http://www.freel2.com/gallery/albums/userp...iagram.pdf

The wiring diagram shows that both Pins 9 and 10 are feed from 15amp fuses from the junction box rear.

The ignition feed (pin 10) has, in series, 2 relays, one controlled by the cars ignition, and one controlled by the trailer module.

The battery feed (pin 9) is always connected to the battery.

In light of this, I believe, to protect your car battery, that it is wise to either charge via pin 10, or use this feed, via a relay, to control a feed from pin 9. They are both the same 15 amps max. FL2 XS SD4 Auto 2010 2012-2017 (21k - 91k miles) (MY2011)
FL2 Metropolis SD4 Auto 2014 2017- (16k - 76k+ miles) (MY2015)

My initial query as to whether pin 10 could be used for charging was was prompted by the fact that it has the relay controlled by the trailer ECU. What purpose does it serve? The normal convention is to use pin 9 - constant supply - to charge the battery, controlled by a relay operated from pin 10. My thinking was to dispense with this relay and just use pin 10 directly. Hence my query.

Prompted by the link to the DC to DC converter above (which doesn’t seem to be an actual battery charger) I found this device - the Votronic VCC 1212 20. It is apparently designed to connect to the 13 pin trailer plug and charge a leisure battery in a caravan (or trailer).

https://www.votronic.de/index.php/en/produ...escription

Not inexpensive at around £150 but it would not only do the job now, it would 'future proof' the trailer in the event of me acquiring a newer tow car with Euro 5 or Euro 6 charging. Time to start saving.

It would seem that the Votronic unit is not suitable for batteries smaller than 50Ah (although I’m still trying to get clarification from Votronic about this) but this unit from Victron apparently has no such constraint - https://www.victronenergy.com/dc-dc-converters/orion-tr%20smart

Seems like the best solution for my needs - YMMV