The S5 can pull up to 1800mA, so a 2A charger is best; that being said, I've charged in 3-4 hours on 1100mA using a proper cable, and even on a PC USB port. This is an example of said cable with a pretty good write up: http://www.ebay.com/itm/181622495649 I also replaced my S5 battery with an Anker one the other day since after a year, the Samsung batteries lose a little zip. Amazon has 2 Ankers and a charger for $25 http://www.amazon.com/dp/B00OUA9M1Y The OEM ones are still good mind you, they just aren't as good at holding a charge as when new. Batteries of this type are rated to 500 full discharge / recharge cycles, so if they didn't replace the battery in this refurb (they usually don't) they might replace it for you under warranty. Usually when a battery is bad, slow charging isn't as typical as not holding a charge, or not fully charging.
Don't bother getting a Samsung battery unless you buy it directly from Samsung. Every other battery out there is a fake.
I have this Anker battery kit and the batteries are every bit as good as OEM of not better. Fantastic customer service, if you ever have any issue with a Anker battery they will replace it fast.
https://www.amazon.com/dp/B00OUA9M1Y/
They do have single batteries but note the single batteries may not have the NFC antenna.
I was also kind of sketched out by the "Samsung" batteries that reviewers said were knock offs.. so I went with a 3rd party called Anker. I've had it since May and it's been great so far. Came with 2 batteries that had 2800mah like the stock battery and a wall charger. Here is the link on amazon.
Edit: appears to be on sale too lol
This is the Anker package I bought for my two S5's - https://www.amazon.com/dp/B00OUA9M1Y/
They have been going strong, no complaints. The batteries in that package have the NFC antenna, I recently found out some of the single batteries from Anker don't have the NFC antenna.
Unless you buy the battery directly from Samsung, you have assume it will be a knock off.
If you want two with a charger I bought this and am very satisfied. I've had occasion to buy Anker products several times now and they've actually exceeded my expectations.
Anker has a two battery kit with a travel charger on Amazon for about 28 dollars. https://www.amazon.com/dp/B00OUA9M1Y/
Those batteries have the NFC antenna, some of the single Anker batteries you can find on Amazon for about 14 dollars may not have the NFC antenna.
I bought these replacement S5 batteries from Anker the other day and they are working well; 2 batteries and a charger for $25 http://www.amazon.com/dp/B00OUA9M1Y The trouble with the cheap "Samsung OEM" batteries is that they are often counterfeit crap batteries. The OEM S5 batteries are only $20 to begin with from Verizon stores, or pretty much whatever carrier you bought the S5 from. If the carrier store isn't an option, a reputable 3rd party like Anker works well, and as I said, their $25 NFC compatible 2 battery+charger combo can't be beat.
The batteries I got have the NFC antenna, this is the exact package I bought from my order history - https://www.amazon.com/dp/B00OUA9M1Y