Only as Strong as the Weakest Link
The electrical potential or voltage of a battery is dictated deterministically by its chemistry. There is a specific range of operating voltages over which each type of chemical cell operates. For example, the nominal voltage of a lead-acid battery cell is approximately 2 volts. Common lead-acid batteries, such as those found in cars, operate at a nominal 12 volts. These are in fact battery packs comprised of six single lead-acid cells connected in series such that their individual voltages are additive. Higher voltage allows for greater efficiency and cheaper, thinner wiring.
This illustrates the need to connect cells in series to achieve a required operating voltage. For reasons of efficiency and practicality, the operating voltage of an electric vehicle (EV) is typically quite high, usually over 300 volts. Depending on the specific chemistry, the nominal cell voltage of a lithium-ion battery varies, but most common chemistries fall within the 3.6-3.8 V range, necessitating the use of many cells connected in series.
A challenge arises when cells are connected in series; within a group of seemingly identical batteries there will be small variations in capacity. This causes the voltage of cells connected in series to differ from one another during use. While the initial variance may be miniscule, over time if left unchecked, these disparities in cell voltage will grow with each charge and discharge cycle, akin to compounding interest. The solution is to equalize the voltage between series connected cells.
With less volatile chemistries, such as lead-acid, the issue of cell voltage drift can be mitigated by maintaining the battery pack at a voltage significantly greater than nominal voltage during charging. This allows the weak cells to reach a full charge, while the more robust cells find themselves being safely overcharged. This solution is dependent on the battery chemistry’s ability to tolerate significant overvoltage. Lithium-ion chemistries do not tolerate this abuse well, and can lead to a cell heating up and eventually to thermal runaway. Furthermore, lithium-ion cells do not tolerate undervoltage. While other chemistries can be discharged fully, lithium-ion batteries should not be allowed to discharge below a certain critical voltage because a permanent decrease in capacity can result, while increasing the risk of catastrophic failure.
A sophisticated scheme is therefore required to ensure maximum capacity and safety for lithium-ion battery packs. A battery management system (BMS) is designed to compensate for small variations between cells, while providing additional safety features. A BMS measures the voltage of each cell in a battery pack and ensures that those values stay within an acceptable range during both charging and discharging. Additional parameters, such as temperature and discharge current are monitored.
While a variety of topologies can be utilized in a BMS to balance cells within battery packs, the most common and easiest to understand is that of top balancing using shunt resistors. In this scheme, when a particular cell reaches its maximum safe voltage, the BMS begins shunting, thereby limiting that cell’s voltage. This allows the other, lesser cells to continue to charge without causing a safety hazard or additional cell degradation. The result is that, at a full charge, every cell in the pack will be at the same voltage and state of charge. Of course, the lowest capacity cell will discharge and reach its low voltage cutoff first, meaning that the higher capacity cells will retain residual energy that cannot be accessed. However, this is an acceptable inefficiency for the increased safety factor and pack life. Upon recharge, balance will be restored once again.
Lithium-ion batteries are playing a crucial role in both mobile and stationary energy storage as we transition to renewable energy sources. Modern battery chemistry requires sophisticated systems to maintain efficiency and safety. To this end, engineering highly efficacious battery management systems is critical to the continued adoption renewable energy technologies.