Dell wont charge.works if connected to power

Hi everyone,
my dell studio 1737 laptop wont charge.. its starts when i connect it to the charger fine , but there is a msg when i switch on that th AC adapter is not the right one ( though i ve been using it for 1.5 years and was the original one ) and it will slow the performance. when i pull the plug off it shuts down off just like the TV woul do if the power gets out...its strange ... please help me figure out the problem ...