So how are all the cells kept matched in a series pack?  The circuit on the left side is the most common method, and implemented in most TI multi-cell gauge and protection devices. If users are recharging a series stack of cells at a 1 amp rate, and cell 2 is just a bit higher in voltage than cell 1, cell 2 reaches 4.2V first, but cell 1 is still only 4.1V. At this point, normally the total current would have to be reduced to prevent Cell 2 from exceeding the charge limit. However, if there is a transistor in parallel with cell 2, then it can be turned on to divert some of the current around the high cell, while allowing the full current to keep going to the lower cell. If 100mA is diverted through Q2, for example, then cell 2 will be receiving only 900mA while cell 1 continues to get 1A charge current. Over time, this will allow Cell 2 to “catch up” and recover to the same voltage as cell 1. Note that a resistance R5 is also required to avoid a short-circuit across cell 2 which is in the range of 4V. So, there will be some power lost across the R5/Q2 combination. The switch mode implementation on the right is used for applications that need very high balance currents, typically greater than several hundred milliamps. This has the benefit of higher efficiency, as energy is transferred from the high cell to the lower cell. The linear / dissipative approach on the left just burns the extra energy in the form of heat, just as in a linear voltage regulator. The switch mode method actually uses the excess energy to restore a weaker cell faster than the strong cell and can equalize the cells much faster. However, the switch mode approach can be very complex and difficult to control. In effect, there needs to be a switch mode converter in parallel with every cell. This can become expensive due to the number of components involved, as well as difficult to control and potentially a source of noise in the system. In general, balance current levels are quite small relative to the size of a battery pack. For a notebook PC with capacity in the 5000mAH range, the balance current is often less than 5mA. This is because the cells normally will start off new as being relatively close in capacity and voltage. The balancing circuit only needs to keep them balanced, correcting for small amounts of mismatch. As long as the balance circuit starts working before the mismatch grows too far, only low currents are required. Balance current is typically only a fraction of a percent relative to the cell’s C-rate capacity. So, for most applications, the size, cost, and complexity of the switch mode approach is not implemented. Depending on the current levels involved, and the specific battery management IC being used, the balancing transistors Q1 and Q2 may be either internal or external to the IC. The standard fuel gauge ICs from TI can handle a few milliamps of balance current, while those designed for high power applications can tolerate in the range of 50 to 100mA bypass current.