Okay i tried but i don't have the work space to make it completely, I managed to make a rudimentary version but it's far from final. I would usually make the switching circuit with a slight hysteresis and a few other modifications here and there but this is the basic circuit. The switching point is around 14V, but to prevent switching back and forth of the relay a slight hysteresis should be added. Typically the relay has hyateresis, but the way it's driven (pulsed) it won't do any good.
