When I installed the 2nd battery in the MK (which I still have) I was advised to use 12mm² wire by an old school auto sparky - I thought this to be overkill but figured over engineering cabling rarely causes problems... I have the wire terminated to an anderson plug in the tub and can install/remove the battery at will... A while back I made up a 6m extension for this with anderson plugs both ends so I could charge the battery next to the camper if required...
Setup:
redarc isolator and 100A circuit breaker under bonnet + 4m of 12mm² to the tub - add 6m of 12mm² wire extension and test the voltage at the end, 10m from the isolator - all connections hex crimped with no solder(I'm lazy)...
Result:
Readings were a constant 14.4v at the end of the wire with engine at ~1500rpm
I ran the car for 40 mins, disconnected the battery(100Ah AGM), placed my 3rd battery on the fridge and let the original battery sit over night to settle - 11.7v at charge commencement, 13.1v the next morning...
Just thought this may make the decision a little easier for those trying to figure out exactly what they need to do the job well and get the best possible performance from their electrickery work...
Blue