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.