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5 FUNCTIONAL DESCRIPTION

5.1 FUEL GAUGING

bq27500

bq27501

System-Side Impedance Track™ Fuel Gauge

SLUS785 – SEPTEMBER 2007

Table 4-7. Data Flash Summary (continued)

Subclass Data Min Max Default

Class Subclass Offset Name Units

ID Type Value Value Value

Ra Tables 91 Pack0 Ra 0-18

Ra Tables 92 Pack1 Ra 0-18

See Note

(2)

Ra Tables 93 Pack0 Rax 0-18

Ra Tables 94 Pack1 Rax 0-18

Calibration 104 Data 0 CC Gain F4 0.1 4 0.47095 m Ω

Calibration 104 Data 4 CC Delta F4 29826 1193046 280932.6 m Ω

Calibration 104 Data 8 CC Offset I2 -32768 32767 –1667 mV

Calibration 104 Data 10 Board Offset I1 –128 127 0 mV

Calibration 104 Data 11 Int Temp Offset I1 –128 127 0 0.1 ° C

Calibration 104 Data 12 Ext Temp Offset I1 –128 127 0 0.1 ° C

Calibration 104 Data 13 Pack V Offset I1 –128 127 0 0.1 ° C

Calibration 107 Current 1 Deadband U1 0 255 3 mA

Security 112 Codes 0 Usealed Key0 H2 0x0000 0xffff – –

Security 112 Codes 2 Usealed Key1 H2 0x0000 0xffff – –

Security 112 Codes 4 Full-Access Key0 H2 0x0000 0xffff – –

Security 112 Codes 6 Full-Access Key1 H2 0x0000 0xffff – –

The bq27500/1 measures the cell voltage, temperature, and current to determine battery SOC. The

bq27500/1 monitors charge and discharge activity by sensing the voltage across a small-value resistor

(5m Ω to 20 m Ω typ.) between the SRP and SRN pins and in series with the cell. By integrating charge

passing through the battery, the cell’s 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 state of charge 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 bq27500/1

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 Term Voltage.

NominalAvailableCapacity( ) and FullAvailableCapacity( ) are the uncompensated (no or light load)

versions of RemainingCapacity( ) and FullChargeCapacity( ) respectively.

The bq27500/1 has two flags accessed by the Flags( ) function that warns when the cell’s SOC has fallen

to critical levels. When RemainingCapacity( ) falls below the first capacity threshold, specified in SOC1

Set, the [SOC1] (“ State of Charge Initial””) flag is set. The flag is cleared, once RemainingCapacity( ) rises

above SOC1 Set. All units are in mAh.

When RemainingCapacity( ) falls below the second capacity threshold, SOCF Set, the [SOCF] (“ State of

Charge Final”) flag is set, serving as a final discharge warning. If SOCF Set = –1, the flag is inoperative

during discharge.

Similarly, when RemainingCapacity( ) rises above SOCF Clear and the [SOCF] flag has already been set,

the [SOCF] flag will be cleared, provided SOCF Set ≠ –1. All units are in mAh.

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