There are a ton of things that could be going on that would prevent your iPhone 6S from charging properly. First things to check would be :: - Battery Health (https://support.apple.com/en-us/HT208387) - Charging Port Debris (https://www.youtube.com/watch?v=OJisCKosTEE) - Charging Port Damage (https://www.ifixit.com/Guide/iPhone+6s+Lightning+Connector+Assembly+Replacement/62572) - Logic Board Damage, TriStar (https://www.youtube.com/watch?v=tGPW6d3oFyQ)
Go get your battery replaced. It might be free or $30
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