I have bought a 18650 2S protection board on eBay (http://www.ebay.it/itm/222383859919) because I need to use and charge 2 18650 batteries in series for a project (a portable audio system) but, searching the protection board model (fdc-2s-2) on google, I have found this site (http://mklec.com/components/battery-protection/lithium-ion-7.4V-3A-protection-pcb) that says that "This module is NOT a charging circuit!". What does this means? Can't I use this for my project? Waiting for an answer. Thank you for your attention.
From a quick read though, it looks like this is meant to be used with a charging circuit to protect your batteries.
You shouldn't be surprised it says the module is not a charging circuit, it is after all called a PROTECTION board, not a CHARGING board.
So yes, you can use it for your circuit, you will just have to make your own charging circuit and use this board as protection to detect over-voltage, over-current and short circuit protection.
It means that it is only a protection circuit. Lithium battery are quite sensitive to overdischarge and overcharge so you need to protect them with a small circuit. This kind of circuit are normally called BMS (Battery Management System) or PCM (Protection Circuit Module). If you need to charge your batteries look for a 2S 18650 battery charger.
Old question, but I just bought the same board off ebay for $0.75 (where it was inaccurately described as a charging board). As its 2S (7.4 to 8.4V), perhaps connecting output directly to a 5V input is not always a good idea?
To turn this into a charger/backup battery for my system, I also bought a $1 "New CC/CV Power Supply Module 1PC LM2596 LED Driver DC-DC Step-down Adjustable", which allows you to set a charging voltage (8.4V into protection circuit) and maximum charging current (say, C/10 or something like that). There is an LED on this board which indicates when charging is (more or less) complete, and instructions for battery charging application are provided in most ebay listings.
The charging input in my case is a 12V mains adapter. The 2S 8.4V output can be fed to another LM2596 DC DC converter ($0.50) if you need a regulated 5V output (not required in my application).