As mentioned in the article "Next Generation Battery Monitors: How to Improve Battery Safety While Increasing Accuracy and Runtime," accurate monitoring of battery voltage, current and temperature can help ensure safe operation of systems suitable for popular consumer products including vacuum cleaners, power tools and electric bicycles. In this article, we will take a deeper look at temperature monitoring of lithium batteries, including the proper configuration for safe system operation.
When lithium batteries operate outside the temperature range specified by the battery manufacturer, there is a risk of thermal runaway, which can eventually lead to fire or explosion. Therefore, to ensure system safety and compliance with various standards, batteries must be disabled whenever they are outside of 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 critical to ensuring safe system operation.
The latest additions to the Texas Instruments battery monitor and protector family, the BQ76942 (3 cells in series [3S] up to 10S) and BQ76952 (up to 3S to 16S), integrate a 16-bit/ 24-bit delta-sigma analog-to-digital converter (ADC) to multiplex between various voltage measurements, including measurements of internal chip temperature and external thermistors.
The BQ76942 (10S) and BQ76952 (16S) include an internal chip temperature measurement based on the ADC's measurement of the ΔVBE voltage using its internal reference. This voltage is converted to a temperature reading that can be read via a serial communication interface.
Both battery monitors support temperature measurements using external thermistors on up to nine 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 for use as battery temperature, field-effect transistor (FET) temperature, or neither.
The protection subsystem uses the measurements specified as cell temperature to identify over/under cell temperature in charging or over/under temperature in discharging, and to determine whether to allow cell balancing. 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 used by the protection subsystem.
The internal chip temperature also determines whether cell balancing is allowed and whether the device should be placed in the off state to avoid erroneous operation if its specified operating temperature range is exceeded.
The thermistor is measured while connected to an internal pull-up resistor connected to a REG18 (~1.8V) low dropout regulator
During operation, the device is automatically biased 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 calculations.
The voltage ADC measures the thermistor pin voltages in proportion to the REG18 voltage. The voltage on each thermistor is measured every one to three measurement cycles. The raw ADC count values are 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 calculate temperature. These devices include default polynomial coefficients for
Semitec 103-AT thermistors using an 18 kΩ pull-up resistor (10 kΩ at 25°C, B25/85 = 3,435 k).
Semitec 204AP-2 thermistor using a 180 kΩ pull-up resistor (200 kΩ 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 in 0.1°K for each enabled thermistor can be read by using the serial communication interface.
Conclusion
The BQ76942 and BQ76952 battery monitors and protectors contain a high-performance measurement subsystem. The subsystem integrates an internal chip temperature measurement and supports up to nine external thermistors for battery or FET temperature measurement. These devices can be used in a variety of applications such as power tools and e-bikes to ensure system safety by monitoring battery temperature and disabling the battery pack when conditions become dangerous.
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