A quick way to check is to pop the bonnet just before you lock up for the night and take a voltage reading between the positive jump start post and a suitable earth. Next morning, before starting the engine, repeat the exercise and note the difference in readings. I suspect you'll find the system voltage displayed is reading lower than the actual on the volt meter.
Elsewhere on this forum are posts explaining that the 'smart alternator ' is designed to operate to maintain an 'overhead/surge capacity' of 20% in the charged capacity of the battery to make space for the 'harvested' charge when the vehicle is slowing down /over-running down a long grade etc. If you watch the behaviour of the charge meter on the display you'll see the charge rate increases significantly at this time.
The idea is to minimise draw from the alternator and supposedly save fuel when under throttle, but get rapid charge in bursts when there's min. throttle required.... fear not! It's a common adaption in many vehicles even without the dual battery arrangement peculiar to the Grenadier.
As explained elsewhere, the primary purpose of the second battery in the Grenadier is to pulse charge the main battery to maintain it in optimal condition and provide backup redundancy. The Smart pass 120s enables both batteries to automatically operate in parallel to provide sufficient juice to start the engine in the event that both have been somewhat depleted....
