Besides the obvious advantages, the beauty of the PowerPump cell-balancing technology is that balancing is possible " regardless of the individual cell voltages. This means that if you decide to make a transfer charge between two cells, it can occur during any sequence of battery operational modes (charge, discharge and rest). Even if the cell that provides the charge has a lower voltage (for instance, due to lower cell resistance while in charging or discharging) than the receiving cell, transfer will occur. Compared to resistive-bleed balancing, energy lost as heat is small.

There are three selectable balancing algorithms:

• Terminal-voltage (TV) pumping

• Open-circuit-voltage (OCV) pumping

• State-of-charge (SOC) pumping (predictive balancing)

The TV pumping is just like the voltage-based passive cell balancing described earlier. As you can see in Figure 4, TV balancing during charge does not always result in a balanced capacity toward the end of discharge. This is due to mismatches in cell impedance we mentioned earlier. The OCV pumping compensates for impedance differences by estimating the OCV, based on pack-current and cell-impedance measurements.

SOC pumping operates in a similar manner as Impedance Trac devices " it determines the exact charge level of each cell and transfers energy between cells so cell capacities are balanced at end-of-charge (EOC) (see Figure 9). Looking at a discharge OCV graph (see Figure 10), predictive balancing each cell is taken to an offset voltage which reflects its capacity. Capacity difference of a few percent makes a large difference at the knee of the discharge curve. If we know the capacity of one to two percentages, we can have a very close match at the end of discharge. This is the area where you want to have the cells most balanced, at the end-of-charge and end-of-discharge, effectively achieved with active balancing.

Figure 9: PowerPumpTM technology provides quick and accurate cell balancing- For higher resolution, click here

Figure 10: Each cell is taken to an offset voltage which reflects its capacity - For higher resolution, click here

PowerPump technology corrects cell imbalance better than conventional passive balancing because a higher balance current can be controlled by changing component values.

The effective balancing current for a notebook PC is typically 25 to 50 mA, which is 12- to 20-times higher than internal-bypass balancing. With this advantage, active cell balancing can correct capacity imbalance within one cycle up to 95% of the time.

In larger capacitive batteries, PowerPump technology makes an even greater difference. Take into consideration the amount of time a battery pack can be balanced with voltage-based passive balancing. The only battery energy level, positive balancing takes place during the tapper off portion of a charge cycle. So out of the entire lifetime of a large battery pack, only a few percentage points of time allow for balance. Thus many pack designers choose to balance over an ampere, even above 10 ampere. This creates many thermal issues and cost for large FETs. Given the literally uninterrupted balancing opportunities with the PowerPump, these design hurdles are minimized.