Lithium-ion batteries offer high energy density (relative to their size and weight), high efficiency, and a long lifespan. Due to these benefits, high-capacity lithium-ion batteries are the technology of choice for most electric vehicles (EVs).
However, lithium-ion batteries have one major disadvantage. They’re susceptible to thermal runaway. The battery cells can still overheat due to physical damage, manufacturing defects, or overcharging. Therefore, temperature monitoring of lithium-ion battery packs is a critical safety function. Detecting temperature rises early in a battery pack minimizes the risk of a cell entering an uncontrolled thermal runaway and igniting a dangerous fire.

Figure 1. Precise thermal regulation in EV lithium-ion batteries is crucial for safety, preventing overheating, and potential thermal runaway. (All images courtesy of Littelfuse, Inc.)
One solution to the thermal runaway challenge is continuously monitoring each cell in a battery pack using the Distributed Temperature Monitoring (DTM) method. This approach employs a conformal tape embedded with closely spaced printed thermal indicators (PTIs), which adhere to all cells within the pack to provide a rapid indication when a cell’s temperature exceeds a preset threshold.
Typically, conventional discrete temperature sensors, like thermistors, are used. But they cannot conveniently be placed on every battery cell, and may fail to detect a temperature rise until a cell reaches an unsafe level. In contrast, distributed temperature monitoring capabilities, such as those offered by digital temperature indicator platforms, enable more precise detection of localized overheating, thereby enhancing battery life and improving the safety of battery installations.
Understanding DTM
Figure 2 shows the two ends of an example DTM solution. The black dots are printed thermal indicators (PTIs). The lower end has solder pads for connection to a circuit.
Figure 3 describes the dimensional details of the DTM example, indicating the input and output solder pad connections. This example includes 10 PTIs spaced 30 mm apart on a tape having a total length of 337 mm. By customizing the design, the DTM can have up to 50 PTIs spaced apart from as close as 10 mm. The total length of custom designs would typically be under a meter.
The narrow spacing between the PTIs allows increased spatial resolution for monitoring the temperature of all cells in the battery pack. Discrete temperature sensors cannot achieve such close monitoring of all individual battery cells.
With a thickness of under 1 mm, the DTM device can conform to the irregular surface of the battery pack and have direct contact with the individual battery pack cells. Pressure-sensitive adhesives bond the DTM device to a surface. The adhesive allows bonding to metal, polyamide, PET, and polyimide surfaces.
How the PTIs work
The PTIs are polymer positive temperature coefficient elements that experience a substantial increase in resistance when exposed to a threshold temperature. The resistance of a PTI can surge by nearly five orders of magnitude, rising dramatically from a low resistance of around 100 Ω.
The threshold temperature is set at 58±3° C, which accommodates any battery cell temperature fluctuations that occur when a battery pack is powering a varying load or is being re-charged. This temperature avoids nuisance alarm conditions and the set point is well below the temperature where thermal runaway occurs. The PTIs are connected in series, so when any single PTI detects a temperature above the threshold, the resistance of the DTM device increases significantly.
The PTIs also have a hysteresis effect. Once a PTI transitions to the high resistance when the threshold temperature is detected, it does not return to the low resistance state until it senses that the temperature has fallen to 42±3° C — a 16° C differential. The hysteresis characteristic avoids rapid on-off cycling of control electronics when the battery temperature is around the threshold temperature. That helps to avoid unnecessary wear on critical components and contributes to the safety and durability of the battery pack.
An example circuit
Although the DTM device in this example monitors several cells, the circuit employs the temperature indicator platform with a single output. A simple logic circuit can indicate the DTM’s status.
Figure 4 shows a circuit with a pull-up resistor. When a PTI detects a threshold-exceeding temperature, VT becomes a logic high above 3.5 V. The output voltage, VT, can connect to a comparator or a microcontroller circuit to initiate cooling the battery, interrupting the load, or turning off the charger should the DTM’s sensor detects a hot spot on the battery pack).
Under normal operating conditions, the circuit consumes around only 20 µA if VDD is 5 V. The plot in Figure 3 shows the hysteresis characteristic and how the voltage remains at a logic high level until the temperature falls to the 42±3° C level and below.
Performance comparison with thermistor sensors
Currently, the typical solution for battery temperature monitoring is the use of thermistors. The challenge is determining the number of thermistors necessary for a battery pack and their placement to comprehensively monitor an EV battery assembly. Discrete sensors complicate connection to the battery pack and add a significant amount of wiring to the electronic monitoring circuitry. So placing a thermistor on every battery cell is not a viable solution.
The example DTM solution responds faster than a thermistor. The PTIs have a response time of under one second to an overtemperature condition. Figure 5 (left) compares the response of a DTM sensor to an NTC thermistor when the two detectors are in contact with a surface that exceeds 60° C. The DTM sensor changes state in under 0.55 s. The thermistor temperature rises to the threshold temperature in over 2.8 s. In this case, the example DTM PTI responds 2.2 seconds faster than the thermistor.
Of greater significance is the simulation shown in Figure 5 (right). If a thermistor is on the battery cell next to a hot cell, the thermistor will require 2.4 minutes to detect the overtemperature condition. If the thermistor is separated by one cell from the hot cell, a thermistor will need 11.4 minutes to indicate an overtemperature condition.

Figure 5. A comparison of the response of the example DTM device’s printed temperature indicator to an NTC thermistor. Left: Responses to a 60° C surface example DTM device versus NTC. Right: Detection times for the example DTM on the hot cell, including the distances from the hot cell.
In contrast, the example DTN PTI attached to the hot cell only needs 0.55 seconds to indicate an overtemperature — which can escalate in minutes.
Only a pair of wires interface the tape with the temperature threshold detection circuit. Unlike discrete negative temperature coefficient (NTC) thermistors, the temperature detection circuit does not require calibrated measurement electronics or a look-up table to output an accurate temperature reading. Additionally, individual thermistors minimally require a two-wire connection.
For the best accuracy, thermistors require a four-wire Kelvin connection. The amount of wiring needed for multiple thermistors can be significant, and potential reliability concerns can arise from poor connections or wires breaking.
Dual-trip temperature DTM
Another DTM application contains two independent series strings of PTIs. Each string increases its resistance at a different threshold temperature.
Figure 5 illustrates an implementation of the logic circuits for the single-trip DTM device and the dual-trip DTM device. The analog inputs of a microcontroller measure the output voltage or voltages of the DTM device. The lower diagram illustrates options for dual temperature threshold levels. For example, the lower threshold temperature could create a warning status indication, and the higher temperature threshold could initiate a power shutdown.

Figure 6. The circuit implementations for single and dual-trip DTM devices and various temperature activation levels.
Applications
In this case, the example DTM platform is AEC-Q200 qualified for use in the automotive environment to protect EV battery packs. It can also protect battery energy storage systems used for back-up power by utilities.
DTM devices can also monitor temperatures on busbars, printed circuit boards, and capacitor banks. Additionally, they can track temperature distribution across motors, such as those in EVs, to detect winding hotspots.
Enhanced safety and maximized battery life
Distributed Temperature Monitoring (DTM) platforms, such as the temperature monitoring tape, can provide high-density temperature monitoring with a fast response to detect battery cell hotspots quickly. Hotspots can result in premature battery module aging and potentially catastrophic damage when not detected.
The small size of the temperature sensors and the tape form factor enables convenient installation and monitoring of every battery cell in a battery pack for early indication of an overtemperature condition. In addition to the base device, an enhanced version combines two series sets of PTIs to allow setting two different temperature thresholds for specific control applications. DTM devices provide a simple, high-reliability method to monitor all the cells in a battery pack, offering enhanced safety and extended battery cell life.
Filed Under: Batteries, FAQs, Thermal Management