I am looking to install a second battery in my 2012 T5 Crew Van 132kw and am looking for some technical advise on the best set up.

I have read all about Split chargers with Isolators for the main battery and have now come across information about CAN-Bus equipped vehicles having variable output alternators. I have read the UK forums on this as well but all this information is leaving me unsure of exactly what I need.

My plan is the install a 100AH AGM battery under the passenger seat. This will have relatively light duties being used to run:
A cooler
Charge mobile phones
Charge computer if necessary
Maybe an inverter to run a heater
some lights

So the actual drain on a 100AH battery should be ok, my question is what is the method of charging the battery.

I have looked at 140Amp split charging systems that draw from the main battery with an low voltage isolator.
or
BC-DC battery charges like the RedArc BCDC1225LV, that is basically a battery charger drawing power from the main battery and amplifying to charge the AUX battery. This type also can run solar panels. (not doing this but maybe later)

If the 140Amp split charging system would work in the van I would prefer to go this way on a cost basis but my real question is would this work.

Any help on understanding how the battery/electrical/alternator system works would be appreciated.
I did a massive search on the weekend for this and really can not find an complete answer.

Cheers
Andy