Problem. These vans drain batteries fast anyway if parked. Theres a multitude of small things that take power such as the remote key entry receiver, radio memory, engine management memory, alarm,clocks, etc. Say in a month parked IF the battery is quality and fresh. Add the computer in the boot, and another key entry rx running all the time and shorten that to 2 weeks.
But the battery deteriorates fast because batteries love being full, hate being even partly discharged. And being too flat to start the engine several times means they are basically ruined. Even though they seem ok once jump started. So it happens over and over. And then its dead in a week.
A PC1500 Odyssey helps, in ONE WAY, in that the battery is a true deep cycle battery, so it doesn't deteriorate as fast. And that even if half flat its got enough cranking amps to start the van. But it wont cure the problem. Only adding power will do that.
I used to leave a maintenance charger connected when not in use. But its a pain. The real cure, is a 50 watt solar panel on the roof at the rear. And a charge controller that CORRECTLY charges and maintains the battery. And a PC1500. I did this years ago and its faultless even in winter after months sat unused.
http://www.ebay.co.uk/itm/Lensun-20W-30 ... xsPenGV4nwThere are cheaper panels. Smaller wont do the job. The flat angle means you get less power. The winter means it can be down to about 2 or 3 watts for just a couple of hours a day. This is efficient, encapsulated, on a fiberglass backing, and will survive years on a car roof. You cannot see it. I wouldn't use anything else. Mine is simply stuck with clear silicone rubber sealant as used on baths. The cables extended, run down the rubber in the tailgate, and the charge controller below set to 13.4V is used. It connects to the wiring just behind the right side plastic side panel, wherever you find battery voltage with the key in your pocket! Actually that bit was dead simple.
http://www.ebay.co.uk/itm/371837291938? ... EBIDX%3AITThere are many charge controllers. I have 6 now. THIS ONE does the job correctly, can be set to a long term low 13.4V float, and has proper solar panel tracking for increased power from the panel, useful in winter when light is low. This lives next to the computer/spare wheel in the rear.
Dont think one of those solar panels you stick in the dashboard and leave in front of the windscreen will do the same job. It wont! Tried a 20 watt one. Useless. They dont work through glass and dont have enough power in winter.