Hello,
Sounds like you have a faulty battery you have two options. Option 1 is you could buy a battery and replace it (Only do this if you know what you are doing Its not difficult). Option 2, You could take your laptop to a PC Repair shop and get them to replace it. They will order the battery and replace it for a fee and it may take them a few days.
According to the internet this is the battery you would need (http
/www.amazon.co.uk/Dell-Inspiron-N3542-Laptop-Battery/dp/B00USQFGZU/ref=sr_1_4?s=computers&ie=UTF8&qid=1447021150&sr=1-4&keywords=Dell+Inspiron+3542+Laptop+Battery+Replacement) But make sure to double check before/if you purchase.
Hope this helps, Lewis W
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