How do you charge and discharge lithium cells connected in series? It's not easy. If you choose to design the circuitry to monitor each cell's voltage, charge current, and discharge current, the circuitry would be quite complex and difficult to optimize for best performance.
However, with an integrated solution, the needed functionality is all on one analog IC, so the overall design becomes much easier — and you can focus on added value elsewhere.
So, why is it difficult to charge and discharge these cells? At first glance, the question may seem simplistic. The answer seems to be contained in the question. You connect them in series and charge them or use them to power the load. And that is absolutely all there is to the problem — as long as you make sure that the cells are absolutely identical, perfectly matched in ampere-hour rating with absolutely identical internal chemistry, composition, and construction.
Why would that matter? Because if the cells are not identical in charge capacity, as you draw energy from them, first one and then another will become completely discharged. Besides the obvious effect of reduced terminal voltage from the battery pack, any cell that is discharged will become reverse biased from the voltage of the remaining cells and the connected load. You can verify this by sketching a simple schematic of cells in series plus a load resister. Assume the voltage across one cell drops to zero and that all that remains is some internal resistance. You will note a polarity reversal at that cell. Pushing current the wrong way through a discharged cell will cause rapid deterioration.
In consideration of the possibility that you can't find perfect cells, the best thing to do is to use cells from the same production lot. The next technique is to switch an additional load onto the stronger cells to bleed them down to match the energy level at the weakest cell when you detect a mismatch in terminal voltage. That will minimize damage, as explained above, but at the cost of making your battery pack as bad as its weakest cell.
There is a better way to do this. Rather than throwing the excessive energy away, using it to boost the weaker cells is the smart thing to do. If you had a special bidirectional circuit that could pull energy from the better cells and supply it to the weaker cells, that would be the ideal solution.
Linear Technology has introduced just such a device in a complete integrated package. The LTC3300-1 monitors up to six cells and has a unique integrated controller that drives a bidirectional flyback switcher with external FETs that draws from the strong cells and charges the weaker cells. For more than six cells, you just stack up another LTC3300 IC.
By controlling the timing of the on-board low-side FETs, one side of the transformer becomes the primary and one side the secondary. Energy can be transferred from right to left or left to right, so any cell can be the current source or sink.
The IC would be useful if you're designing electric vehicles or backup battery systems (e.g., a UPS for a server). The IC has a SPI interface, so it can communicate with your system and report battery status. It's spec'd at a maximum temperature of 125°C, so it's clearly intended for automotive applications. Note that the IC is intended for use only with Li-Ion or LiFePO4 batteries, not NiCd or nickel-metal-hydride.