David brought next idea :
Does this work directly off the alternator, or via a 12v cigar lighter
socket?

The requirement is for something cheap and simple which does not involve
any extra wiring in the car.

I realise the output from a 12v socket will not have the same
characteristics as a direct feed from the alternator via a charge
controller.

It would provide a more rapid and consistent charge for a second
battery. If you tow the caravan and have the 12S socket or the fully
wired 15(?) pin socket, then you should have a voltage controlled relay
already system wired up in boot.

I have and tapped off that with a none reversable plug and socket, to
allow a battery to be charged in the boot in emergencies.

The problem with using lighter sockets is they are current limited,
there will be lots of voltage dropped in the wiring and the connections
are just not reliable at the plug and socket.