The HTC One M8 has a slightly larger battery than last years model, but still not enough for heavy users. And unfortunately it does have a non-removable battery so you won't be able to put on an extended battery from the likes of Zero Lemon and others. But thankfully there are plenty of portable battery chargers available that work with just about any Android device, including the new HTC One M8.
Powerocks 7800mAh External Battery Pack with Dual-USB ports
Here's a nice external battery pack from Powerocks. I actually reviewed a few of their battery packs earlier this year. All of which I was very impressed by. This one is a 7800mAh battery pack, which can charge many phones at least once and maybe a second or third time. That all depends on your phone. Like if it's the DROID Maxx, it probably won't charge it more than once. This one also has dual-USB ports so you can charge two devices at once. Which is pretty nifty. All this for under $50!
7800mAh External Battery Pack Dual USB ports and Charger for iPad2, iPad, iPhone 4S 4 3Gs 3G, iPod Touch (1G 2G 3G 4G 5G), Motorola Droid, HTC Android Phone, Blackberry, Samsung Galaxy Tab 10.1, HTC flyer, and much more
Anker Astro Slim2 4500mAh Ultra-Compact Portable External Battery Backup Charger
The Anker Astro Slim2 is one of many external battery packs that Anker has available. This one is a pretty slim and compact one that works pretty well. It's not the largest capacity, but it'll work to get you through the rest of your day. It works with just about every microUSB device, including Apple products, however you'll need to supply your own cable for iPhones, iPods, and iPads.
Anker® Astro Slim2 4500mAh Ultra-Compact Portable External Battery Backup Charger Power Bank Charger for iPhone 5S, 5C, 5, 4S, 4 (Apple adapters not included), iPod, Samsung Galaxy Note, Galexy S4, Galaxy S3, Galaxy S2, Galaxy Nexus, HTC One X, One S, Sensation G14, ThunderBolt, Nokia N9 Lumia 920 900, Blackberry Z10, Sony Xperia Z; Google Glass, GoPro and More
PowerGen 12,000mAh External Battery Pack
Here's a massive battery pack from PowerGen. You may not want to carry this one everywhere as it is probably pretty heavy. After all it is a 12,000mAh battery pack. Think of it as carrying about 4 of the Galaxy Note 3 batteries with you. Good thing about this is it has 3 USB ports for you to charge your phones, and tablets from. And if you have the HP Chromebook 11, you'll be able to charge that one as well.
EC TECHNOLOGY 22,400mAh Portable Battery Pack
This one is actually the biggest capacity external battery pack I've seen. This one is a 22,400mAh battery. Which is a little less than double the previous battery pack from PowerGen. This one is pretty slim too. Although, I'm sure it'll be pretty hefty as well. It also has 4 Smart LEDs which indicate the power left, and it also has a built in flashlight.
RAVPower Element 10,400mAh External Battery Pack
RAVPower's got a nice external battery pack as well. This is the Element which has a 10,400mAh battery backup inside to charge all your devices. Now it's not as big as the previous one we listed, but it is about half the price. So we can't really argue with that, now can we?
RAVPower Element 10400mAh External Battery Pack Portable Charger Power Bank (Dual USB Outputs, Ultra Compact Design), for iPhone 5S, 5C, 5, 4S, 4, iPad Air, 4, 3, 2, Mini 2 (Apple adapters not included); Samsung Galaxy S4, S3, S2, Note 3, Note 2; HTC One, EVO, Thunderbolt, Incredible, Droid DNA, Motorola ATRIX, Droid, Moto X, Google Glass, Nexus 4, Nexus 5, Nexus 7, Nexus 10, LG Optimus, PS Vita, GoPro and More
EasyAcc 12000mAh Power Bank
This power bank from EasyAcc has about 4 different USB ports, so you can really charge just about everything all at once. It'll work with just about every microUSB device out there, and it is also fulfilled by Amazon. So you know what that means? Amazon Prime's free two-day shipping is available.
EasyAcc® 12000mAh Power Bank 4 USB 3.5A Output Portable Charger High Capacity External Battery Pack for iPhone 4S 5S 5C, iPad, Nexus 7 10, Galaxy S4 S3 Tab 3 2,Andorid Smartphone, 5V Tablets, Bluetooth Speakers headset, Google Glass, Gopro etc.[Output: 0.5A ~ 2.1A; Black]
Tylt Upplant 2
Here's a pretty small one from Tylt. They actually don't make a whole lot of external battery packs yet. They mostly make cases with batteries inside for certain phones. But that might change. I actually reviewed this one earlier this year and actually quite liked it. It's pretty lightweight and it does have a 5200mAh battery inside.
Limeade Blast 15,600mAh Battery Pack
The Limeade Blast is a pretty slim batteyr pack that does come in different sizes. There's a 6000, 13000, 15600, 18000, and a 24000mAh version of this battery pack. That's some serious juice right there. This particular one is the 15,600mAh battery pack and it's on sale for $50. Which isn't bad at all.
USB External Battery Pack 15600mAh (Black Limeade Blast L156X) Charger with built-in Flashlight for iPhone 5, 4S, 4, iPad Mini, iPods; Samsung Galaxy S4, S3, S2, Note 2; HTC One, EVO, Thunderbolt, Incredible, Droid DNA; Motorola Moto X, ATRIX, Droid; Google Nexus 4 ; LG Optimus
New Trent iCarrier 12000mAh Portable Dual-USB Port External Battery Charger
New Trent is also a pretty familiar company for creating great external battery packs. This one is a 12000mAh battery pack that has two USB ports so you can go ahead and charge two devices at once. Which is a pretty big deal. Now you won't have to worry about which device should you charge first.
Anker Astro E5 15,000mAh Portable Battery Pack
Here's my personal favorite, this one's from Anker. It's a pretty large battery pack, but it's a 15,000mAh battery pack so it'll last you a while. It does have two USB ports so you can also charge a couple of devices at once using this one. There's also LED lights showing how much juice you've got left.
Anker® Astro E5 15000mAh Portable Ultra-High Density High Capacity External Battery Backup Charger for iPhone 5 (Apple adapters not included), 4S, 4, iPad 4, 3, 2, Mini, iPods, Samsung Galaxy S4, S3, S2, Note 2, Note 8, Tab 3; HTC One, EVO, Thunderbolt, Incredible, Droid DNA, Motorola ATRIX, Droid, Google Glass, Nexus 4, Nexus 7, Nexus 10, LG Optimus and More - Black
- New Trent PowerPak Ultra 14000mAh Portable Dual USB Port External Battery Charger/Power Pack for Smartphones, Tablets and more. (Grey/Black)
- EasyAcc® 5000mAh Brilliant (White+Orange) Ultra-Slim Power Bank Portable Charger External Battery Pack For Most Smartphones, Samsung Galaxy S3 S4 Note 3, HTC One Mini , Google Nexus 5, Nokia Lumia 520 1020, LG G2; iPhone 5 5S 4S, iPod, Bluetooth Headset, Google Glass
- Voltsaf - Portable High Capacity Dual-Port External Battery Pack Power Bank Backup Charger for Smartphones
- New Trent PowerPak Xtreme 12000mAh Rugged Water/Dirt/Shockproof Dual USB Port External Battery Charger/Power Pack for Smartphones, Tablets and more
- New Trent PowerPak+ 13500mAh Portable Dual USB Port External Battery Charger/Power Pack for Smartphones, Tablets and more (Upgraded Version of NT120T)