Thermal-Monitoring "Tape" Helps EV Batteries Beat the Heat

This article is part of the TechXchanges: Cool Designs and EV Battery Management.

Lithium-ion (Li-ion) batteries have their pros and cons. But their unique combination of high energy density, moderate self-discharge rate, and endurance to handle many thousands of charge-discharge cycles before reaching the end of their lifecycle has turned the battery chemistry into the gold standard of the electric-vehicle (EV) era.

Li-ion batteries are only getting better over time. However, despite the power-handling capabilities of these batteries, they have deficiencies that must be carefully managed.

One of the risks inherent to Li-ion batteries is their tendency to be highly combustible. If the cells comprising the high-voltage battery pack inside the EV are damaged or incorrectly charged or drained, they can overheat uncontrollably or even erupt in flames. Given that explosive potential, it’s important to have accurate thermal monitoring of the battery cells at any instant in their lifecycles to ensure safe operating conditions.

To help enhance the thermal management of high-voltage EV battery packs, Littelfuse launched a new thermal-monitoring device called TTape. The thin, flexible tape is embedded with a series of tiny, printed temperature sensors that can be wrapped around several Li-ion cells at a time. By tracking the local temperature of the cells instead of monitoring the entire pack, the tape can detect overheating before it brings about a thermal runaway, offering superior safety and battery life enhancement.

With a response time of less than a second, the tape can alert the battery-management system (BMS) to abnormal levels of heat building up in the cells faster than existing solutions, such as thermistors, said Littelfuse.

The tape offers more flexibility in terms of its placement in the battery pack. It’s possible to use a single strip of temperature-monitoring tape to monitor several cells in the pack at the same time, increasing the “spatial resolution” of overtemperature monitoring. This feature also simplifies the cell installation process. The tape is backed by adhesive so that you can peel and stick it directly on the Li-ion battery cells in the EV.

According to Littelfuse, the thermal sensing tape is suited not only for Li-ion batteries in consumer and commercial EVs, but also large-scale commercial batteries that act as energy storage for the grid.

Battery-Management Systems Keep the EV Cool

Every EV is packed with as many battery cells as possible—physically and economically—to increase the storage capacity of the packs housing them, which can account for approximately 30% to 40% of the EV’s total production cost.

Since the capacity of Li-ion batteries varies over time and usage—and even more so when operating under harsh conditions out on the road—every cell must be carefully managed and regulated to perform at its best and protect the battery cells from permanent damage. If the cells are damaged in any way, the EV battery can suffer from reduced driving range and a shorter useful lifespan, or it can raise the risks of explosion.

The cells are typically wired to a BMS that plays the pivotal role of monitoring the voltage, current, and other facets of each cell over its lifetime. It’s also engineered to adapt to the constantly shifting conditions that the EV battery cells are exposed to and, in that way, to maximize the state of charge (SOC) of the cells at any point in time and over their lifetime, also called state of health (SOH).

The primary role of the BMS lies in balancing the level of charge stored in each battery cell, as overcharging or undercharging the battery cells may also cause physical stress, leading to premature cell aging. Cell balancing helps prevent cell degradation. The system’s other role lies in correcting the temperature fluctuations inside and outside the cells comprising the battery pack, which in some cases can imply the onset of a thermal runaway.

Exposure to severe heat or cold can also lead to large variances in a battery’s performance and lifespan that must be accounted for. Frigid temperatures, for instance, can inhibit chemical reactions in the battery cells, sapping their usable power and slowing down the charging process.

While there are many ways to keep tabs on the temperature of the battery’s cells, one of the most widely used is a type of thermal sensor called an NTC thermistor. A thermistor is a type of temperature sensor that exhibits a large change in resistance proportional to a change in temperature. Thus, by measuring the resistance, you can figure out the amount of heat afflicting the system.

However, these thermal sensors may not be the best fit for the Li-ion cells in EV batteries. Specifically, thermistors lack the rapid response times and accuracy necessary for safe battery management, Littelfuse said. They also tend to take a broader view of the temperatures affecting the battery pack. Placing these bead-, disk-, or cylinder-shaped devices inside the battery pack in such a way to perform "localized" cell monitoring is a major challenge, the company said.

Distributed Temperature Monitoring at the Cell Level

However, you can never be too careful when it comes to protecting a Li-ion battery from harm. The tape is intended to supplement the thermistors and other sensors in EVs instead of replacing them. Since the tape has a faster response time, the BMS may start cooling the EV battery cells at an earlier stage and help stop thermal runaways before they get out of control and negatively impact the battery’s longevity.

Because no calibration or temperature lookup tables are needed, the tape can be easily integrated into existing battery-management units along with NTC thermistors, delivering enhanced overtemperature detection at the cell level.

In the event it detects overheating in the battery cells, the tape wakes up the MCU at the heart of the BMS from sleep mode over a single input. The device uses a dual-wire interface to sense if any of the cells in the battery runs over a critical temperature limit. The tape measures 10 mm wide and stretches up to 1 m. The product comes in tape-and-reel or box packaging, depending on the length required.

The thin form factor of the tape is also ideal for “conformal installations,” meaning it can bend around sharp corners, rounded edges, or pouch-shaped bellies of the Li-ion cells in battery packs. As industry insiders point out, EV batteries are becoming huge clusters of high-voltage energy storage. The 400/800-V batteries being plugged into the latest EVs typically comprise 200 cells connected in a series.

Since the tape is automotive grade, it’s rigorously “qualified” against a long list of industry standards to ensure durability and reliability for cars. It can tolerate operating temperatures of −40 to 85°C.

Read more articles in the TechXchanges: Cool Designs and EV Battery Management.