Hi! If the J7 model has the features "lift to wake" and "double tap to wake", it's possible they'd act up a bit and turn the phone on. Go to Settings > search "Lift to wake" then turn it off. Same step goes for the double tap feature. That's my idea anyways. Hope this helped!
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It's likely a faulty battery, I'd recommend you return it and buy a new one, since you only recently bought it, or buy a different phone.
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