Cell Temperature Sensing - Battery Design

Cell temperature sensing is a critical function of any Battery Management System (BMS) this is because the cell temperature needs to be kept within a band to maintain safe operation. This band is narrower still to maintain the lifetime of the cell and hence reduce ageing.

Cell Temperature

For each cell the manufacturer will define temperature limits for normal and safe operation.

Some of these temperatures are hard limits for the continued safe operation of the cell.

For most cells they will operate best between 15°C and 35°C.

Jinasena et al [1] break the sensing down into Hard and Soft Sensors. Using this as an initial list we can extend this further into a more complete list of sensors:

Hard Sensors
- Contact
  - Thermocouples
  - Thermistors
    - Thermocouple vs Thermistor
  - Resistance Temperature Detectors
  - Optical Fibre Sensors
- Contactless
  - Electrochemical Impedance Spectroscopy
  - Johnson Noise Thermometry
  - Infrared Laser Thermometer
Virtual Sensors
- Mathematical Model Based Estimators
  - Bulk Internal Temperature Estimators
  - Distributed Internal Temperature Estimators
- Hybrid Model Based Estimators

How Many Temperature Sensors are Required?

The exact number of temperature sensors you need depends on the size of your battery pack, its physical configuration, rate of charge/discharge (C-rate), and the required level of thermal observability.

In commercial Battery Management Systems (BMS), it is common practice to use one temperature sensor for every 4 to 20 cells, depending on the specific pack design. Installing a physical sensor on every single cell in a high-capacity pack is generally avoided because it introduces excessive manufacturing costs, wiring complexity, and added weight.

Firstly though we should consider this from the thermal viewpoint. The temperature gradient across a cell needs to be <2–3°C as a maximum excursion, and the temperature difference between cells also needs to be <2–3°C. If the temperature gradients and differences are too high, the cells and pack will age very fast, it will be difficult to deliver the energy and power, and maintaining the safety of the pack will require throttling the performance envelope. This means that the electrical and thermal design of the complete battery system needs to be designed around these requirements, and the cell capabilities need to be fully understood.

→ How Many Temperature Sensors are Required?

Hard Sensors

This is an actual physical sensor that is in contact with the cell or contactless. In the case of hard sensors there is also the number and location to be considered.

The location and number of temperature sensors is important to consider. Often it is not practical to fit a temperature sensor on every cell, also, extra sensors might not give you more detailed information.

Smart Battery Development

Research is ongoing to put sensors inside the battery cell, thus giving the ability to measure key internal variables such as electrode potentials, current, temperature, mechanical stress and internal pressure.

You do need to consider redundancy as a single thermistor is very cheap as a part, but could be very expensive to replace. Hence often two sensors are used for a group of cells with the BMS able to use those two signals to check operation and one failure can be managed.

comparison of thermistor and thermocouple

Thermocouple vs Thermistor

The near linear response, robustness and wide operating temperature range make the thermocouple ideal for test environments. However, the need for more electronics and overall higher cost mean that the thermistor is better suited to the production environment.

Contactless

Estimating Battery Internal Temperature Through Electrochemical Impedance Spectroscopy

Critical findings include:

Temperature sensitivity: -0.35°/K for temperatures below 23°C and -0.17°/K above 23°C
Universal applicability: These sensitivities remain remarkably consistent regardless of cell capacity or chemistry
Measurement precision requirements: To achieve sub-1 K temperature uncertainty in large-format cells above 23°C, EIS systems must maintain measurement uncertainty below 4 μΩ for both real and imaginary impedance components

Virtual Sensors

Process or plant models designed to estimate many parameters, including the temperature of the cell.

Downside is that this is specific to the cell and is likely to be dependent on a number of parameters, including the SOC and SOH of the cell. Hence it is likely that a large test data set will be required.

The upside is that these soft sensors don’t have the failure modes associated with physical sensors that can become disconnected or just break.

→Virtual Temperature Sensors

Calibration

With any type of sensor we need to calibrate the system against the pack design. This will require the pack to be instrumented with lots of temperature sensors and for the pack and cooling system to be operated over the full envelope.