OK if you Are travelling along and th emain battery is fully charged, otherwise it won't switch over to charge the leisure battery.
Got myself one of those tiny maintenance chargers today from Halfiords £17.
A little larger than a phone charger, with connection wires to hardwire directly to leisure battery, then plug it in one of the 3 pin sockets inside the van. Then when on leccy hook up you just flick the on switch
and charge and maintain the batteries at the campsite expense.
If everyone knows this, then OK iv'e bored you.
But I thought it was a good gadget.
