Voyager Flat Battery
 

I have fitted a new battery to our '98 3.3 Voyager but it has gone flat after standing for just 3 days.
I've used jump leads to start it and there is a good charge rate, 14.5V, so I don't suspect the alternator.
I habe noticed the small red light on the dashboard staying on sometimes. What is the light for, and could this have anything to do with the problem?
Any help greatly appreciated.