I want to charge a second battery in the boot of my freelancer 2 sd4. The idea is to have a battery in a box in the boot connected to the car with a Litton plug so I can take it when on holiday in my caravan so when out for the day it will charge up in the car then can be used in the caravan on our return. I already have solor panels and two battery's in the caravan but early and late in the year they can struggle to keep up with swmbo's TV watching. Question could I take my charging feed off number 9 pin wire (13 pin towing electrics fitted) in the boot using a mayflower 30 amp auto charging relay. Turn off your TV turn on your brain
List of previous vehicles
Series 3 short wheelbase
Range Rover v8
Disco300tdi
Disco td5 best landrover ive owned ever
Now Freelander sd 4 xs

What you need is a split charging system.
This allows your main vehicle battery to charge, when fully charged, switches over to your auxiliary battery (in the boot).
You must maintain the vehicle battery at maximum.

The split charge system is similar to that in your caravan and when you have electric hook-up and the vehicle is connected to the caravan, it will also charge.

I have a set-up in my vehicle which does the very same, as generally I run Off Grid.

Remember to vent the area where the battery is located.