8.4.1 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.

8.5.1 Standard Data Commands

The fuel gauge 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. Additional details are found in the SLUUB04, bq27532-G1 Technical Reference Manual.

8.5.2 Control( ): 0x00 and 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 fuel gauge during normal operation and additional features when the fuel gauge is in different access modes, as described in Table 2. Additional details are found in the SLUUB04, bq27532-G1 Technical Reference Manual.

8.5.3 Charger Data Commands

The charger registers are mapped to a series of single-byte Charger Data Commands to enable system reading and writing of battery charger registers. During charger power up, the registers are initialized to Charger Reset State. The fuel gauge can change the values of these registers during the System Reset State.

Each of the bits in the Charger Data Commands can be read or write. Note that System Access can be different from the read or write access as defined in bq2425x charger hardware. The fuel gauge may block write access to the charger hardware when the bit function is controlled by the fuel gauge exclusively. For example, the [VBATREGx] bits of Chrgr_Reg2 are controlled by the fuel gauge and cannot be modified by system.

The fuel gauge reads the corresponding registers of Chrgr_Reg0( ) and Chrgr_Reg2( ) every second to mirror the charger status. Other registers in the bq2425x device are read when registers are modified by the fuel gauge.

8.5.4 Communications

8.5.4.1 I²C Interface

The fuel gauge 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 fuel gauge 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):

8.5.4.3 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 fuel gauge. In addition, if the SCL clock frequency (f_SCL) is > 100 kHz, use individual 1-byte write commands for proper data flow control. The following diagram shows the standard waiting time required between issuing the control subcommand the reading the status 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.