Every battery chemistry has its tradeoffs, but the tradeoffs with lithium-ion (Li-ion) batteries are more favorable than most. Li-ion batteries are the gold standard in virtually all electric vehicles (EVs), largely due to a combination of high energy density by volume and weight, moderate self-discharge rate, and the ability to sustain thousands of charge-discharge cycles before reaching the end of their service life.
But since they account for approximately 30% to 40% of the total cost of the average EV, such batteries must be carefully monitored and managed to extract as much power as possible on a single charge and protect them from permanent harm. Despite the power-handling properties of the latest formulations, they tend to be highly combustible and thus must be managed with care to prevent safety hazards.
NXP Semiconductors is bringing higher levels of safety and reliability to EV battery management with its next-gen battery cell controller IC, the MC33774. Rated for ASIL D functional safety, the company said the analog IC has the ability to accurately measure the voltage of a battery cell to within 0.8 mV. It also can monitor up to 18 cells at the same time and run passive cell balancing to maximize their usable capacity.
The chip can be used to calculate the state-of-charge (SOC) of battery cells at any point in time and over their lifetime—a metric called state-of-health (SOH). It’s also engineered to adapt to the constantly shifting conditions that EV batteries are exposed to and, in that way, maximize both metrics.
The BMS: The Backbone of Electric Vehicles
Battery management isn’t getting any easier given the rising complexity and density of EV batteries.
Every EV today is packed with as many battery cells as possible to increase the storage capacity of the packs housing them. These bulky systems can weigh up to thousands of pounds and cost thousands of dollars, with the cells adding up to hundreds of volts at a time. As NXP pointed out, the 800-V Li-ion battery packs being built into the latest EVs typically comprise 200 individual cells connected in a series.
Since the capacity of Li-ion batteries varies over time and usage—even more so when operating under harsh conditions out on the road—every cell must be managed and regulated to perform at its best. Each cell is typically wired to a battery-management system (BMS) comprised of chips such as NXP’s MC33774. It plays the pivotal role of monitoring voltage, temperature, and other facets of each cell over its lifetime and under varying conditions to keep them from wasting precious energy.
Even minute improvements in the measurement accuracy of battery-monitoring ICs can add up, extending the driving range of electric vehicles by several percentage points in a single leap.
One major challenge in EV batteries revolves around balancing the charge stored in each cell. These cells also lose storage capacity over time due to the stress of repeated charging and discharging, and they degrade at different rates.
For safety, EV battery packs tend to stop supplying power when the weakest cell in the bunch is fully drained. Uneven charging of the cells can cause permanent harm or cause safety hazards. Unless the battery cells are charged and drained uniformly, the EV will not last as long per charge.
The other role the BMS plays in the EV involves correcting the temperature fluctuations inside and outside the battery. Exposure to heat and cold can lead to large variances in its overall performance and lifespan.
A modern BMS must also measure what’s happening inside the battery cells to mitigate risks—both in the short and long term. One of the risks associated with high-voltage EV batteries is thermal runaway. If the battery cells are damaged or incorrectly charged or drained, they can overheat uncontrollably and even erupt in flames. Overcharging or undercharging the cells comprising the battery may also cause physical stress, leading to premature charge termination and even a reduction in its useful lifespan.
Moreover, NXP pointed out that it’s necessary to maintain accuracy and precisely measure and adapt to the internal state of the battery cell at any instant over the years-long lifecycle of these Li-ion batteries.
Battery Monitoring Made to Handle Harsh Conditions
According to NXP, its latest battery-cell controller more than fits the bill for modern EV battery management.
Based on its SmartMOS silicon-on-isolator (SOI) technology, the analog front-end IC delivers voltage measurement accuracy at the cell level within 0.8 mV, and it can monitor and manage up to 18 separate cells at a time over a wide automotive-grade temperature range. The device can also be daisy-chained in strings of up to 62 devices for use in safety-critical, high-voltage Li-ion battery packs.
NXP said the MC33774 was rigorously tested and qualified for the rigors of the road. The IC is rated for less than 1.5 mV of voltage measurement error over the full temperature range of −40 to +125°C. And, regardless of variances in the battery’s cell voltage over its lifetime, it enables precise predictions of the EV’s range. Though very precise with voltage measurements, the IC lacks the ability to sense current.
The accuracy of the IC’s assessments helps ensure lifetime performance over a wide range of battery-cell chemistries, said NXP. These include nickel manganese cobalt (NCM) and lithium iron phosphate (LFP).
The battery-cell controller also features MOSFETs that can output currents of up to 300 mA per channel to correct differences in the charge of the battery cells in a pack, a process called passive cell balancing.
The cells at the heart of a battery pack are never completely identical. They differ slightly in capacity, internal resistance, self-discharge rate, and other factors that are largely due to inconsistencies in manufacturing. At any given time, a single cell can have less charging capacity than the other cells in the battery pack. Consequently, the "weak" cells in the system limit the overall performance of the Li-ion battery.
Despite the slight imbalances in internal resistance and storage capacity, the battery cells in a series will all maintain the same charging and load current. Thus, not all cells will be refilled at the same rate when charging. Faults can occur when batteries are overloaded, which shuts down charging current if one cell is fully charged before the others in the pack. Passive cell balancing solves the problem by equaling out the amount of charge in the battery cells so that they’re fully replenished and safely depleted.
Housed in a 64-pin LQFP-EP package, the new NXP chip adds a fully redundant and independent internal architecture that supports ASIL D, the most rigorous standard for functional safety, when measuring the voltage and temperature of the cell connected to it. The I2C interface gives it the ability to control other devices inside the BMS. NXP said the MC33774 also enables communication over the SPI bus at 2 Mb/s.
A Battle Ahead in EV Battery-Management ICs
As battery management becomes more of a make-or-break feature in EVs, major players in the power semiconductor industry, such as Analog Devices, Infineon Technologies, NXP, and Texas Instruments, are wrestling to win more of the fast-growing market for battery-management ICs.
For its part, NXP is launching the battery-cell controller IC as a core component of its BMS chipset for EV batteries. The offerings include the MC33777, its future battery junction box controller for pack level measurement, and the recently released MC33665, which acts as a communication gateway within the BMS, linking together several MC33774s to monitor cells bundled in larger quantities inside the EV.