Cell offset calibration provides improved accuracy over using the fixed offset adjustment procedure. During field operation, the calibration involves a similar offset adjustment of each cell voltage reading from the device. However, cell offset calibration also requires the measurement of each device on the customer production line.

The customer must prepare a fixture for the production line that generates 16 simulated cell voltages at a nominal voltage approximately in the middle of the expected cell voltage range. For example, if LFP cells are to be used over a range from 2.7 V to 3.7 V per cell, a fixture can be prepared that provides an output of 3.2 V per cell. If, instead, Li-ion cells are used over a range from 3.0 V to 4.3 V, the fixture can provide outputs of 3.7 V per cell. Figure 7-4 shows an example diagram of this fixture.

Figure 7-4 Example Fixture for Cell Offset Calibration

The differential voltage of each simulated cell output is measured with a precision voltmeter (Vsim1 – Vsim0, Vsim2 – Vsim1, and so on) to determine each voltage generated. It is not critical that each output is exactly the target voltage, only that its value is precisely known. If a high quality voltage regulator is used, customers need to only measure the fixture outputs once after it is assembled.

On the production line, the fixture is connected to each PCB after assembly at room temperature before cells are attached, as shown in Figure 7-5. The BQ76972 is powered, and measurements of each cell voltage are read and stored (Cell 1 Voltage(), Cell 2 Voltage(), and so on). Multiple readings for each cell voltage can be collected and averaged to reduce the effect of noise that may be present on the production line.

Figure 7-5 Fixture in Use During Calibration

The offset for each cell is calculated as follows:

• Cell 1 Offset = Cell 1 Voltage() – (Vsim1 – Vsim0)
• Cell 2 Offset = Cell 2 Voltage() – (Vsim2 – Vsim1)
• . . . and so forth

The 16 values of Cell # Offset are unique for each device and are stored in the host processor. During field operation, each reported cell voltage measurement is offset using the stored Cell # Offset values by the host processor:

• Final Cell 1 Voltage = Cell 1 Voltage() – Cell 1 Offset
• Final Cell 2 Voltage = Cell 2 Voltage() – Cell 2 Offset
• . . . and so forth

Alternatively, the customer can collect offset data on a number of devices and analyze the resulting offsets, to determine whether using a fixed set of offsets may still provide sufficient cell measurement accuracy. This would then be equivalent to the fixed offset adjustment described earlier, but instead using a set of fixed offsets from the customer's boards and design.