What 26-Cell Battery Monitoring and EIS Mean for High-Voltage BMS Design
Key Highlights
- Electrochemical impedance spectroscopy (EIS) gives battery-system designers information from inside the cell, going beyond traditional external temperature and voltage monitoring.
- TI’s 26-cell battery monitor integrates more monitoring channels, which can reduce the number of chips required in high-voltage battery management systems and help contain BMS cost as battery packs add more cells.
- EIS can provide battery-cell core-temperature information that helps designers adjust fast charging and use closed-loop feedback to protect battery health.
- Precise EIS measurement depends on closely synchronizing voltage and current measurements; if they are not aligned, the measured impedance may not reflect the intended value.
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As high-voltage battery packs incorporate more cells, battery management systems must provide precise cell monitoring without driving up system cost. Higher-cell-count battery monitors can reduce the number of monitoring devices required, while electrochemical impedance spectroscopy (EIS) can give designers greater insight into conditions inside individual battery cells.
Brian Burk, senior product manager at Texas Instruments, says EIS can provide information such as the core temperature of a battery cell, enabling closed-loop feedback that can inform charging decisions and help protect battery health. In this conversation, Inside Electronics host Alix Paultre talks with Burk about TI’s new 26-cell battery monitor and how EIS changes what designers can learn about battery-cell behavior.
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Alix Paultre: I understand you’ve recently come out with a new device that's the first 26-cells-in-series channel battery monitor. Can you tell us a little bit about that and how its core technology could differ from some of the other competing monitoring technologies?
Brian Burk: Our new battery monitor is the highest cell count on the market. What that really means is just a lower cost to implement the BMS. Every channel has slightly lower cost, because we've reduced the components around the BMS chip.
We've also allowed you to reduce the number of chips in the system as well. In a traditional battery system, you may need 10 chips today, using old generation 10 to 12, just depending. But now you can use maybe eight, and so that helps reduce some of the costs there.
The reason why that's important is we see an evolution in voltage of the battery packs, both in energy storage and in EVs. Traditionally, it's been in EVs 400, especially here in North America, but then we also see it evolving into 800 volts around the globe now, even in North America, and energy storage is even going higher — it's going to 1,500 volts. That means you're getting more and more cells. So now we've progressed from 100 cells in a pack to 200 cells in a pack.
And now we'll see 250 cells, maybe 300 cells in a pack. With this new chip, we can help contain that cost and reduce it by integrating more channels.
Paultre: Having that information and doing such sophisticated analysis of how the cell is behaving — and making accurate predictions based on performance — that can significantly improve safety and performance.
Burk: What most people don't know is most of the monitoring that we do is external to the cell, so we're monitoring the temperature of the cell. We're using a temperature sensor that's basically taped to the cell, and then we're monitoring the voltage of the cell. But there's a lot of noise in the system, especially from the engine or the inverter of the system, so it's very hard to get an accurate measurement just with our traditional methods.
Being able to see inside the cell with EIS is really what is groundbreaking about the EIS technology. Think of it like an EKG. We send a signal through your body, and it tells us a little bit about your heartbeat and what's going on inside your body. It's the same thing with batteries. We're actually sending signals through the batteries so we can see inside what's going on, which gives us more information — and more accurate information.
Paultre: Are there any other aspects that distinguish it from simple passive? I mean, there's a proactive as opposed to a reactive application, right?
Burk: By looking inside the cell, we can look directly at the core temperature of the battery. Rather than having that temperature sensor outside the cell, like I mentioned, you're getting the core temperature of the battery. When you start charging the cell, the cell is going to naturally heat up.
Especially when you fast charge, you're dumping a lot of electrons into the cell, and there's a lot of things happening inside that cell to basically absorb those electrons. We're actually heating the cell by doing that, and by using EIS, we can get that core temperature of the cell to make sure that the battery's not overheating while charging.
Just imagine charging in the middle of a frigid winter up in Minnesota, in the U.S. It gets to negative temperatures all the time, and you don't want to charge your battery super fast in those frigid temperatures, because the battery's not ready to absorb the electrons, and the lithium's a little bit slower. In those cases, we want to back off a little on fast charging to preserve battery health.
But if you don't have this closed-loop feedback that EIS offers, it leads to exactly what you mentioned, which is sometimes thermal runaway.
Paultre: How does this new technology fit within TI's battery management approach?
Burk: We're talking a little bit more about this battery monitor that we've recently released, but it really is a chipset. It comes with a battery monitor; there's a pack monitor that does current sense.
Those two things go kind of in tandem, and by using them together, we've added features that let them synchronize very closely, so we can enable precise EIS measurement. Not only is measuring EIS important, but also synchronizing all these chips together. That's important because if your voltage and current are not aligned, you'll start to measure stuff that's not real, and so you won't be able to know if the impedance you're measuring through EIS is actually the impedance you want.
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[Technical Article] Enabling the Next Phase of Battery Intelligence
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