How to Boost Safety and Lifespan of EV Lithium-Ion Batteries

­The rapid adoption of electric vehicles (EVs) brings increased focus on lithium-ion battery safety and lifespan. Lithium-ion batteries are the dominant choice for electric vehicles (EVs) with their high power density. Yet, they pose serious risks such as thermal runaway—where a cell overheats, leading to potential smoking, charring, and other critical thermal events like fire due to catastrophic failure. Mitigating this threat has become a key challenge for engineers, especially as batteries pack more energy into tighter spaces.

One breakthrough solution is the TTape™ Distributed Temperature Monitoring Device, designed to enhance safety by rapidly detecting localized hotspots. This technology safeguards against critical battery failures and extends battery life.

Lithium-Ion Batteries: High Power, High Risk

Lithium-ion batteries stand out for their high energy density, allowing EVs to travel farther on a single charge with lighter, more compact batteries. However, these advantages come with the risk of thermal runaway, which can be triggered by overcharging, physical damage, or high temperatures. In thermal runaway, the battery’s internal temperature can spike uncontrollably, sometimes igniting the electrolyte and resulting in catastrophic failure.

The challenge for EV battery management systems (BMS) is to detect and mitigate these overheating events before they escalate. Traditional monitoring systems rely on discrete sensors, which, while effective, can miss critical localized heating. Enter TTape™, a high-density temperature monitoring solution that offers comprehensive coverage that rapidly identifies hotspots across the entire battery pack.

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Figure 1. TTape Distributed Temperature Monitoring Device showing closely spaced thermal indicators

TTape™: High-Density Temperature Monitoring in Action

TTape™ is a flexible, printed temperature indicator strip designed to be affixed directly to battery cells. Unlike conventional thermistors or temperature sensors that provide data at a few discrete points, TTape offers continuous, distributed monitoring. Its slim profile (less than 1 mm) and adaptability to various battery pack configurations allow it to cover irregular surfaces, ensuring that no critical spot is left unchecked.

The TTape strip is embedded with Polymer Positive Temperature Coefficient (PPTC) material, which functions as a highly sensitive temperature sensor. When a specific temperature threshold is exceeded (e.g., 58 ± 3°C), the material's resistance skyrockets, providing an instant alert to the BMS. With a spacing of 10mm between sensors, TTape detects localized temperature increases much faster than traditional systems. For comparison, a thermistor located a cell away from a hotspot may take over two minutes to react, whereas TTape sensors located directly on a cell respond in less than one second.

Extended Temperature Options for Versatility

TTape devices are available in multiple temperature thresholds to meet the specific needs of different applications. In addition to the 58°C version, it also comes in 75°C, 85°C, 130°C, and 175°C variants.

·       58°C: Designed to minimize battery cell aging and extend battery life.

·       75°C and 85°C: Ideal for thermal runaway protection, providing advanced warning of dangerous temperature increases.

·       130°C: Used to trigger a fire extinguishing system in extreme thermal runaway conditions.

·       175°C: Suitable for monitoring the rotor/stator temperature in electric motors.

TTape can also be configured with dual-temperature sensors, offering two discrete temperature monitoring points within a single tape, with separate output signals for each threshold.

Precision Design for Maximum Efficiency

TTape samples for initial evaluation are available in standard lengths of 337 mm. The final product – which will always be a custom design to fit specific battery packs and applications perfectly – can extend up to several meters, enabling monitoring configurations for diverse battery architectures. The Printed Temperature Indicators (PTIs) within the TTape generic samples are spaced at 30 mm intervals and have a diameter of 5 mm. With this layout, the 337 mm strip includes 10 PTIs. At the same time, custom designs can accommodate up to 50 at arbitrary positions along the tape (the minimum distance between PTIs is 10 mm), making it ideal for high-capacity battery packs.

The tape also features a pressure-sensitive adhesive that is compatible with a wide range of materials, including metal, polyamide (PA), PET, and polyimide (PI) surfaces. This adhesive ensures a strong bond to any battery module configuration. TTape’s form factor is flexible enough to conform to cylindrical, prismatic, and pouch cells, making it adaptable to a variety of battery types and applications, from EVs to power distribution and e-motors.

Electrical Properties and Circuit Implementation

The printed thermal indicators are Polymer Positive Temperature Coefficient (PPTC) elements, and their resistance increases by a factor of at least three decades when a temperature threshold is exceeded. This significant change in resistance allows the system to detect overheating conditions instantly.

A recommended temperature detection circuit involves a simple logic circuit with a pull-up resistor. When the temperature is normal, the circuit remains at logic low. If the temperature at one of the PTIs exceeds the threshold, the output voltage (VT) switches to logic high, triggering an alert. This design consumes only about 20µA of current at 5V, making it highly energy-efficient for continuous monitoring.

The Importance of Hysteresis and Reliability

One key feature of TTape is its built-in hysteresis, which prevents the system from cycling on and off too frequently when temperatures hover near the threshold. This design avoids unnecessary wear on battery management components and minimizes false alarms. The 58°C sensors feature a 16°C hysteresis range, meaning the system only resets when the temperature drops below 42°C. This ensures that the battery is fully cooled before normal operation resumes.

Simplified Circuit Integration

Integrating TTape into an existing battery management system is simple. A standard 200KΩ resistor is recommended in the logic circuit to monitor the system's state. If a hotspot is detected, the logic signal switches from low to high, and this output can be fed into an analog-to-digital converter (ADC) in a microcontroller, forming part of the BMS. Figure 2 in the original schematic demonstrates how TTape’s output provides real-time thermal data without complex temperature conversions.

Unlike traditional NTC thermistors, which require calibration and involve temperature conversion, TTape operates as a two-state device. This simplicity reduces processing time and ensures that even the smallest temperature spikes provide immediate feedback to the BMS.

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Figure 2. TTape safety circuit and hysteresis curve

A Step Beyond Traditional Sensors

While effective in localized sensing, traditional discrete thermistors cannot match the coverage and speed of TTape. Discrete sensors typically detect thermal events only when the temperature rise occurs directly underneath them. TTape’s continuous monitoring addresses this gap by ensuring rapid detection, regardless of the location of the heat source. This high-resolution monitoring not only enhances safety but also prevents premature battery aging by catching and mitigating excessive heat before it can cause long-term damage.

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Figure 3. Example TTape installation on a scooter battery pack

Bottom line:By using the TTape platform, designers of battery packs and BMS can initiate early countermeasures against any over-temperature events, subsequently preventing these and related critical situations. Other technologies can often only detect critical situations and not prevent them.

Extending EV Battery Lifespan and Safety

TTape helps engineers enhance EV batteries' overall safety and efficiency by providing precise and reliable thermal monitoring across entire battery packs. Its low-cost, lightweight design makes it a viable solution for automotive applications, and its AEC-Q200 qualification ensures it meets rigorous industry standards. Whether incorporated into new designs or retrofitted into existing systems, TTape represents a critical step forward in EV battery safety.

For technical details and application-specific advice, engineers can refer to the Lithium Batteries: Enhancing Protection and Control presentation and the TTape™ Distributed Temperature Monitoring Device Application Note. For additional resources and support, visit the TTape product page on the Littelfuse website.