Another consideration specific to Dell is if it has a "smart" power supply. Usually indicated by a barrel type plug with a thin pin in the middle.
There's a chip in there that identifies the supply to the BIOS, if the BIOS doesn't recognize it, it will not charge the laptop battery. It may also post "F1 to continue" BIOS messages, disable the use of port replicators etc. Might want to google your model to see if they have it. My 8600 does. I found an adapter made by batterybiz that worked and powered the port replicator (~$50USD), Dell now makes one as well but I believe it's closer to $75-100usd, not sure if it's powerful enough for running a port replicator or docking station.
If you have this type of supply, beware of auto-air adapters that are "Charge only"
Might want to read this thread.
Inspiron 8500 Power Question