Smart battery charging on the GC is complex, very complex!
The engine has a smart alternator that like all modern vehicles, attempts to minimise fuel consumption by "pulse" charging. Basically it tries to only load the alternate when you are decellerating, ie it recovers energy from the Kinetic energy in your vehicle at speed.
note the use of the word "attempts" because this ^^^ rule is the most basic rule ie try not to charge unless the vehicle is decelerating.
BUT, there are a HOST of other rules that come into play
1) don't charge if batteries are already charged
2) try to keep all batteries at 80% State of Charge (SoC) to ensure there is always spare capacity in the battery to accept a charge when you do slow down
3) maximise battery life by keeping the voltage within certain limits expecially when the battery is fully or nearly charged (float voltage)
4) make sure there is ALWAYS sufficient battery power to start the engine
And of course, in a GC, we have TWO batteries as standard, linked with a charging relay, controlled by the "J608 Special vehicles" control unit. Both batteries have negative terminal sensors that measure the voltage, current and temperature of each battery, and the system uses those measurements to estimate both the State of Charge and the State of Health (capacity) of each battery. The starter battery is connected with the alternator to the main engine control unit (underbonnet) and the leisure battery is connected to the Special Vehicles control unit under dash.
Added to which, there is an onboard charger, that when the vehicle is plugged to an Electrical Hook up, charges both the batteries, and you may also have a solar panel on the roof and a charge controlled in the back charging the leisure battery as well, depending on how sunny it is!
What this all means in practise is that exactly when your alternator charges, and the instantaneous system voltage of those batteries at any given moment varrys a LOT and is difficult to guess!
Normally, to get a positive current output from the alternator ie an output voltage demand that is HIGHER than the current battery voltage (drives current out of the alternator and into the battery(s) you need at least one of the batteries to have an SoC below 80%. If both batteries are above 80% SoC, no charging is requested (unless you have the "boost charging" button on the dash pressed that increases the idle speed and raises the SoC threshold to around 95%. If batteries are at above 95%, pressing that button does increase idle speed but no additional charging takes place.
Once some charging has occured, charging will be stopped when both batteries raise above 80% SoC (unless boost charging is pressed then it's 95%). If a high current demand (looks to be approx >15A alternator output) is present when the batteries reach there charged thresholds (80 or95%) the system will drop into a medium level where the alternator holds a voltage output level which is sufficient to supply the current demand, but not high enough to drive a large amount of charging current into the batteries, this is around 13v typically)
As the split charge relay is controlled by the logic, normally once the engine is running, both the starting battery and the leisure battery get coupled together by that relay closing, so typically the SoC of each battery converges, ie the higher SoC battery bleeds charge into the lower SoC one, even when the alterator is not charging.
So, that having all been said, you can do a couple of things to prove your alternator system is working (honestly, if you're batteries aren't totally flat, it's almost certainly working!)
1) leave the vehicle parked, at night with the ignition on and with some electrical load on (headlights etc) to discharge the batteries below 80%. Probably 15min with headlights and fans running ought to be enough, if you have heated seats, these can be turned on to draw more current
2) monitoring the voltage of the battery, start the engine and drive off. If SoC is low (approx <60%) charging will commense immediately, even at idle (>14v showing) if it is between 60% and 80% charging should only happen when you drive off and slow down, ie lift off the accelerator in gear using engine braking
3) after driving for a while, normally around 30min, make sure everything is off that can be (lights, heaters, heated seats). You should see the system stop charging, dropping to approx 12.5v
If the voltage stays at over 14v continuously, you have two possibly outcomes
1) the system things your batteries are effectively end of life, ie the State of Health is less than 65% (approx). The system will disable smart charging to try to ensure with these degraded batteries you can still start the engine always
2) there is a fault with the system (or it's been deliberately bypassed by someone) that prevents the system from getting data from either battery smart sensor or from the alternator itself. Wiring faults on the LIN data bus between these devices and the engine controller and/or special vehicles control unit will put the system into the safe fall back state where smart functions are disabled and the alternator simply outputs approx 14v at all times. Some people deliberately disable smart charging by disconnecting parts of the system, normally the battery terminal sensors to force 'dumb'charging!
