Test purpose and process based on DS2786 open circuit voltage circuit board

The DS2786 fuel gauge based on open circuit voltage (OCV) loads the default OCV characteristics and default configuration into the EEPROM at the factory. In order to improve the OCV fuel gauge and adapt the DS2786 to specific applications, it is necessary to reprogram the EEPROM of the DS2796 if necessary. This article describes how to program the EEPROM and how to test the installed circuit board.


Plate test


The following gives an example of testing based on the DS2786 OCV circuit board before installing the battery pack. Fig. 1 is the circuit schematic diagram of the circuit board, used all the functions of DS2786. All important test points (7 in total) in the figure are marked with circled numbers. The test procedure assumes that all discrete components in the circuit have been tested. Therefore, the purpose of the test is to confirm the circuit connection and verify whether the installed circuit board is correct.


Test purpose and process based on DS2786 open circuit voltage circuit board


Figure 1. Circuit nodes that must be verified


Test step 1: Test initialization. The purpose of this step is to determine whether there is a direct short circuit in the circuit board and whether it can communicate. After the device communicates successfully, the voltage register reading can be read to verify the connection between SDA and SCL (node 1), the connection between Pack+ and VDD pins (node 2), and the connection between Pack- and VSS pins (node 3). Is the connection correct? In addition, by reading the voltage register and confirming whether the test is valid, it can be verified whether the VIN pin (node 4) is connected correctly.
Pack+ and Pack- indirect 4.0V power supply.
Wait 880ms. Wait for voltage conversion.
Read voltage register: 2 bytes.
If no communication occurs, the circuit installation has failed.
If the voltage reading is incorrect, it indicates that the circuit installation has failed.
Test Step 2: Verify SNS (Node 5). An effective current test can verify that the SNS pin is connected correctly.

Pack+ and Pack- indirect 4.0V power supply.
The current between Pack- and system VSS is 1.0A.
Wait 880ms. Waiting for current conversion
Read current register: 2 bytes.
If the current reading is inaccurate, it indicates that the circuit installation has failed.
Test step 3: Verify the connection of auxiliary inputs AIN0 and AIN1 (node 6). Through effective resistance measurement, it can be verified whether the AIN0 and AIN1 pins are connected correctly. This step is optional.
Connect a 10kΩ resistor between PackID and Pack-.
Connect a 10kΩ resistor between the Therm terminal and Pack-.
Pack+ and Pack- indirect 4.0V power supply.
Wait 880ms. Waiting for auxiliary input conversion.
Read AIN0 and AIN1: 4 bytes.
If the AIN0/AIN1 reading is inaccurate, it indicates that the circuit installation has failed.
Test step 4: Verify VPROG and program EEPROM (node 8). Need to provide a test point for connecting the programming voltage to the VPROG pin to program the EEPROM of the DS2786. By writing EEPROM and copying EEPROM, you can verify that the connection is correct and verify that the EEPROM has been updated. The EEPROM includes the Current Offset Bias Register (COBR), so it is beneficial to calibrate the COBR before programming the EEPROM.
Pack+ and Pack- indirect 4.0V power supply.
Calibrate COBR. If you want to perform COBR calibration, you can refer to the detailed description below.
Write parameter EEPROM area: 32 bytes.
Copy the parameters to the EEPROM.
Wait for 14ms. Waiting to copy EEPROM.
Write 0xFFh to the parameter EEPROM area: 31 bytes of non-memory address 0x7Dh)*.
Recall parameters from EEPROM.
Read parameter EEPROM area: 32 bytes.
If none of the 32 bytes read from the EEPROM can match the previously written 32 bytes, the circuit installation has failed.
*Do not write 0xFFh to the memory address 7Dh, otherwise the slave address will change and the device will stop responding to the current slave address.


Calibration of Current Offset Bias Register


Through the current offset bias register, the DS2786 current measurement results can be adjusted from +3.175mV to -3.2mV in 25μV steps. The factory default value of COBR is 0x00h. The following example lists the steps to calibrate the DS2786 current offset bias register:
Power up the DS2786 and make sure that there is no current in the sense resistor.
Write 0x00h (memory address 0x60h) to COBR.
Wait 880ms until the next conversion cycle arrives.
Read current register.
According to actual needs, repeat steps 3 and 4 several times until the average current reading is obtained.
Write the corresponding value of the average current reading into COBR.
Copy COBR value to EEPROM (this step should be combined with copying all values to EEPROM).


to sum up


To correctly verify the assembled OCV fuel gauge DS2786, each solder joint in the circuit needs to be tested. Test steps 1, 2 and 3 can be combined into one step to shorten the test time, especially the conversion delay time.
In addition, programming the EEPROM during the test can provide a more effective test process, and at the same time can provide enough time to apply the saved parameters (including the current offset bias) to the programming voltage required by the EEPROM.