One of the key aspects of electrified vehicles is the battery management system (BMS). Despite the progress that has been made reducing the cost of batteries, they remain the most expensive component of an electric vehicle and if not handled properly, they can also be the most fragile. Batteries are also still shockingly low in energy density compared to traditional fuels. Chip maker NXP is doing its part to help the BMS make EVs more practical for drivers in partnership with automakers like Volkswagen.
The purpose of the BMS is to track and manage the flow of energy into and out of the battery pack and estimate its state of charge. That state of charge is itself important to understand not just because it can inform the driver of how far they can go but also, but also to get the most out the battery on the current drive as well as over the life of the vehicle.
We’ve likely all experienced the annoyance of a rechargeable battery that loses its capacity to hold a charge. It’s bad enough that you might have to spend $100 to get a battery replaced in a smartphone or laptop. Doing the same in an EV will cost many thousands of dollars. One of the major causes of battery degradation is overcharging or drawing down the state of charge too deeply.
One way automakers try to extend the lifespan of batteries is to use a buffer at the upper and lower limits of state of charge. For example, they may only charge it 90% and deplete it to five percent to ensure the limits are never exceeded. However, that means the vehicle can only use 85% of the full capacity of the battery and thus has 15% less range than it might if all the battery is used. The car is now carrying around extra mass that never gets used and adds to the cost.
A more accurate measurement of state of charge could allow those buffers to be reduced, enabling more range without the risk of damaging the battery. However, measuring state of charge is problematic. Unlike many old-fashioned consumer batteries, the voltage of lithium ion batteries doesn’t decline linearly as the charge drops. It tends to stay relatively constant through most of a charge cycle. Thus the BMS contains a software model that tracks the flow of electrons in and out of the battery to estimate the state of charge. The more accurately that flow can be measured, the closer that state of charge model is to reality.
Prior to NFC’s acquisition of Freescale and its processor business in 2015, much of the company’s legacy was in analog electronics. That analog expertise plays a key role in its development of modern BMS. NXP has developed 14-channel analog devices that track and manage individual cells within a battery pack down to 1 mV.
NXP’s reference design for battery