Charger and/or wire.
Explanation for the wire part:
Laws of physics , the thicker the wire the more the current. Recently I was interested in knowing how much input my phone was getting from the charger , so I started testing it out. I used 2 wires , one which was the standard charging cable and one which was short and thick. I have a 2 amp output charger. To my surprise , the thin cable gave me current of around 900mah while the thick one was able to deliver current up to 1800mah. So yeah that may also do the difference.
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