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BQ78350-R1A

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SLUSE05 – DECEMBER 2019

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Copyright © 2019, Texas Instruments Incorporated

9.2.3.1 Measurement System

9.2.3.1.1

Cell Voltages

The device is required to be configured in the AFE Cell Map register to determine which cells to measure based

on

the

physical

connections

to

the

BQ76920

AFE.

The

cell

voltage

data

is

available

through

CellVoltage1()…CellVoltage5(). The cell voltages are reported as they are physically stacked. For example, if the

device is configured for 3-series cells connected to VC1, VC2, and VC5 per the AFE Cell Map, then the cell

voltages are still reported via CellVoltage1(), CellVoltage2(), and CellVoltage3(), respectively.

For improved accuracy, offset calibration is available for each of these values and can be managed through the

BQSTUDIO tool. The procedure for calibration is described in the BQ78350-R1A Technical Reference Manual

(SLUUBD3) in the Calibration chapter.

9.2.3.1.2

External Average Cell Voltage

This is enabled by default (DA Configuration [ExtAveEN] = 1) and uses the external resistor divider connected

to the VEN and BAT pins to determine the average cell voltage of the battery pack. The average cell voltage is

available through ExtAveCellVoltage().

CAUTION

Care should be taken in the selection of the resistor and FETs used in this divider

circuit as the tolerance and temperature drift of these components can cause

increased measurement error and a gas gauging error if CEDV Gauging Config

[ExtAveCell] = 1 (default = 1).

For improved accuracy, offset and gain calibration is available for this value and can be managed through the

BQSTUDIO tool. The procedure for calibration is described in the BQ78350-R1A Technical Reference Manual

(SLUUBD3) in the Calibration chapter.

9.2.3.1.3

Current

Current data is taken from the BQ76920 and made available through Current(). The selection of the current

sense resistor connected to SRP and SRN of the BQ76920 is very important and there are several factors

involved.

The aim of the sense resistor selection is to use the widest ADC input voltage range possible.

To maximize accuracy, the sense resistor value should be calculated based on the following formula:

RSNS(min) = V(SRP) – V(SRN) / I(max)

(1)

Where: |V(SRP) – V(SRN)| = 200 mV

I(max) = Maximum magnitude of charge of discharge current (transient or DC)

NOTE

RSNS(min) should include tolerance, temperature drift over the application temperature,

and PCB layout tolerances when selecting the actual nominal resistor value.

When selecting the RSNS value, be aware that when selecting a small value, for example,

1 mΩ, then the resolution of the current measurement will be > 1 mA. In the example of

RSNS = 1 mΩ, the current LSB will be 8.44 mA.

For improved accuracy, offset and gain calibration are available for this value and can be managed through the

BQSTUDIO tool. The procedure for calibration is described in the BQ78350-R1A Technical Reference Manual

(SLUUBD3) in the Calibration chapter.