Since I already reviewed a single replacement Anker 2800 mAh for S5, I will quote parts of my original review in here.
It's not a speculation but a fact - sooner or later your shiny new Samsung battery will start showing signs of aging and reduced performance. It's also a fact that some of us wants to keep S5 slim without extra thickness or weight of the extended battery, and would rather prefer to have a spare one they can charge externally and swap when necessary. Either way, a number of S5 owners will be looking for a replacement OEM battery and that's where the nightmare begins. You can cross out most of the cheap ebay or other on-line retailers, especially those claiming 2x capacity in the same footprint as OEM battery - those are all marketing tricks where often you won't even get the original capacity. Going on Amazon used to be a safe choice, but not anymore. The same ebay sellers list their junk under the brand names and people don't bother to check when looking for a bargain price.
This battery/charger combo from Anker arrived in a sturdy box with two batteries, a travel wall charger, and a nice set of instructions all separated inside of the partitioned plastic tray. Each battery was shrink wrapped, sealed airtight, and actually half way pre-charged. The wall charger is generic re-branded type with a plug that folds flat thus making it convenient for traveling. This charger actually has a dual functionality, to charge a battery (at 600 mA) and to charge any other device connected to external usb port (at 1A). I know this is not the fastest speed, but it's a convenience of been able to charge a spare battery externally outside of your phone. A lot of people prefer not to have a charging downtime when using their phone, and find it a lot easier to just open the battery cover and replace with a spare charged battery.
Putting Samsung original battery next to Anker reveals an identical outline shape in length, width, and thickness. Everything aligns with the same contact spacing, the same capacity and NFC functionality, and the same fitment. The only difference was a water tamper seal sticker on Samsung which is not present on Anker. Regarding the performance, out of the box I didn't see or expected to see any difference in comparison to a stock battery. The battery was recognized, ran cool, and lasted the same amount of time relative to our typical use with a stock battery.
Overall, this battery is not about double capacity or some new extraordinary functionality. It does what it supposed to do and comes in a convenient double packaging with a bonus travel charger. Furthermore, it's from a trusted company that earned its reputation with thousands of 5-star comments/reviews on Amazon over the last 3+ years. I'm not hyping it up, just stating the obvious by searching on Amazon as well as forums in our community. Anker actually has a really good quality control, a great tech support, a very reasonable competitive pricing, and 18-months hassle free warranty. Definitely gets my recommendation!
Here are the pics.