It could be a software optimization issue which is why we’re sure HTC One M9 owners will be pleased with an upcoming update scheduled for the handset. According to a recent tweet by @LlabTooFeR, it looks like HTC is currently working on an update for the phone that will supposedly improve upon its battery life.
The update appears to be targeting the baseband and will bump it up to version 1.40. No word on when the update will be released, but if battery life on your HTC One M9 is one of your biggest gripes about the phone, then hopefully this update will help improve it for you and this is an update you’ll want to keep an eye out for.
@LlabTooFeR claims that users in Taiwan have already started to receive the update, so there is a good chance that it will eventually roll out to the rest of the HTC One M9 owners around the world in the near future.