I did some tests on a simple (and very unoptimized) switched capacitor balancer on a pair of 2300 mAh A123 cells. A switched capacitor balancer wastes no energy as heat. It shuffles charge from the higher voltage cells to the lower ones by switching a capacitor between adjacent cells. It has no precision components and makes no measurements on the cells.
Its main disadvantage is rather small balance currents... this one is around 1 mA per 10 mV of imbalance. But it can run whenever the pack is being charged/discharged/or sitting idle. By keeping it connected while running or charging, it helps keep the pack from becoming unbalanced in the first place.
First I fully charged the cells, then I drained 750 mAh from one and 1300 mAh from the other. I placed them on a 7.3V, 1A cv/cc lab supply to charge. At 1 hour the cell voltages were 3.32 and 3.30 volts. At 1.5 hours the cell voltages were 3.90V and 3.34V. They essentially stopped charging and stayed at these voltages overnight. Next I discharged them to 2.50V. I got 2356 mAH and 1777 mAh. Clearly the cells stayed unbalanced. Bad news if your pack was in this condition.
Next I fully charged and drained the cells to the same condition as in the first test. I then placed them on the lab supply with the capacitor balancer connected. At 1 hour the cells were at 3.38V and 3.36V. At 1.5 hours they were at 3.82 and 3.38V. At 2 hours they were at 3.76 and 3.47V. At 2.5 hours they were at 3.63 and 3.62V... both cells fully charged and balanced. I drained them to 2.5V and got 2256 mAh and 2306 mAh.
Its main disadvantage is rather small balance currents... this one is around 1 mA per 10 mV of imbalance. But it can run whenever the pack is being charged/discharged/or sitting idle. By keeping it connected while running or charging, it helps keep the pack from becoming unbalanced in the first place.
First I fully charged the cells, then I drained 750 mAh from one and 1300 mAh from the other. I placed them on a 7.3V, 1A cv/cc lab supply to charge. At 1 hour the cell voltages were 3.32 and 3.30 volts. At 1.5 hours the cell voltages were 3.90V and 3.34V. They essentially stopped charging and stayed at these voltages overnight. Next I discharged them to 2.50V. I got 2356 mAH and 1777 mAh. Clearly the cells stayed unbalanced. Bad news if your pack was in this condition.
Next I fully charged and drained the cells to the same condition as in the first test. I then placed them on the lab supply with the capacitor balancer connected. At 1 hour the cells were at 3.38V and 3.36V. At 1.5 hours they were at 3.82 and 3.38V. At 2 hours they were at 3.76 and 3.47V. At 2.5 hours they were at 3.63 and 3.62V... both cells fully charged and balanced. I drained them to 2.5V and got 2256 mAh and 2306 mAh.