Figure 2-1 shows the wiring used for collecting impedance data. The BQ35100 EVM is connected using I2C communication, and data memory is connected using SPI communication, or any other available method. The BQ35100 EVM already implements pull-up resistors on the I2C communication lines so no external pull-up resistors are needed.

Any controller or MCU can be used for collecting the impedance data, there are only a few requirements:

• The ability to save logged data read from the BQ35100
• Ability to communicate over I2C to read data and send commands
• GPIO functionality for the GE and ALERT functions

Figure 2-1 Wire Connection Block Diagram

Scaled resistance and measured impedance values are only updated when the gauge is in EOS mode. Using BQStudio is the easiest way to configure the gauge to EOS Mode.

First, set the gauge to unsealed mode if it was previously sealed by pressing UNSEAL in the Commands tab. Verify the gauge is unsealed by checking the [SEC1,0] bits of ControlStatus(). The gauge is unsealed when SEC1 is set high and SEC0 is set low. If the gauge was previously sealed, re-uploading the default .SREC file which was never sealed will stop the gauge from booting up in a sealed state.

Figure 3-1 UNSEAL and [SEC1,0] in bqStudio

To gather resistance and impedance data, the microcontroller uses the addresses specified in the Technical Reference Manual (TRM). The I2C 8-bit address for the BQ35100 is 0xAA. The 7-bit address is 0x55. This address cannot be changed.

For both scaled resistance and measured impedance, the data is stored in little-endian format as unsigned integers. The command 0x16 is used to gather the scaled resistance data, and the command 0x22 is used to gather the measured impedance data. This is the most critical data to be extracted to calculate the appropriate EOS configuration for the system.

While in EOS mode, the gauge needs to be enabled before any major discharge of the battery occurs. For this test, the MCU gathers impedance data and writes the data to a data memory device. The testing procedure consists of the following steps:

1. Wake up the gauge before any major discharge using the GE pin
2. Send the GAUGE_START command
3. Send the GAUGE_STOP command after the major discharge
4. Wait for the G_DONE bit to be set to one
5. Read the scaled resistance and measured impedance from the gauge
6. Save the scaled resistance and measured impedance to a data memory device
7. Put the gauge back to sleep using the GE pin

For the gauge to take an accurate measurement of the voltage and current for estimating the resistance of the cell, the pulsed load must be at least 100-ms long and cause a 100-mV drop of the battery voltage. This constitutes the minimum requirement for a major discharge.

It is common for end equipment to use separate resistors from the expected load burst, which is typically RF dynamic load, to provide consistent current bursts for the gauge to learn the resistance. The gauge thrives on consistency of the measurements and does not require significant drain on the battery to perform the learning load burst.

Figure 6-1 shows an example of the impedance data that is collected by the gauge. This data shows a lithium thionyl battery starting at full capacity to be completely discharged. Towards the end of the battery’s life, there is a spike in the impedance which is characteristic of lithium thionyl chloride batteries and a strong indicator that the battery is nearing the end of battery life.

For testing purposes, the battery was discharged by 1% SOH at the maximum continuous discharge rate specified by the battery manufacturer after each learning pulse. After the 1% SOH discharge the battery was relaxed for 5 hours. Each learning pulse was 500 ms long and the current was set to 100 mA to meet the requirements of the pulse length and amplitude.

Figure 6-1 Scaled Resistance Profile of LS14500 Battery with EOS Flag Thresholds