[entwisi]

Dodgy alternators don't always knacker batteries, its when teh recitifier is knackered that they tend to take batteries out as this tends to overcharge the battery(or feed AC current to them instead of DC). If the windings on the alternator breaks down then it tends to drop the output voltage (which you rightly say is ~14.5V). However if this was the case teh van would never be starting as teh battery wouldn't be being recharged at any point.

I still reckon there is something left on that is bypassing the ignition switch, Vans often have a small light in the back so they can see what they are doing, these often get forgotten. Or some people wire stereos in incorrectly such that they are permanently live as opposed to just the memory lead being connected.

I think you were also linking to not jump starting modern vehicles as the ECUs can be damaged when you do this and they often cost ££££s to replace.

One other point, you should always remove the earth (-ve) lead first when disconnecting a battery.