Battery Light
 

1999 Voyager 2ltr Petrol
10 days ago (driving at night) the battery light came on while driving. Slowly everything died.I had pulled in when it became obvious that my lights were gonna go. Engine never stalled but electrics went dead. Then all of a sudden 'bingo' the whole lot came back to life after about 15mns all the while the car was on idle.
Then for 10 days not a problem....then the battery light came on and repeat as above.
Now from reading the results of a search on here the alt may not the problem.
The only thing I have checked is the alt belt tension.
Any pointers before I start at point a) which will be install new belt and check connections.
The only light that come on are to indicate this prob is the battery light (which I presume means its not getting a charge from the alt)
Thoughts?