Which battery should you use with the A3 flight controller, or - is there a way to use the (for example) M600's batteries with its Smart Connector?
Even the lowest 'low battery' warning settings of the A3 seem quite high with a normal 6S. I hoped this could be circumvented with DJI's own batteries. But there seems no way to connect these in the first place. Does anyone know more?

There is no need to charge your A3 flight controller, as it is already integrated into the M600 and runs off of the 6 intelligent batteries.