MarkF that makes more sense to me. I assume that 1R0 is 1W means 1 WATT resistor.
errr nope. 1R0 = 1.0 ohms (at 1W as stated)
Working with V=IR, as current increases, voltage across the resistor also increases following that formula. At near 0.6V ~= 600mA, it will start to conduct
and start bringing down the Vop. If you need more current,
decrease the resistor value and vice-versa
Note: That type circuit is really only for "Trickle charging" at ~ 13.8V, it has no boost capability, as usually ~14.4V. That gets trickier :-)