The x3300w bb control system is an all-rounder. From measuring fps, counting shot bbs, counting back bbs and even functioning as a tracer unit.
The control unit can indicate your battery voltage itself, and thus warn you of low battery voltage.
The only thing missing is a black cap at the top. You could have this 3D printed. It has never been used and is still new in the box. New price was € 149.00.