I had my iPhone 6S battery replaced in store less than 11 months ago but it still drains within hours

See if these tips improve performance.

Batteries - Maximizing Performance - Apple

If they do not, make an appointment at the Genius Bar for a hardware evaluation of the device.

I face the same issue. It started since I upgraded to ios 11. The Apple service center ran a diagnostic and confirmed that the battery condition is good therefore battery replacement may not resolve the problem. Apparently the problem is with the ios11 version. So now I have trashed the iPhone and moved to Android.

Me too! For 12 years I have supported Apple and its products. Purchasing iPhones from the 3 to the 6s, in addition to iPads, Ipods, MacBooks and Macs. Under Mr Cooks tenure I was badly affected by Apples decision to drop support for 'Aperture' but the recent duplicity demonstrated by Apple over the iPhone battery scam is the last straw. I had ordered a new iPhone X, but I have now cancelled the order and currently using my Son's old Samsung phone. I will be buying a new phone, but it won't be another Apple!