Just bought this battery at Best Buy:

Rocketfish™ - DSi Power Shell with Rechargeable Backup Battery

(my BB has it for 99 cents). I don't have a DSi, but I bought this thinking it would work for my 3DS, considering they use the same charger. The battery doesn't fit, so I took it apart. Just wondering if I could use this in a project, if I solder on a charge port of some sort (probably mini USB). The way this one works is it acts as a second battery in the dsi. It goes in the slot of the dsi where the original battery would be, and then the original battery goes in a slot in this thing. Since it's designed in this way, it has no charging port of its own and uses the dsi charging port.

Wondering if anyone knows if I take a li-ion battery such as this and add solder a charger/ charging port will that charge it, or does it have to be designed to be charged in this way?

Well, you need a smart charger. That battery is terrible anyway.

Ah, ok I will keep looking. It does seem to work pretty well on a DSi, according to the reviews on Best Buy.

The DSi doesn't use a whole lot of power, that's why.

Oh, well I probably should have said what I wanted this for. Oops. I want to make a wii remote/ classic controller hybrid with built in rechargable battery. The wii remote runs on 2 AAs (or AAAs if I use the smaller Power A brand Wii remote).

So what exactly is a smart charger then?

I'm not sure, that battery may be okay then.
http://forums.benheck.com/viewtopic.php?f=43&t=34576