Split charge system for my defender?

Hi Guys, I have a bit of extra kit fitted (or soon to be fitted) to my defender, just wondering if it is worth fitting a split charge system, and if so, what should it and the second battery be rated to?

Extra kit:
6 x spot lights
300w inverter
compressor
rear work light
fridge
winch
USB and cigarette lighter ports

Cheers.

For my second battery I went with a voltage sensitive relay that detects when my main battery has 13.3v or more then it lets charge go through to my second battery, but when my main battery gets down to 12.8v it cuts off power to my second battery.

Do you intend to only use the kit when the engine is running or for camping?
You need to work out your current draw for each piece of kit your running and roughly how long each will be on for.
If your running them with the engine on, what is your current rating of your alternator.
To work out what the current draw is for the things your going to plug into the inverter use the formula.

Amp = watts / volts

Say a 240 volt portable led tv is 36 watts and you want to run it through the inverter you will do. 36 watts divided by 12 volts = 3 amps draw.

My intention is to use most of the kit when the engine is running, except the fridge, work light and possibly the inverter for short periods.

The simplest method would be to use a split charge relay for towbars. These usually have 2 voltage-sensitive outputs. You'd connect one to the second battery and the other to pin 6 of the 12S towbar socket if you've got one fitted. This way the two batteries are only connected when the alternator is charging. The fridge and anything plugged into the 12v sockets can then be left running without draining the starting battery.

Any kit that you want to run only when the ignition is on can be switched through a relay which is triggered by the ignition switch. This makes sure the batteries are kept separate.

+1 for a towing relay, if you need more amps join both outputs, if you need more again then use it to switch a large relay