Thats very helpful Joe thank you. I am sure you are right about the cabling. You can see from my earlier posts that I had already reached that conclusion. I just looked up current ratings for cable and agree with your sizing. The charge cable will end up bigger than the starter motor cables but I guess they only have to pass current for a short burst. The charging line cables have to pass current for extended periods....
Yes the voltage mentioned is at the alternator and showing up on the engine panel. I actually have three batteries on a 3 way split diode. There is a shunt on each line from the diode splitter to each battery which is also showing a high voltage (15V) but I am advised that this is probably due to the sensor wire going to the battery side of the diode rather than the battery terminal itself. This I shall also change. The shunts are wired directly to a Stirling battery management panel which allows me to see volts and amps to each battery. The sensing wire goes to the battery supplying the fridge, eberspacher, lights etc, which is going to be the highest discharged. The second battery serves instruments and essential services, The third battery is for engine cranking only and seldom gets heavily discharged.
There are 2 sets of dip switches on the Stirling. One is set for voltage re wet or gel batteries. The other controls the charging cycle. for a small alt on big bats it extends the boost cycle time. The other way around reduces it. They are the ones I will need to reset to avoid boiling batteries with a bigger alternator.
I can't try the old alternator Joe, it does not work! The whole point of the exercise is to aviod spending money on repairing it when it's too small anyway. I think in a way it's a blessing in disguise that it has failed as I now have an excuse for buying a bigger one and upgrading the installation generally.
Thanks a whole lot for your very helpful input (and that of others of course) that in true forum tradition has helped me to get the details clear in my mind. I shall post again when the project is completed and tell you how I got on.