How long do you leave your 991 undriven before charging the battery?  The manual says six weeks, which seems too  lonng.  If I park it for six weeks, should i use a trickle charge like the porsche one that plugs into the cigarette lighter?

http://www.amazon.com/dp/B001684YCK/ref=pe_385040_30332200_pe_309540_26725410_item