Make an Inexpensive Lithium-Ion Battery Pack

There are two ways to charge a battery like this. You can either apply 4.2 volts to individual cells or you can apply 16.8 volts to the whole battery. While the latter is more simple, in this case the former is better because we are using used cells. When new packs are made, they all use new batteries of the same amp hour capacity. This way they can be charged hundreds of times without going out of balance. However, balancing will extend the service life of any pack. With our pack, the batteries are in various states of wear and amp hour capacity, therefore balancing is a must. A balanced battery is one in which all the cells remain the same voltage.

Let me explain how packs can become unbalanced and why that is really bad. Batteries will inevitably wear our at slightly different rates. Say the voltage of each cell in your pack is 4.2 volts after charging when new.

Cell 1 is 4.2
Cell 2 is 4.2
Cell 3 is 4.2
Cell 4 is 4.2
Whole battery is 16.8 volts

Now say one of the batteries has started to wear out. They manifest their wear by sometimes refusing to charge all the way to 4.2 volts. Now cell 4 only charges to 3.8 volts. The charger doesn't know this but still charges to 16.8 volts and the other cells are charged higher to compensate.

Cell 1 is 4.3
Cell 2 is 4.3
Cell 3 is 4.3
Cell 4 is 3.8
Whole battery is 16.8 volts

You can see how one bad cell could destroy the good ones because the others are overcharged. Balance chargers never charge cells above 4.2 volts and they can tell you if one cell refuses to charge completely. My charger has four lights and shows blue for fully charged and red for not there yet. They make a volt meter here that is perfect for monitoring the health of cells.

Balance chargers are more common in hobby batteries.  People use lithium polymer (LIPO for short) batteries to power model helicopters, airplanes, boats, etc.  LIPOs are cheaper and tougher, better for surviving crashes.  They require more balancing then lithium ion.  Some have C rates of 50!  That means a 5 amp hour battery could supply 250 amps and completely discharge in only a few minutes!  The chemistries are almost the same so the chargers are interchangeable.  I suggest looking through Hobbyking so you know what all is out there.  They have an especially dizzying array of chargers.