Most of the time you don't need that third "pin" when using litium single cells like cellphone batteries etc. you can buy generic chargers which have sliding pins to touch just the positive and negative terminals.

Actually I've just noticed that 12ianma answered my question about parallel cells. I somehow missed it, so my bad, and thanks a lot for the help! However it doesn't change the point for stargazer's issue.

I'd say it's not safe to assume anything in case of LiPo batteries since if you wire them wrong you might end up burning the battery and most likely damage the device. I'm sorry I can't really see what's written on this chip and I'd would be good to know it's specification. As I said it might be temperature sensor. What worries me in this case is that the third pin is there for a reason. It means that something must be wired there to control how the battery charges/discharges.

So to summarize - I wouldn't use the battery unless I know everything about it. Simply for the safety sake. Imagine you left the thing at home at it burned and your desk or whatever caught on fire (or is it too paranoid? O_o). Can you get something similar for you needs from batteryspace? There you can purchase a battery with known number of cells and get a protective boeard for your pack.

Well I don't really know about all of that, but here's what I'm talking about:

Is it safe to assume that this circuit is a charging circuit? Would a charging circuit be more voltage tolerant than the cell itself?

The voltage written on the battery will be the voltage of the battery independently on whether it has protective circuit or not. So if it's a 3.7V battery with a circuit it will say 3.7V. But you could measure it with the multimeter couldn't you?

Third pin however might or might not indicate the presence of protection. It might be just dummy pin which is quite typical for cheap aftermarket stuff. It could be simple temperature sensor, or smart full-featured protective board. So if you buy a battery from a good source and the battery has the spec - just check with it. Otherwise I don't see how to verify all that without dismantling the battery and even then you need to reverse engineer the board (if one is present) and figure what does it do. Does it protect from both overdischarging of overcharging? Or is it just for discharging. Do you see what I mean?

For example I've got hold of a DSi battery, but have no idea how many cells are there and whether a circuit is there or not. It has 3 pins all right, but it could be a dummy pin attached to ground or always returning OK status. So although I like the form-factor of this battery I won't be using it since I don't have the spec. If say it has more than 1 cell, then I need to take care of how they charge and discharge (balance them in other words) and the way they must be balanced depends on how many fucking cells are there

Anyways... this is the stuff I've read from various resources like battery university, but dunno how is it on practice. That's why I brought up this originally, but got no answer apart from accusation of asking pointless questions (it actually becomes quite typical for this forum which is a shame, I guess fantastic expert gurus sitting here won't descent to a noob like me).

3.7v. I guess maybe I'm incorrectly assuming that if it has a circuit built in, the circuit is a lot more voltage tolerant than the cell itself?

I answered his question earlier.

Stargazer, how many volts does the battery say on it, my guess is it could be 2c2s

That escalated quickly.

I do have a li-ion question though. Generally speaking, if there are only 3 contacts on a battery, is it probable that it has a charging circuit built in? I'm thinking about a psp battery, which, to my knowledge, does have a built in circuit. And is there any good way to confirm/test that without a) blowing the battery (or yourself) up, or b) opening the battery?

LOL!!!

Yah as I said I'll manage on my own. Thanks for you help.

If it was multi-cell, and you know that they are in a parallel circuit, your question is redundant. Your question is pointless.

What? It's exactly what I wrote, and that's why I'm asking about the number of cells... Anyways I'll figure on my own, thanks, this discussion becomes pretty pointless.

Rule of thumb: Lithium Ion Battery = YOU SHOULD ADD A PROTECTIVE CHARGING CIRCUIT.

Lithium cells can be hooked up in parallel and charged like one cell. they self balance when hooked up in parallel, I charge 6 lipos at a time for my one helicopter since they are single cells. It's important that the cells are close in voltage before you hook them up though, however if they come together they are already fine.

The only time balancing is needed (while charging) is series litium, as if you charge a 2 cell pack to 8.4v (the full charge) one cell may be 4.1v and the other 4.3v (where 4.2v is max on the cell) and over time this gap can grow and eventually ruin one or both cells.

Well the reason I'm asking is that I want to hook it up with a usb charger and a protective circuit that prevents overcharging and overdischarging. This is necessary with li-on/po batteries otherwise the thing might heat up too much and damage the actual device it is powering. So I need to know do I need a protective board for a single cell or for 2 or more cells. It might be that the battery already has the protective board, but I it could by that all the protection is handled by DSi itself.

Now since it provides 3.7 volts it's either a single cell or a pack of cells in parallel.

PS I probably didn't state it clear but I'd like to install it in a game boy pocket, not SP.