7.3.1 Functional Description

The fuel gauge measures the cell voltage, temperature, and current to determine battery SOC. The fuel gauge monitors the charging and discharging of the battery by sensing the voltage across a small-value resistor (5 mΩ to 20 mΩ, typical) between the SRP and SRN pins and in series with the cell. By integrating charge passing through the battery, the battery SOC is adjusted during battery charge or discharge.

The total battery capacity is found by comparing states of charge before and after applying the load with the amount of charge passed. When an application load is applied, the impedance of the cell is measured by comparing the OCV obtained from a predefined function for present SOC with the measured voltage under load. Measurements of OCV and charge integration determine chemical SOC and chemical capacity (Qmax). The initial Qmax values are taken from a cell manufacturers' data sheet multiplied by the number of parallel cells. It is also used for the value in Design Capacity. The fuel gauge acquires and updates the battery-impedance profile during normal battery usage. It uses this profile, along with SOC and the Qmax value, to determine FullChargeCapacity() and StateOfCharge(), specifically for the present load and temperature. FullChargeCapacity() is reported as capacity available from a fully-charged battery under the present load and temperature until Voltage() reaches the Terminate Voltage. NominalAvailableCapacity() and FullAvailableCapacity() are the uncompensated (no or light load) versions of RemainingCapacity() and FullChargeCapacity(), respectively.

The fuel gauge has two flags accessed by the Flags() function that warn when the battery SOC has fallen to critical levels. When RemainingCapacity() falls below the first capacity threshold as specified in SOC1 Set Threshold, the [SOC1] (State of Charge Initial) flag is set. The flag is cleared once RemainingCapacity() rises above SOC1 Clear Threshold.

When the voltage is discharged to Terminate Voltage, the SOC will be set to 0.

7.4.1 Power Modes

The fuel gauge has different power modes:

• BAT INSERT CHECK: The BAT INSERT CHECK mode is a powered-up, but low-power halted, state where the fuel gauge resides when no battery is inserted into the system.
• NORMAL: In NORMAL mode, the fuel gauge is fully powered and can execute any allowable task.
• SLEEP: In SLEEP mode, the fuel gauge turns off the high-frequency oscillator and exists in a reduced- power state, periodically taking measurements and performing calculations.
• SLEEP+: In SLEEP+ mode, both low-frequency and high-frequency oscillators are active. Although the SLEEP+ mode has higher current consumption than the SLEEP mode, it is also a reduced power mode.
• HIBERNATE: In HIBERNATE mode, the fuel gauge is in a low power state, but can be woken up by communication or certain I/O activity.

The relationship between these modes is shown in Figure 6.

7.4.2 SLEEP+ Mode

Compared to the SLEEP mode, SLEEP+ mode has the high-frequency oscillator in operation. The communication delay could be eliminated. The SLEEP+ mode is entered automatically if the feature is enabled (CONTROL_STATUS [SNOOZE] = 1) and AverageCurrent() is below the programmable level Sleep Current.

During SLEEP+ mode, the fuel gauge periodically takes data measurements and updates its data set. However, a majority of its time is spent in an idle condition.

The fuel gauge exits SLEEP+ mode if any entry condition is broken, specifically when:

• Any communication activity with the gauge, or
• AverageCurrent() rises above Sleep Current, or
• A current in excess of I_WAKE through R_SENSE is detected.

7.4.3 HIBERNATE Mode

HIBERNATE mode should be used when the system equipment needs to enter a low-power state, and minimal gauge power consumption is required. This mode is ideal when system equipment is set to its own HIBERNATE, SHUTDOWN, or OFF mode.

Before the fuel gauge can enter HIBERNATE mode, the system must set the CONTROL_STATUS [HIBERNATE] bit. The gauge waits to enter HIBERNATE mode until it has taken a valid OCV measurement and the magnitude of the average cell current has fallen below Hibernate Current. The gauge can also enter HIBERNATE mode if the cell voltage falls below Hibernate Voltage and a valid OCV measurement has been taken. The gauge remains in HIBERNATE mode until the system issues a direct I²C command to the gauge or a POR occurs. Any I²C communication that is not directed to the gauge does not wake the gauge.

It is the responsibility of the system to wake the fuel gauge after it has gone into HIBERNATE mode. After waking, the gauge can proceed with the initialization of the battery information (OCV, profile selection, and so forth).

7.5.1 Standard Data Commands

The bq27530-G1 uses a series of 2-byte standard commands to enable system reading and writing of battery information. Each standard command has an associated command-code pair, as indicated in Table 1. Because each command consists of two bytes of data, two consecutive I²C transmissions must be executed both to initiate the command function, and to read or write the corresponding two bytes of data.

7.5.2 Control(): 0x00/0x01

Issuing a Control() command requires a subsequent 2-byte subcommand. These additional bytes specify the particular control function desired. The Control() command allows the system to control specific features of the bq27530-G1 during normal operation and additional features when the device is in different access modes, as described in Table 2.

7.5.3 Communications

7.5.3.1 I²C Interface

The bq27530-G1 supports the standard I²C read, incremental read, quick read, one-byte write, and incremental write functions. The 7-bit device address (ADDR) is the most significant 7 bits of the hex address and is fixed as 1010101. The first 8 bits of the I²C protocol are, therefore, 0xAA or 0xAB for write or read, respectively.

The quick read returns data at the address indicated by the address pointer. The address pointer, a register internal to the I²C communication engine, increments whenever data is acknowledged by the bq27530-G1 or the I²C master. “Quick writes” function in the same manner and are a convenient means of sending multiple bytes to consecutive command locations (such as two-byte commands that require two bytes of data).

The following command sequences are not supported:

• Attempt to write a read-only address (NACK after data sent by master):

• Attempt to read an address above 0x6B (NACK command):

7.5.3.3 I²C Command Waiting Time

To ensure proper operation at 400 kHz, a t_(BUF) ≥ 66 μs bus-free waiting time must be inserted between all packets addressed to the bq27530-G1. In addition, if the SCL clock frequency (f_SCL) is > 100 kHz, use individual 1-byte write commands for proper data flow control. Figure 8 shows the standard waiting time required between issuing the control subcommand the reading the status result. A DF_CHECKSUM subcommand requires 100 ms minimum prior to reading the result. An OCV_CMD subcommand requires 1.2 seconds prior to reading the result. For read-write standard command, a minimum of 2 seconds is required to get the result updated. For read-only standard commands, there is no waiting time required, but the host must not issue any standard command more than two times per second. Otherwise, the gauge could result in a reset issue due to the expiration of the watchdog timer.

Figure 8. Standard Waiting Time