The HTC One M9 is one of the better reviewed new flagships from this year's batch. But one of the most common complaints about this is that the battery life isn't all that great. So unless you want to constantly be looking for a wall socket to charge your device when you're on the go, your best bet would be to either get a power bank or to just get one of those battery cases, like the Mugen Power 3700mAh Extended Battery Case.
This particular battery case of course is only fit for the HTC One M9, and it claims that it can give the flagship twice the normal charge when used. This will then allows you to use your device even when you're on the go, and at the same time, gives your smartphone extra protection. Mugen assures the public that the case has been extensively tested and underwent various QA experiments and is compliant to CE requirements.
The case also contains a power meter button at the back to let you know how much juice your Mugen still has. There are LED indicator lights that indicate the level of percentage remaining on the case. The One M9's original battery has 2840 capacity but the Mugen 3700mAh has almost twice that amount.
The Mugen Power 3700mAh Extended Battery Case for HTC One M9 will set you back just $89.50 if you buy it directly from their site. No word yet as to how much other retail stores will be carrying this all-important battery case.