ST STEVAL-ISQ008V1 User manual

September 2010 Doc ID 15176 Rev 1 1/28

UM0612

User manual

STEVAL-ISQ008V1, fuel gauge implementation

based on the STM32F103x

Introduction

This user manual describes the functions of a single cell Li-Ion battery fuel gauge

technology using an MCU within the STM32 (ARM Cortex™-M3 core) family. The fuel gauge

demonstration board is used to show the capability of an STM32 microcontroller to be used

for battery fuel capacity monitoring. The fuel gauge implementation is based on the STM32

microcontroller; the MCU is used for monitoring the capacity of a single cell Li-Ion battery.

An on-chip 12-bit ADC from the STM32 family is used for battery capacity calculation.

The fuel gauge technology is a smart system which monitors battery capacity, predicts

remaining capacity, and displays its status. The basic ideology used for battery capacity

measurements is Coulomb counting, in which the current flowing through the battery is

continuously monitored and integrated to calculate the capacity. Current measurement is

done when the battery is both discharging and charging. Therefore both the IN and OUT

current are taken into account to calculate the remaining capacity of the battery. Battery

capacity is measured in mAh.

Figure 1. STEVAL-ISQ008V1, fuel gauge demonstration board

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Contents UM0612

2/28 Doc ID 15176 Rev 1

Contents

1 Getting started . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.1 Package contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.2 Hardware description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.3 Power supply unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.3.1 Battery supplied block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.3.2 External supplied block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2 Fuel gauge application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.1 Demonstration board sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.1.1 Microcontroller section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.1.2 Operational amplifier section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.1.3 Battery charger section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.1.4 LCD display section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.2 Starting the fuel gauge application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.2.1 Battery state of health not known . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.2.2 Battery state of health is known . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.2.3 Alarm condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.2.4 LCD messages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.3 Compensations in fuel gauge calculations . . . . . . . . . . . . . . . . . . . . . . . . 13

2.3.1 Temperature compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.3.2 Self-discharge compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3 Flow chart for fuel gauge demonstration board . . . . . . . . . . . . . . . . . . 15

4 The STM32 as fuel gauge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4.1 ADC error minimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4.1.1 Dual polarity measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4.1.2 Amplification of low voltage values . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

5 Demonstration board schematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

6 Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

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UM0612 Contents

Doc ID 15176 Rev 1 3/28

7 Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

8 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

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List of tables UM0612

4/28 Doc ID 15176 Rev 1

List of tables

Table 1. Power selection jumper positions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Table 2. Jumper J4 connections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Table 3. Battery capacity variation due to temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Table 4. Self-discharge rate of Li-Ion battery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Table 5. BOM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Table 6. Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Table 7. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

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UM0612 List of figures

Doc ID 15176 Rev 1 5/28

List of figures

Figure 1. STEVAL-ISQ008V1, fuel gauge demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Figure 2. Connector for battery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 3. External supply selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Figure 4. Charging and discharging path selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 5. Capacity learning for the first time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Figure 6. Flow chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Figure 7. Microcontroller section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure 8. Power section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Figure 9. OP-AMP-section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Figure 10. USB section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Figure 11. LCD section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Figure 12. JTAG-CONN section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

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Getting started UM0612

6/28 Doc ID 15176 Rev 1

1 Getting started

1.1 Package contents

The fuel gauge package comes with hardware and supporting documentation.

●Hardware: one demonstration board (along with onboard battery charger)

●Documentation: user manual for operating the demonstration board

1.2 Hardware description

The fuel gauge demonstration board has onboard charging and discharging capabilities.

Major onboard components are:

●STM32F103C8, 32-bit microcontroller

●TS941B, operational amplifier

●L6924D, battery charger

●LD1086XX33, voltage regulator

●STLQ50XX25, voltage reference

●STT5PF20V, P-channel MOSFET

●16 x 2 alphanumeric LCD

●Mini USB connector

●External power supply adapter jack

●100 mΩsense resistor

1.3 Power supply unit

The power supply unit for the demonstration board is divided into two blocks:

1.3.1 Battery supplied block

This block is powered up using the battery supply.

The battery used for capacity monitoring is also used for supplying the load (if resistance

connected at J8).

There is a jumper connection (J3) used for connecting the positive and negative terminal of

the battery. Battery connections are shown in Figure 2. Therefore during the charging of the

battery the current is flowing from the charger to the battery and then through the 100 mΩ

sense resistor. During discharge, current is flowing from the battery through the externally

connected resistance connected at J8 and through the 100 mΩsense resistor.

Voltage drop across the 100 mΩsense resistor is used to evaluate the capacity of the

battery.This voltage drop is measured by ADC of the STM32 MCU and from this measured

voltage, the battery current is calculated.

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UM0612 Getting started

Doc ID 15176 Rev 1 7/28

Figure 2. Connector for battery

1.3.2 External supplied block

External 5 V of power is used for supplying the following components:

●Battery charger

●Back light of LCD

●LCD display.

Note: 1 The DC adapter used for external power should be of 5 V

2 The USB Connector is used for external power supply source only.

Table 1. Power selection jumper positions

Switch jumper SW2 Action

PIN 1 and 2 shorted USB power supply selected

PIN 2 and 3 shorted External DC power source selected

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Getting started UM0612

8/28 Doc ID 15176 Rev 1

Figure 3. External supply selection

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UM0612 Fuel gauge application

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2 Fuel gauge application

This fuel gauge demonstration board is based on the calculation of current flowing through

the 100 mΩsense resistor using the on-chip 12-bit ADC of the STM32 32-bit MCU. The

voltage drop across the sense resistor is measured by the on-chip 12-bit ADC at periodic

intervals of 1 second and this voltage is used for current calculation from which battery

capacity is derived. The current accuracy for this solution is up to +/- 3 mA.

2.1 Demonstration board sections

The fuel gauge demonstration board is split into 4 sections:

●Microcontroller

●Operational amplifier

●Battery charger

●LCD display.

2.1.1 Microcontroller section

The STM32 microcontroller is used for fuel gauge calculations. The features of the STM32

used for this application are:

1. On-chip ADC used for voltage, current, and temperature measurements

2. On-chip RTC used for the time base for fuel gauge computations

3. Input/output ports used for the LCD display

NTC is also connected to one ADC channel of the microcontroller for monitoring battery

temperature.

2.1.2 Operational amplifier section

The TS941BIDT operational amplifier is used as an amplifier and for dual polarity detection

of current flowing through the battery. Input to the operational amplifier is the voltage drop

across the 100 mΩsense resistor and output of the operational amplifier is connected to the

ADC input channel of the microcontroller. Both positive current (in case of battery charging)

and negative current (in case of battery discharging) is measured using this operational

amplifier.

2.1.3 Battery charger section

The L6924 present on the demonstration board is used for charging the battery. Jumper J4

is used for connecting the battery to the resistive load or to the battery charger, as described

in Ta b l e 2 .

Table 2. Jumper J4 connections

Jumper J4 Action

PIN 1 and 2 shorted Battery connected to battery charger

PIN 2 and 3 shorted Battery connected to load at jumper J8

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Fuel gauge application UM0612

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The charger supplies current to the microcontroller circuit and also to the battery, maximum

charging time is fixed at 4 hours. The charger can be powered up either from an external DC

adapter (5 V) or using USB power (shown in Figure 3). Jumper J5 is used for the shutdown

pin of the charger, it should be shorted using the jumper to enable the charger. There are 2

LED's present on the demonstration board (D4 and D5). When charging, the ongoing D4 is

in the OFF state and D5 is in the ON state.

If charging time has lapsed or the battery is not connected to the charger then both D4 and

D5 are in the ON state. Figure 4 shows jumper J4 for selecting the charging and discharging

path.

Figure 4. Charging and discharging path selection

2.1.4 LCD display section

An alphanumeric (16 x 2) LCD is present on the demonstration board. It displays the voltage

of the battery and current flowing through (in/out) the battery. Also the capacity of the battery

is shown as a percentage of full capacity.

If the battery voltage goes below 3.1 V, the “Battery Low” message is displayed on the LCD

and the D2 LED starts blinking.

2.2 Starting the fuel gauge application

The demonstration board has the provision of learning the capacity of a new battery (if not

previously known) or if the user is confident that the new battery is fully charged then the

capacity learning phase can be skipped, but in this case the battery state is considered as

fully charged with a remaining capacity of 900 mAh.

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