Hello guys, I wonder if you can help me at all. I have received a battery charger pack (6000mAh) for mobile phones, tablets and cameras, here is a screenshot of it
It has several connectors and can charge differend brands like apple android etc. OK, so I have a OPO device with android (battery 3100mAh), and I'd like to use the battery pack to charge it but as you can see from the picture it has 2 outputs: 5V 1A and 5V 2.1A. Unfortunately electronics isn't my thing, so I had a look on google but frankly I got more confused than before, and read that i need to make sure I use the right voltage otherwise I screw the phone battery: so, my question is, which output should I use with my OPO phone, and, more generally, which output is used for which device? Is there a rule of thumb of some kind?

They are both 5 volts. One will provide a higher current, hence shorter charge time, than the other. The only thing you need to verify is what the maximum charge rate (amperage) is that your device supports. The 1Amp rate should be safe for everything. The 2.1Amp rate may not be.

Well, yup @rubberman i scorrect over here both of then are 5v one will send the charge out faster means faster chanrging while with the other one it will take a little more time to ...

thanks, so how/where do I check the maximum charge rate?
Also, I've heard about underchargin and overchargin and problems that they could cause to batteries, is that true?

Well, I would say don't worry much about the overcharging problem because modern batteries have inbuilt circuit which prevents the device from overcharging and what does checking the charging rate means over here ?? I would say its already written at the USB ports i.e 5 and 2.1 ...

When charging via a dedicated power supply or USB you do not need to worry about the battery voltage and supply voltage output, as long as the current supply capability of the source is high enough. All chargers in devices such as your OPO are "smart" in that they will control the charging rate via voltage applied to the batteries themselves. The battery fully charged voltage may be 1.2, 1.5, 3.0, 3.6.... you don't need to concern yourself about it. It may even be higher than the 5V USB supply - a boost circuit would take care of that.
3100mAh... say 20% of that for a quite safe charging rate is equiv to just .62A => 5 hours roughly.