When the lithium battery is working beyond the temperature scale specified by the battery manufacturer, there is a risk of thermal runaway, which may eventually lead to fire or burst. Therefore, to ensure the safety of the system and to meet the requirements of various standards, it is necessary to disable the battery 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 maintainer temperature measurement subsystem, which is critical to ensure the safe operation of the system.
As mentioned in the article "Next-Generation Battery Monitors: How to Improve Battery Safety While Improving Accuracy and Extending Operation Time," accurate monitoring of battery voltage, current, and temperature can help ensure safe operation of systems for mass consumer products including vacuum cleaners, power tools, and electric bicycles. In this article, we'll take a deeper look at temperature monitoring of lithium batteries, including the proper equipment for safe system operation.
The newest products in the Texas Instruments family of battery monitors and maintainers, the BQ76942 (3 cells in series [3S], up to 10S) and the BQ76952 (up to 3S to 16S), integrate 16-bit/ 24-bit delta-sigma analog-to-digital converters (ADCs) to multiplex between various voltage measurements, including measurements of internal chip temperature and external thermistors.
Both battery monitors support temperature measurement using external thermistors on up to nine device pins, which allows system planners to more sensitively select where in the battery pack to measure temperature. Separate thermistor measurements and internal chip temperature readings can be specified for battery temperature, field effect transistor (FET) temperature, or neither.
The BQ76942 (10S) and BQ76952 (16S) include an internal chip temperature measurement based on the ADC using its internal reference measurement of ΔVBE voltage. This voltage is converted to a temperature reading that can be read via the serial communication interface.
The maintenance subsystem uses the measurement specified as the cell temperature to identify over/under cell temperature in charging or over/under temperature in discharging, and to confirm if cell balancing is allowed. A thermistor designated for FET temperature is used to identify FET overheating. Any thermistor that is enabled but not specified for battery or FET temperature will be used for temperature reporting, but will not be applied by the maintenance subsystem.
The internal chip temperature also determines whether the cell balance is allowed and whether the device should be placed in a closed condition to prevent erroneous operation when its specified operating temperature range is exceeded.
During operation, the device automatically biases one thermistor at a time using an internal pull-up resistor programmable to 18kΩ or 180kΩ. The pull-up resistor is measured during factory commissioning and its value is stored digitally in the device for temperature accounting.
The voltage ADC measures the thermistor pin voltages in proportion to the REG18 voltage as a reference. The voltage on each thermistor is measured every one to three measurement cycles. The raw ADC count value is available through the DASTATUS6() subcommand. In normal mode, the device converts these measurements to temperature every 250ms; in sleep mode, the device converts these measurements to temperature every other measurement.
The BQ76942 and BQ76952 use a fifth-order polynomial based on ADC measurements to account for temperature. These devices include default polynomial coefficients for
Each enabled thermistor accounts for temperature in units of 0.1°K and can be read by using the serial communication interface.
Semitec 204AP-2 thermistor with 180 kΩ pull-up resistor (200 kΩ at 25°C, B25/85 = 4,470 k).
Semitec 103-AT thermistor with 18 kΩ pull-up resistor (10 kΩ at 25°C, B25/85 = 3,435 k).
Custom coefficients optimized for use with other thermistors can also be written into registers or one-time programmable memory.
Conclusion
The BQ76942 and BQ76952 battery monitors and maintainers include a high-performance measurement subsystem. The subsystem incorporates an internal chip temperature measurement and supports up to nine 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 risky.
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