Alternator Voltage / Replacement

[Ammonite]

From what I understand this is a standard option on Volvo engine alternators, as can be seen in the illustration here:
Volvo Penta Alternator fits most Diesel and Early petrol engines. Valeo Paris rhone comes with sensing wire for split charging.
The sensing wire is usually yellow.

Thank you! That's reassuring. Having said that you are only sensing one battery (unless the 1-2-Both is on Both) which doesn't sound like a great idea but at least I now understand how my setup is configured. Having taken a closer look at my alternator I beginning to think it's been modified to up the voltage judging by some distintly DIY soldering which in combination with the sensing wire setup would explain what I've been experiencing.

 

[BabaYaga]

Your setup, having both a low loss splitter AND a 1/both/2/off-switch, seems unusual and, as noted above, without information on how it is wired it is difficult to give further advice.