Brando has released a battery case for the HTC One M8, the Power Jacket For HTC One (M8) with 3200mAh rechargeable battery. Here's more info:
An ergonomic design of portable back-up battery pack for HTC One (M8). It can be used as a protective cover, protects your HTC One (M8) from strong momentum and provides excellent console protection. Prop your device at a comfortable viewing angle with built-in stand.
Charge the Power Case by USB cable. Plug the power station to charge your HTC One (M8) at any time and any where. To charge the phone, simply slide in the phone into the portable battery and slide ON-OFF switch to on.
Features: - Recharge HTC One (M8) On the Go - 3200mAh High Power Capacity - Charge with Micro-USB Charging Port Only - Lightweight and Protective - With Battery Power Indicator