by the dutch guy » Sat Jun 23, 2018 11:34 pm
because a lot will lower the voltage to 13.8~14V when the starter battery is full again. but it will still go up to 14.5V just after starting and activate the relay. it will then charge both batteries until it sees the batteries are full and lower the voltage.
some relays will switch off below 14V and cut the aux battery from the alternator while the engine is still runnning. it is not really a problem because the aux battery is already full and under driving you dont have any load on the aux battery anyway.
test the relay and at what voltage it cuts off and see what voltage the battery terminal have holds after 30min of driving and without any loads while the engine is idle. if its more then the relay trigger voltage you are good.
Mk6 SWB TDCI 125