Spec-wise, HTC has once again given the battery a slight capacity boost—up to 10.87 Whr from yesteryear's 9.88, but still a little shy of the 11.1 Wh in the iPhone 6 Plus. (The original One sported a measly 8.74 Whr battery.)
AnandTech's review found the M9's battery life slightly disappointing compared to the M8. It seems HTC made an effort to stop the bleeding with a capacity bump, but it's ultimately not enough to surpass its predecessor.
Add the fact that the M9 supports Qualcomm's new Quick Charge 2.0 spec, but ships with a conventional 5 V, 1.5 A charger that can't provide the quick juice, and we're a little disappointed.