In recent years the Lithium-Ion (Li-Ion) battery has gained in popularity because the market for battery powered portable products is increasing significantly. The superior characteristics of Li-Ion cells are high gravimetric and volumetric energy density, low self-discharge rate and no memory effect. The consumer electronics world relies upon Li-Ion as the main portable power source. Cell phones and laptop computers are the dominant user of Li-Ion cells year over year. As Li-Ion cells evolve and different chemical compositions grow in popularity, other equipment can take advantage of Li-Ion properties. Power tools, electronic vehicles, medical and industrial tools now can also use Li-Ion batteries as their power source.

Consumer applications can require a single Li-Ion cell (cell phone), or three in series and two in parallel (notebook). Adopting this new equipment requires higher power, higher capacity and more rugged battery packs. Cells are added in series to raise voltage and added in parallel to add capacity. The resulting battery pack grows from six cells for notebooks to several hundred cells for automotive. These requirements create new design hurdles for battery designers by magnifying the less than ideal qualities of Li-Ion cells.

These large batteries require advanced management to ensure a quality design. We must consider proper temperature, voltage and current measurements. Thermal management, pack reliability, cycle life and cell balancing require more attention as the Li-Ion packs become larger. In fact, cell-to-cell differences in temperature, capacity and series impedance are a major concern as more cells are required in a pack. We will spend the majority of this article addressing the effects of these differences and how to manage these in a battery design.

The Issue: Cell Conditions Mismatch

The purpose of a battery is to store and deliver energy to its host. We want to be able to put in and take out as much energy from the pack as possible. A major item preventing a multicell battery pack from doing this is cell imbalance. Let us take a look at how this affects delivering energy to the host.

In Li-Ion battery packs there are predefined voltage minimums and maximums that each cell in series will be allowed to reach. This is a safety feature managed by an IC inside the battery pack. Take a look at Figure 1A. This battery pack will be able to discharge and charge as long as each cell stays in-between the over-voltage and under-voltage cutoffs. Should one cell reach either threshold, the entire pack would be shut down (under voltage), which would leave stranded charge in the pack that should be available for the host (Figure 1B). Or, this would not allow the charger to place as much energy into the battery pack as it should (Figure 1C) (over voltage).

Figure 1: The effects of cell imbalance on battery capacity utilization - For higher resolution, click here