Sudden Battery Drain out problem with L1520

I have a Lumia 1520. Whenever I am using the phone with Data connection being on the phone with say Battery around 90% the phone randomly switches off and then when it restarts it shows battery at 2-5% level. I put it on charge and with 30-40 min i gets fully charged. I have closed all background app and use one single app and still having the problem. It it Win 10 OS.

Can some one help.

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