OK, I'm getting my act together slowly, and I can run the laptop via the DC/DC converter and it draws ~21W from the 12V battery, nice!

Problem is, the start-up current of the converter is so high that it immediately triggers the load short-circuit protection on the solar controller.  So I can't just treat it as a normal load from the controller.

Therefore I'm going to have to modify my circuit be its own low-voltage drop-out with power drawn direct from the battery, and I think I'll have to use FETs as you suggest.

Can I simply put a P-channel FET in the positive line from the battery, with the gate pulled up to the battery positive voltage with a resistor, and 'let go' to cut the current to the load (and pull down with an open-collector output to allow current to the load)?  Or will that move the gate voltage too slowly and cause it to overheat?

My DC/DC converter has a peak load of 150W (though I shall never go over about 60W) and an input fuse of 15A, so allowing healthy margins all round I've picked out an Infineon SPU30P06P 30A (60V max), and would pull up its gate with something like a 10K resistor.  I'd pull it down via an open-collector 'voltage OK' output in series with an LED for a small 'current on' indicator.

I propose to set the drop-out voltage at 12V and the reconnect voltage at 12.8V to 13V for some decent hysteresis.

Does that sound sane?

Never used a bare p-channel power FET before...  Your guidance would be much appreciated.

Rgds