I use LI-ion cell phone batteries in the Roomba robot, the EZ-B, and the Wifi board. All of the batteries have built in Power control boards that monitor charge and discharge and will open each batteries electrical path if exceeded. That way I get to use the standard NiMH, NiCD power supplies. You can buy bulk cell phone batteries for about $1 each and a lot of them have 1200 to 1600 mAH capacity. Then it's just a matter of series/parallel arrangements to obtain the voltage and current needed for your projects.
The cell phone batteries installed in the Roomba are a 4(S) 2(P) 1250 mAH cells that provide 14.8 volts @ 2500 mAH. I leave the stock Roomba battery connected all the time unless actually using the Roomba. I have two 7.4vdc 2600 mAH LI-ion packs from a Cannon camera in parallel to provide the EZ-B with 7.4 vdc @ 5200 mAH and 2 3.7vdc 1600 mAH cell phone batteries in parallel for 3200 mAH current for the WiFi board. All battery packs have 2.5mm jacks so that the standard 22.5 vdc .75 amp Roomba power supply can be used to charge them all. Actually all packs have their own Roomba power supplies.
I work for a cell company and I tear those things apart , there's no charging controller in the battery packs of cell phones , the charging circuit is in the phone itself. If you Notice there are always two sets of contacts , two are for charge monitoring and two are the pos and neg output. If you can get them cheap enough making your own batteries would be great. Remember each battery is a single lipo Cell 3.7 volts min charge 4.2 max. Remember this when arranging them.
I agree with jstame1 about the charging smarts but there is also a charge/dis-charge Power Control Board that is built into the cell phone LI-ion packs that I use. I get them them for $1 each if 100 are purchased. They are rated @ 1600 mAH and 3.7vdc LI-ion packs. I use a standard Roomba power supply to charge these packs when they are wired in series to obtain 14.4vdc. I also double up two series groups to obtain 3200 mAH capacity for longer run times.
And I've also worked for several cell phone providers like AT&T, Lucent, Rochester Tel, US West, QWEST, Bell South, and LEACO.