Option 2 may be called a split charge relay by that vendor, but it’s not. It’s just an alternative way of connecting two batteries in parallel for charging once the tow car's engine is running, i.e. when the ignition light goes out. A split charge system charges one or the other, but not both, and is superfluous nowadays with high output alternators.
But to go back to the original question, the standard way of charging an additional battery in a trailer is to connect a relay - the 'habitation relay' - to pin 10 and have this relay connect pin 9 to the battery. But given that as far as I could determine when sorting this out on my V50, pin 10 is live once the ignition is switched on and not just once the engine is running, it may be advisable to fit a voltage sensing relay to pin 10 so that power to the habitation relay, and thus to the auxiliary battery, only comes on once the alternator is charging.
In my case, the battery in the trailer is only for powering a winch and there was no habitation relay so I just connected the trailer battery to pin 10 via a voltage sensing relayed mounted on the trailer rather than in the car. This ensures that if I use another car to tow the trailer it will still charge the trailer battery without risk of trying to draw power from it when starting.
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Simon J
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