[dmwahl]

For an analog design, the TL431 is the basis for most designs. The high current shunt regulator here, figure 21 will do nicely. You'll just need to add a resistor in series with the PNP transistor.

I designed one a couple of years back, details on the endless sphere forum here: http://endless-sphere.com/forums/viewtopic.php?f=14&t=48961

 

[dmwahl]

I should mention that if you're looking to design the balancer as a learning exercise then great, if you want to save money or time just buy one.