Thank you very much guys for the responses.
but do I need a special circuit in order to cut-off the power once it battery reaches a certain level?
This is usually handled by the charging chip but when you buy batteries from a reputable source, they usually ask if you want a 'PCB' (protection circuit) embedded in the battery.
Is it safe to assume the camera batteries I linked to have this protection circuit embedded? Or should I distrust anyway and use what kind of circuit?
The component you linked to, you say is for charging, and maybe I haven't made myself clear, I don't need help charging, I'll use the original charger for the camera batteries. I'm also not attempting to.. how do you call that, trickle charge? (Use while charging at the same time).
I just need to be sure the battery won't explode while using it, that's all.
That seems like what I would be needing, but I got confused with your comment "bundled in the outer sheath of the battery". In the end, are these circuits I'd use between the Arduino and the Li-Ion battery? Or are they just saying those red 18650 Sanyo batteries have them already on? Or is it, that they sell them packed with the circuit or...?
Also, they say
When the voltage and/or current return to acceptable levels, the safety circuit automatically resets allowing normal operation of battery assembly.
Does it mean the Arduino would stop functioning just as if I had removed the battery or is it left in standby mode or...? It is not very clear, I guess I'd want the cut off circuit to just shut down it all (probably I know I'd have to read something from the Arduino pins then stop the arduino via software???)
If you want to stick to Li Ion then your will probably end up with the 18650 format cell (in link above) unless you go for a proprietary format/charger such as camera batteries. If so you will need a holder such as these:http://www.digikey.com/product-detail/en/BK-18650-PC4/BK-18650-PC4-ND/2330513
This will allow you to remove the batteries and charge them individually if you want. (Be sure of both batteries getting full charge).
Yes exactly, I don't really feel like using 18650s since they seem a bit untrustable. I think I'd be more confident with a proprietary format battery pack.
LiPo is useful if you need high current. If not, NiMH may work better. But, if you feel you must use LiPo, I would suggest you use LiFePo4 as it's the latest technology and doesn't have the "explode" factor that standard LiPo does. LiFePo4 are readily available outside the US (as most are made in China and HK). Anyway, they have a little lower voltage (3.7/3.3 per cell) but that actually works out well for battery powered systems.
As far as charging. It's fairly straight-forward. I typically do a trickle charge via solar with a voltage limiter with a simple voltage devider. This works with any type of LiPo battery. If you go with a trickle charge and voltage limit, the main thing to worry about is low voltage. You need to devise a circuit that will terminate when the voltage is lower than a certain amount. If not, you'll kill your LiPo battery.
No actually I feel like i must not use LiPo, that's more or less the reason for this topic, since it's popular enough to be on every single google search, and I barely could find info on using simple camcorder batteries by themselves (except for a project where some guy uses an Arduino to do something in a camera, so thanks to that guy all hits in google "camcorder arduino battery" link to his project).