Share some tips to improve battery safety and accuracy

1 Introduction

The current flowing into or out of the battery pack is measured and used for a number of different purposes. For example, if a power tool's removable battery pack accidentally shorts out, a huge amount of current could flow and cause an unsafe condition. Large currents can similarly flow if a device with an embedded battery, such as a vacuum cleaner, malfunctions internally, potentially exceeding the level of current it is designed to safely withstand. These two examples illustrate why it is important to monitor current below excessive levels and include protective devices (such as series field effect transistors, relays, or fuses) to open the circuit and prevent the flow of current if unsafe operation is detected. Most designs typically use comparators to monitor current for safety purposes to enable fast detection .

2.Battery pack testing method

Battery pack current measurement is also important for measurements such as determining the state of charge or health of a battery and predicting the remaining capacity or run time of a system. Most advanced battery fuel gauges, such as those using Impedance Track™ technology or the Compensated Termination of Discharge (CEDV) algorithm, rely on coulomb counting for calculations.

A Coulomb Counter is a specialized current measurement ADC that continuously measures the current flowing in a battery pack, typically by measuring the differential voltage across a small series sense resistor. The sense resistor used in typical systems is usually in the range of 1 mΩ or less, while for high current systems it may be less than 100 µΩ. Low resistance is necessary to avoid excessive heat generation in high-current systems, but it also means that under high loads, the voltage developed across the sense resistor may be limited to 50 mV or less.

Most battery metering algorithms use accumulated, integrated charge flowing into or out of the battery in their calculations. This accumulated charge is calculated by integrating the current measurement over time. In calculations like this, the offset in the current measurement is important because it actually behaves as a phantom current, which when integrated can produce a large amount of error charge over time.

Measuring small voltage ranges with high resolution and low offset is a powerful design challenge that often makes the coulomb counter the highest performing subsystem in a battery monitor.

To address this challenge, TI's BQ76942 (3 seconds to 10 seconds) and BQ76952 (3 seconds to 16 seconds) battery monitors integrate a 16/24-bit delta-sigma coulomb counter ADC that can measure up to ±200 mV differential voltage. Low-side sense resistor. These devices also include current protection features that use comparators to detect discharge short-circuit and overcurrent conditions in both charge and discharge directions.

3. Current ADC measurement

The BQ76942 and BQ76952 offer several versions of digital current measurement, optimized for specific uses of the data:

·  CC1 Current()  – Provides a current measurement every 250 milliseconds or every 4 seconds when the device is in normal mode (when actively charging or discharging) and in sleep mode (when not actively charging or discharging). This value is used by some current-based integrated protections as well as accumulated charge integration. CC1 Current() data is reported in 16-bit format via the DASTATUS5() subcommand . The accumulated integrated charge is reported using the 64-bit format via the DASTATUS6() subcommand .

·  CC2 Current() – provides this current measurement  every 3 ms in normal mode and every Power:Sleep:Voltage interval in sleep mode . The value is reported in 16-bit format using the Current() command , and the raw 24-bit data is also provided in 32-bit format for additional post-processing. by settingConfiguration bits that can change the 3 ms conversion to a 1.5 ms conversion, thereby reducing the conversion resolution.

·  CC3 Current()  – This measurement is the average of multiple CC2 Current() measurements, which provides higher resolution data at a slower output rate for further processing. Use the Settings:Configuration:CC3 Samples setting to set the average number of samples from 2 to 255. The DASTATUS5() subreports the resulting data in 32-bit format.

·  The DASTATUS1~4() subcommand reports the measured battery voltage in 32-bit format synchronized with the raw current ADC reading. These values ​​can be used to analyze battery impedance used in certain measurement algorithms.

The BQ76942 and BQ76952 provide selectable units for current measurements to accommodate a different range of current levels. For example, CC1 Current() reported in 16-bit format can represent a current between -32.768 A and +32.767 A when using 1 mA units . If higher currents are expected, the units can be changed to 10 mA, which then allows currents ranging from -327.68 A to +327.67 A to be represented.

As shown in Table 1, Settings:Configuration:DA Configuration:[USER_AMPS_1:0] configuration settings set the unit. These units apply to CC2 Current(), CC1 Current() , and CC3 Current() values.

Table 1: Currently reported programmable units

The BQ76942 and BQ76952 require a current gain value ( Cal:Current: CC Gain and Cal:Current:Capacitance Gain ) to convert the voltage measured across the sense resistor into a current value. You can set these gain values ​​based on the nominal values ​​of the sense resistors used in the system, or calibrate them for each printed circuit board (PCB) to be stored in the device memory. These devices also include board-level offset current settings that can be determined for each PCB and stored in memory. Multiple measurements can be captured on the production line ( the number of samples set in Calibration: Current Offset: Coulomb Counter Offset Samples), and the sum of the measurements stored in Calibration: Current Offset: Board Offset . In When reporting current, the device will subtract the value of the board offset / coulomb counter offset sample from each reading before scaling by CC gain .

4. Synchronized voltage and current measurements

The BQ76942 and BQ76952 support simultaneous measurement of current and each cell voltage using two delta-sigma ADCs. Raw 24-bit ADC readings of each cell voltage and sync current are stored in the device and can be read out as a sync pair. You can use this data to analyze cell impedance or interconnect resistance.

5. Accumulated charge measurement

The BQ76942 and BQ76952 continuously integrate the Coulomb countercurrent data to produce a cumulative charge value; the host can use subcommand 0x0082 RESET_PASSQ() to reset this integrator as needed. These devices also include a timer in seconds that resets simultaneously with the integration of the coulomb count. Dividing the accumulated charge value by the timer will calculate the average current over the time interval since the timer was started.

The accumulated charge is provided as two 32-bit values. The first 32-bit (signed) data reports the cost in userAh, while the second 32-bit (unsigned) data reports the fractional cost in userAh/2 32. Both the 64-bit accumulated charge data and the timer value are reported by the 0x0076 DASTATUS6() subcommand.

The BQ76942 and BQ76952 battery monitors and protectors implement a high-performance measurement subsystem that includes a precision coulomb counter. The current measurement subsystem in these devices is highly configurable, allowing you to make trade-offs between speed and resolution. Multiple current readings are provided, each optimized for measurement, post-processing, and current-based battery pack protection.