Battery packs are among the most expensive components in battery-powered products such as power tools, scooters, and electric vehicles (EVs). Battery pack performance significantly impacts vehicle-level concerns for EVs, including driving range, battery pack lifespan, and charging time, not to mention vehicle safety and reliability. Therefore, it's no surprise that battery management has become a subject of in-depth research and ongoing development.
From a vehicle system perspective, key performance indicators (KPIs) for battery packs include parameters such as DC-DC link voltage, energy density, specific power, and battery life. To date, lithium-ion (Li-ion) batteries have provided good performance; however, lithium-ion chemistry places a considerable burden on the "maintenance and power supply" of battery packs for automotive electronics.
The use of lithium-ion batteries requires the battery management unit (BMU) to push the limits of measurement accuracy in noisy electrical environments with common-mode voltages exceeding several hundred volts. In addition to monitoring battery voltage and temperature, the BMU must perform critical functions such as battery balancing and coulomb counting, while ensuring that the entire battery pack operates within safe operating ranges that meet the stringent functional safety requirements of ISO 26262.
Why lithium-ion batteries?
Energy density (Wh/L) and specific power (Energy/kg) are two key quality factors in EV battery design. These quality factors are driven by several vehicle-level performance parameters; perhaps the most important is the range per charge. To optimize the driving range per charge, energy storage must be compact and lightweight.
Higher energy density means a greater capacity of energy that can be delivered within the vehicle; this, combined with higher specific power and a lighter payload, increases the vehicle's driving range. Besides affecting driving range, the compactness of the battery pack also makes room for other key EV systems, such as onboard chargers and traction drive systems that convert electrical energy into motion. This enables the so-called "skateboard" configuration popular on multiple EV platforms.
We compared several common battery technologies. Currently, lithium-ion is the clear choice, and its use is widespread in today's automotive electrification. Nevertheless, lithium-ion batteries also have drawbacks. Charging is demanding, and it's difficult to measure the state of charge of a lithium-ion battery pack.
Lithium-ion batteries can be tricky, as evidenced by thermal runaway issues in consumer products such as hoverboards. Finally, lithium-ion is an expensive technology, not only because of the special materials that make up the batteries, but also because of the complexity of the battery and thermal management systems that must be in place to optimize performance and safety.
Lithium-ion characteristics
We have described the charging and discharging characteristics of a typical lithium-ion battery. Once the battery reaches saturation during charging (and even during discharging), the battery voltage remains almost constant for most of the operating range. This flat discharge profile makes it an attractive energy source for electric vehicles, as the battery provides nearly constant energy over a wide operating range.
However, this characteristic, along with other inherent qualities, presents challenges for battery management. More importantly, battery characteristics largely determine a vehicle's range, battery lifespan, safety, and vehicle availability. For example, it's necessary to know how far a user can travel before charging.
Different materials are used for the anode/cathode, which affects battery characteristics. For example, a lithium-ion battery can be charged to 3.8 V to 4.2 V with a tolerance of approximately ±50 mV, depending on the anode/cathode materials used. A battery is considered fully charged when the charging current is less than 3% of its rated current. While increasing the charging current does not affect the total charging time, it can accelerate the time to reach the plateau of approximately 70% capacity.
In fact, charging the battery to below 100% is preferable to extending its lifespan, as lithium-ion batteries cannot tolerate overcharging without causing damage and/or compromising safety. Therefore, system designers must weigh parameters such as range/charging, battery lifespan, safety, and charging time.
There are other challenges and nuances to consider. The batteries are connected in an array—a combination of series and parallel connections to increase voltage and capacity—which complicates managing overcharging or undercharging. BMU implements “battery balancing” to ensure that all cells in the battery pack (multiple cells connected in series) are actually at the same level of charge.
Monitoring battery temperature is important for several reasons. A significant temperature rise during charging indicates a malfunction. Furthermore, lithium-ion batteries do not charge well at low temperatures (e.g., freezing). In such cases, the battery control unit (BMU) can compensate by heating the battery.
Finally, even with tightly controlled charging and discharging, battery capacity will decrease over time as it undergoes numerous charge and discharge cycles. Electric vehicles compensate for this by conveying the vehicle's remaining range rather than battery capacity or state of charge. A brand-new car might be charged to 70% and discharged to 30%. As the battery pack ages and its capacity decreases, the BMU can extend the charging and discharging window to allow the vehicle to maintain its "fully charged" driving range for the vehicle's lifespan.
Battery cell management
Automotive EV/HEV batteries contain hundreds of lithium-ion cells connected in series and parallel. As the challenges already discussed clearly demonstrate, safe and lifespan-optimized operation can only be maintained through proper battery management. Each cell in the series connection must be individually diagnosed and balanced.
If the goal is to optimize vehicle-level KPIs, the signal path must provide the accuracy required to estimate the state of charge. Specifically, the measurement accuracy of battery voltage and battery pack current is crucial due to the flatness of the battery's charge/discharge curve. Furthermore, battery management solutions sometimes incorporate coulomb counting—measuring the ampere-second flow rate into and out of the battery pack—as a cross-check to estimate the state of charge of the entire stack.
