i got one from ebay that connects to the d+ wire on the alternator will never get a flat battery in van as it dosent connect to the main battery wires it connects to the d+wire which when you switch your engin on it makes the d+wire live when switched off it is not live i think the d+wire is the one that goes to your instrument pannel but don,t quote me on that
may be someone like jeff the electic windowcleaner can throw some light on this
hope it helps