Iphone 6 A1549 battery drain

I'm using iPhone 6 64GB A1549 model since December 2014. Now, I'm experiencing my battery easily drains fast even the battery % is still 100%. I cannot used my phone without my power bank. I noticed it when I started upgrading my iOS to 9.3. Is this a bug for A1549 model? I already tried downgrading the iOS but there's an error whenever I downgrade it using iTunes. Tried resetting my phone to factory default settings, put to low-power mode, turned-off background app refresh that causes the battery drain but still won't work. Also, this was bought in US but I'm here in the Philippines and I already went to Apple Service Center and they said that it can only be repaired in US based on the model itself. They advised me to ship it there. My Apple Care was already been lapsed/expired and might shouldered the fees. Can someone help me on this issue? 😟 Thanks in advanced.