Week 8 Multi cell Battery Pack

No two cells are identical. There are always slight differences in the state of charge, self-discharge rate, capacity, impedance, and temperature characteristics. This is true even if the cells are the same model, same manufacturer, and the same production lot. A weak battery cell will charge and discharge faster than stronger or higher capacity cells and thus it becomes the limiting factor in the run-time of a system.

Most typical battery chargers detect full charge by checking whether the voltage of the entire string of cells has reached the voltage regulation point. Individual cell voltages can vary as long as they don’t exceed the limits for overvoltage protection. However, weak cells (ones with lower capacity / higher internal impedance) tend to exhibit higher voltage than the rest of the series cells at full charge termination. These cells are then weakened further by continuous overcharge cycles. The higher voltage of the weaker cells at charge completion causes accelerated capacity degradation. If the maximal recommended charging voltage is exceeded even by as little as 10 percent, it will cause the degradation rate to increase by 30 percent.

On the discharge side, the weak cells tend to have lower voltage than the other cells, due to either higher internal resistance, or a faster rate of discharge that results from the lower capacity. This means that if any of the weak cells hit the cell under-voltage protection limit while the pack voltage is still sufficient to power the system, the full capacity of the battery will never be used as the pack protector will prevent over-discharge (which would damage the cell) by stopping the discharge of the whole pack when one cell voltage goes below the cell under-voltage threshold.

The solution to this is cell balancing techniques:

The fundamental solution of cell balancing equalizes the voltage and state of charge among the cells when they are at a fully charged state. Cell balancing is typically categorized in two types:

1. Passive: Passive balancing allows the stack to look like every cell has the same capacity as the weakest cell. Using a relatively low current, it drains a small amount of energy from high SoC cells during the charging cycle so that all cells charge to their maximum SoC. This is accomplished by using a switch and bleed resistor in parallel with each battery cell. The high SoC cell is bled off (power is dissipated in the resistor) so that charging can continue until all cells are fully charged.

2.Active: Active cell balancing, which utilizes capacitive or inductive charge shuttling to transfer charge between battery cells, is significantly more efficient because energy is transferred to where it is needed instead of being bled off. Active cell balancing redistributes charge during the charging and discharging cycle, unlike passive cell balancing, which simply dissipates charge during the charge cycle. Thus, active cell balancing increases system run-time and can increase charging efficiency. Active balancing requires a more complex, larger footprint solution; passive balancing is more cost-effective.

Active cell balancing using inductor: