iPhone 6/6s battery draining after replacement

I have two iPhones: a 6 and a 6s, both running ios 11.3. I got both batteries replace a couple months ago and soon aferword had to get the 6s battery replaced a second time since it was draining quickly once it dropped below 50% and the camera and other features stopped working. It worked for about a week after it was replace and started acting up again. The 6 was showing no signs of this until the last week when it started draining quickly and the camera stopped working after it dropped below 20%. I never use cellular on the 6 but do on the 6s. They both have been draining while off. Is there any fix for this, or should I send them in?