Maybe the AC adaptor is dead. Can you run the notebook direct from the AC adaptor without the batteries?
If this works I suspect you're looking at a failed charger module. I forget whether this is on the motherboard or a daughter-card, but its somewhere inside the case. If you want to investigate this further you should find the maintenance manual on the Dell site, probably under Downloads or Support but they often need a determined search. Spares probably available from PartsPeople - or try ebay if you like the risk of getting a dud.
You will need a motherboard, as the charger is built in. Also, be aware there are many different part numbers, and not all work with all CPU's fitted. Make sure the part number is the same, or be certain it will work.