minutes or even tens of minutes.

You would also want to set a minimum on and off time so that neither the inverter or charger is cycled too quickly. This would be best implemented by limiting the upper and lower limits of the duty cycle and forcing either 100% or 0% when the bounds are reached.

Victron may well have a product that can do either of the above but if not it wouldnt be hard to manage the stuff you have in this wy.

Hope this helps

Al