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One of the most exciting developments brought to electric vehicles in the last few years is the development of lithium iron phosphate batteries as an alternative to traditional lithium-ion chemistries that use minerals such as nickel, manganese and cobalt. Now, new battery management systems (BMS) mean that the range of EVs with these batteries can be more accurately predicted.
Why LFP?
Until last year, LiFePO4 (LFP) batteries were mostly the domain of Chinese EV makers, as a series of exclusive patent licenses were signed with US and Canadian researchers who originally developed the technology. But those patents are expiring and automakers outside China are starting to adopt his LFP batteries.
LFP cells dislike very cold weather and store less energy than comparable NMC or nickel-cobalt-aluminum cells. But the last part may actually be an advantage of this chemistry. There is little need to surround the LFP pack with a heavy protective shell, as there is no danger of the LFP pack burning or exploding on impact.
This means that the energy density of the cell will be lower, but the energy density at the pack level will actually increase. This is because more volume is allocated to the battery cells rather than the collision structure. LFP batteries have a longer service life than NMC and NCA packs. LFP cells were about 20% cheaper per kWh than NMC cells in 2022.
So it’s no surprise that Tesla will switch to LFP cells in many of its cars in 2021, while Ford will add LFP cells to its Mustang Mach E this year and its F-150 Lightning pickup truck in 2024.
More Accurate Measurements Means More Accurate Distance Predictions
To take full advantage of the LFP battery chemistry, Texas Instruments has developed a new BMS (both battery pack and cell) that is much more sensitive than existing systems. “In the previous generation, we could go as low as 3.5 millivolts, which is state-of-the-art technology. Now, with this latest generation, it’s about 3.5 times better, 1 millivolt,” explains Sam Wong. At TI he works on BMSes.
This is necessary because the discharge curve of LFP cells is very flat. Mark Ng, his EV powertrain marketing manager at TI, explains: “When you have a state of charge of 70% to 30% in your area, [the discharge curve] It stays flat and that’s where you need this 1 millivolt accuracy. Because if I’m not correct, I have no idea if I’m right. [at] 70 percent or 40 percent charge,” Ng told me.
Its inaccuracy can be as high as 63 miles (100 km) at 300 miles (483 km) EV compared to BMS with only 10 mV accuracy. More accurate BMS also helps with NMC or NCA chemistries, but to a much lesser extent, improving the error margin from ~6 miles for 300 miles of EV to ~0.5 miles.
The new BMS, which meets the strictest ASIL-D automotive safety standards, also offers functional safety benefits. “With this device, we can use redundancy in the chip to measure the voltage twice and if they don’t match each other, we can tell the system that something is wrong. It’s almost like a voting system. This battery says 4 volts and the other side says 3 volts.”—Something must be wrong [the battery]” Ng explained.
Additionally, the new BMS can accommodate both 400 V and 800 V packs and can be specified wirelessly. General Motors is using an early iteration of TI’s technology to adopt what it already employs in its family of Ultium packs, reducing the need for copper. Wiring within the pack reduces both cost and weight.
