I use surplus LI-ion cell phone batteries in whatever series/parallel arrangement is needed for an application. What's nice is that each pack has its own power control board that prevents both overcharging as well as discharging too low. I use iRobot Roomba power supplies to charge them.

@Kenny, I am sold on LiPos for the build now. I have one 5000mAh 7.4v pack although, depending on budget and space, this may be increased to two or more.

Charging wise I've figured it out now which is great and will work very well with my planned charging station/home base.

However, I also have a LiPo balance port voltage tester which I want to build in to the robot, it's just one of the common 7 segment display types that you plug in to the balance port, but I don't know if it's safe to cut this in to the balance port wiring and have it connected whilst charging, does anyone know if that would pose a problem?

I also thought about using the balance port wiring to go on to two of the ADC ports (2 cell battery) so ARC can monitor each of the cells too, again something I could do with a little advice, or confirmation on.

@Robot-Doc I am really interrested in your re-use of cellphone LiON batteries. How do you connect them to the ezb (there is 3 connectors on the battery and ezb needs 2 wires: + & -). Could you post a picture of the assembly? Thank you very much.

I use a LiPo 7.4v 1500mAh. This battery was used RC helico (crashed!). Works great on my robot.

check this out battery protect

@fredebec You only need the (+) and (-) connection, The third connection is not used. Connect as many cells as you require then use a 2.1mm plug and wire the output from your assembled LI-ion pack to then connect to your EZ-B.

Thanks Robot-Doc. Is it possible to use different models of cells simultaneously (all my cells are 3.7V, but go from 1200mAh to 1500mAh)?