Iphone 6 with IOS 11 will not stay charged

It is very interesting as both my husband's and my Iphones came off contract and now we cannot keep them charged. This was also when we upgraded to the IOS 11. We have to charge our phones 4-5 times a day now. We both did a factory reset, but that did not help at all. We are beginning to think we need to go away from the Iphones - as this is very frustrating! Any solutions?