“As mentioned in the article “Next Generation Battery Monitor: How to Improve Battery Safety While Improving Accuracy and Extending Running Time”, accurate monitoring of battery voltage, current, and temperature helps ensure that it is suitable for use in vacuum cleaners, vacuum cleaners, The safe operation of systems for mass consumer goods such as electric tools and electric bicycles. In this article, we will study the temperature monitoring of lithium batteries in more depth, including the correct configuration of the system for safe operation.
As mentioned in the article “Next Generation Battery Monitor: How to Improve Battery Safety While Improving Accuracy and Extending Running Time”, accurate monitoring of battery voltage, current, and temperature helps ensure that it is suitable for use in vacuum cleaners, vacuum cleaners, The safe operation of systems for mass consumer goods such as electric tools and electric bicycles. In this article, we will study the temperature monitoring of lithium batteries in more depth, including the correct configuration of the system for safe operation.
When the lithium battery is operated beyond the temperature range specified by the battery manufacturer, there is a risk of thermal runaway, which may eventually cause a fire or explosion. Therefore, in order to ensure the safety of the system and meet the requirements of various standards, the battery must be disabled whenever the battery temperature exceeds the specified temperature range. However, knowing when to disable the battery depends on the accuracy of the battery monitor and protector temperature measurement subsystem, which is essential to ensure the safe operation of the system.
The latest product of Texas Instruments battery monitor and protector series BQ76942 (3 batteries in series[3S], Up to 10S) and BQ76952 (up to 3S to 16S), integrated 16-bit/24-bit delta-sigma analog-to-digital converter (ADC), multiplexing between various voltage measurements, including measuring internal chip temperature and External thermistor.
BQ76942 (10S) and BQ76952 (16S) include an internal chip temperature measurement based on the ADC using its internal reference to measure the ΔVBE voltage. This voltage is converted into a temperature reading, which can be read through the serial communication interface.
Both battery monitors support temperature measurement using external thermistors on up to 9 device pins, which gives system designers more flexibility in choosing where to measure temperature in the battery pack. Separate thermistor measurements and internal chip temperature readings can be specified to be used as battery temperature, field effect transistor (FET) temperature, or neither.
The protection subsystem uses the measured value designated as the battery temperature to identify the battery temperature is too high/low during charging or the temperature is too high/low during discharging, and to determine whether to allow battery balancing. The thermistor that specifies the temperature of the FET is used to identify the FET overheating. Any thermistor that is enabled but not designated for battery or FET temperature will be used for temperature reporting, but will not be used by the protection subsystem.
The internal chip temperature also determines whether battery balancing is allowed and whether the device should be placed in the off state to avoid erroneous operation when it exceeds its specified operating temperature range.
The thermistor is measured when it is connected to the internal pull-up resistor connected to the REG18 (~1.8V) low dropout voltage regulator, as shown in Figure 1.
Figure 1: Temperature measurement using an external thermistor
During operation, the device uses an internal pull-up resistor programmable to 18kΩ or 180kΩ, automatically biasing one thermistor at a time. The pull-up resistor is measured during factory debugging, and its value is stored digitally in the device for temperature calculation.
The voltage ADC uses the REG18 voltage as a reference to measure the thermistor pin voltage proportionally. The voltage on each thermistor is measured every one to three measurement cycles. The original ADC count value can be obtained through the DASTATUS6() subcommand. In normal mode, the device converts these measurements into temperature every 250ms; in sleep mode, the device converts these measurements into temperature every other measurement.
BQ76942 and BQ76952 use fifth-order polynomials based on ADC measurements to calculate temperature. These devices include default polynomial coefficients for:
Semitec 103-AT thermistor using 18kΩ pull-up resistor (10kΩ at 25°C, B25/85 = 3,435 k).
Semitec 204AP-2 thermistor using 180kΩ pull-up resistor (200kΩ at 25°C, B25/85 = 4,470 k).
Custom coefficients optimized for use with other thermistors can also be written into registers or one-time programmable memory
The temperature calculated by each enabled thermistor is in units of 0.1°K and can be read by using the serial communication interface.
The BQ76942 and BQ76952 battery monitors and protectors include a high-performance measurement subsystem. This subsystem integrates an internal chip temperature measurement and supports up to 9 external thermistors for battery or FET temperature measurement. These devices can be used in various applications such as power tools and electric bicycles to ensure system safety by monitoring battery temperature and disabling the battery pack when the situation becomes dangerous.