I'm having an issue today where upon starting my van after 20 to 30 seconds the battery warning lights will come on have looked under the Bonnet and can see no significant problems i.e. fan belt hanging off. And have not had any running issues as of yet.
Any ideas what this might be?
2007 Ducato 2.2

Edit. Have noticed the battery light will go off over exactly 3000 revs

Bad earth engine to body, bad connections to alternator, or even the alternator itself on the blink, is where I'd start investigating. Some pointers to get you started anyway !

Stick a voltmeter across the starter battery terminals and get somebody to start the engine.
Watch what happens to the voltage.
It should rise pretty quickly to 13.8 to 14.5 volts (dc) and stay there whilst the engine's running.
The battery light will only illuminate if the alternator charge voltage is less than or equal to the standing battery voltage before starting.
An analogue meter (one with a needle) is easier to interpret than a digital one as you can easily spot the charge voltage trend.
The auxiliary ribbed alternator belt should have about 15mm flex on the longest run between pulleys.
A dip in charge voltage should be simultaneous with the lamp illuminating if you have an intermittent alternator problem.
Is the battery keeping its charge ok?
Are the battery terminals clean and secure?
The battery warning light is crude but effective differential voltage detector.

probably alternator (diode) as suffer from water ingress despite the rather poor afterthought shield.