If the adapter is not recognized then it is usually 1 of 3 things. The adapter is defective, the DC Jack needs replacing, or the MB charging circuit is defective. Check the adapter plug and make sure the centre pin is not bent or missing. If possible try another Dell adapter. If trying another adapter does not help then replace the DC jack. If that does not help then the MB is the source of the problem. 
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My dell Inspiron is showing plugged in but not charging and am using the same 
power adapte