Random thoughts on buying/charging batteries.  

Both sealed and wet lead acid are self balancing as long as you drop the current down to below ~C/5 once you hit a predefined voltage (for example, 14V on a 12V car battery).  Lead acid self balances by converting the excess charge passing through the cell into heat - without further increasing - which is why you simply need to lower the current... otherwise, the heat will damage the cell.  The voltage will not rise above final charge unless the current is too high.

Commercial batteries are cheaper than individual cells due to volume.  If you buy less than several thousand individual cells, there's a huge transactional cost; batteries are a commodity item.  Buy someone else's battery pack and use the cells inside.  DeWalt used to be the de-facto source for A123 cells, but they've migrated over to Panasonic cells, which are also good, but can't carry as much current. 

Lithium cells don't regulate voltage by converting excess charge into heat.  Lithium's energy storage mechanism  doesn't roll off the voltage before the cell is damaged.  Thus, as the cell reaches full, the voltage will begin to rapidly rise (within 30 seconds or so).  Exceeding some nominal voltage (3.65V for LiFePO4) rapidly degrades the cell.  For LiPo, after a minute or so, the cell can ignite... the voltage rises rapidly enough on a charged cell that the constant current ends up dumping considerable power into the charged cell, which heats it real quick. 

There are several off the shelf charging solutions for cell packs less than 6/8S.  Check out the RC helicopter scene for recommendations.  Most of these charges are very inefficient, as they simply place a transistor across the battery leads and partially enable it through a resistor as the cell gets full.  Thus, as soon as the first cell gets full, you've got to drop the charge current to whatever your transistor can handle... since you're just moving the heat source from inside the battery to outside the battery... at the end of the day, you've got current moving across a voltage (the cell's), which equals power (heat) that you've got to get rid of.   

There are more elegant methods to using this excess charge rather than turn it into heat, one of which I'm working on a TBA design for, and the other of which requires isolated charge pumping of charge from one cell to another via a transformer or capacitor... see the LTC3300 for a nice new design from Linear Tech.