A battery management system (BMS), in addition to many other functions, has to closely monitor voltage, current, and the temperature of battery cells and packs. Temperature measurement is important in preserving the operational characteristics of both the cells and the BMS itself, as well as optimizing the state of health (SOH) by preventing degradation, especially during the phases of fast charging and discharging.
Temperature measurement is generally performed by reading the voltage of a device with temperature-dependent properties – most often resistive devices such as thermistors or RTDs. Other technologies like thermocouples require cold junction compensation and proper shielding with millivolt readings, while diode/BJT-based sensors need constant current excitation. The main benefits of using NTC thermistors are their high sensitivity, good accuracy, price-performance ratio, and versatility. Their available executions allow for ease of contact measurement to offer the best temperature sensing option for each spot or area that has to be monitored. A comparison of different contact temperature measurement technologies can be found in Table 1 below. Thermocouples are often used during the design phase.
In high power battery packs, the BMS requires multiple temperature sensor inputs to guarantee the best overall performance, due to the size of the pack and possible thermal gradients inside the pack that could come from individual cells and/or charge/discharge conditions.
A negative temperature coefficient (NTC) thermistor presents a non-linear exponential decreasing resistance/temperature characteristic, as shown on Figure 1 and Equations 1 and 2.
An advantage of NTC thermistors is the possibility of producing them with different resistance values (R25) and slopes (B-value) in many different styles, from surface-mounting on a (remote) PCB to a highly insulated surface sensor that can be mounted by a screw or even welded to a connecting bar.
Used in a resistor voltage divider network, as shown in Figure 2a, the thermistor voltage will depend on the temperature with an S-shaped form (see Figure 2b and Equation 3).
The relation between temperature and Vtherm in Figure 2b can be established either in a lookup table (LUT) or by using an algorithm (2)+(3), which will allow an ADC and controller IC to apply the pre-defined strategy to control the different phases of the charging or health conditioning of a battery pack.
As a simple illustration, we can use the LTC4071 from Analog Devices, which is a charger IC for Li-ion and Li-polymer battery packs, used for energy harvesting applications and in embedded automotive systems.
The simulation is reprinted in Figure 3. Basically, the schematics replicate the SPICE macro model of the LTC4071 provided by Analog Devices and a model of a Li-ion battery.