(A) Check your battery is not at the end of its useful life, (B) check your adapter is functioning correctly. On later model DELL laptops that come with 70w adapters, you have to turn the computer off (whilst still plugged into the AC adapter) in order to charge. With 90w adapters, they will charge while the computer is on, but only if the adapter recognition circuit is working. If it fails (common problem on PA10 adapters), the recharge circuit is closed off and you can't recharge at all (either with the computer on or off).  Try; http://www.excelcomputerinc.com/html/xlsearchdc.asp for a 7500 Dell powerboard