BQ27510_08 Datasheet, PDF (20/36 Pages) Texas Instruments – System-Side Impedance Track™ Fuel Gauge With Direct Battery Connection

bq27510
SLUS816A – MARCH 2008 – REVISED JUNE 2008.......................................................................................................................................................... www.ti.com
FUNCTIONAL DESCRIPTION
FUEL GAUGING
The bq27510 measures the cell voltage, temperature, and current to determine battery SOC. The bq27510
monitors charge and discharge activity by sensing the voltage across a small-value resistor (5 mΩ to 20 mΩ typ.)
between the SRP and SRN pins and in series with the cell. By integrating charge passing through the battery,
the battery’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 bq27510 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 bq27510 has two flags accessed by the Flags( ) function that warns when the battery’s SOC has fallen to
critical levels. When RemainingCapacity( ) falls below the first capacity threshold, specified in SOC1 Set
Threshold, the [SOC1] (State of Charge Initial) flag is set. The flag is cleared once RemainingCapacity( ) rises
above SOC1 Set Threshold. The bq27510’s BAT_LOW pin automatically reflects the status of the [SOC1] flag.
All units are in mAh.
When RemainingCapacity( ) falls below the second capacity threshold, SOCF Set Threshold, the [SOCF] (State
of Charge Final) flag is set, serving as a final discharge warning. If SOCF Set Threshold = –1, the flag is
inoperative during discharge. Similarly, when RemainingCapacity( ) rises above SOCF Clear Threshold and the
[SOCF] flag has already been set, the [SOCF] flag is cleared. All units are in mAh.
IMPEDANCE TRACK™ VARIABLES
The bq27510 has several data flash variables that permit the user to customize the Impedance Track™ algorithm
for optimized performance. These variables are dependent upon the power characteristics of the application as
well as the cell itself.
Load Mode
Load Mode is used to select either the constant-current or constant-power model for the Impedance Track™
algorithm as used in Load Select (see Load Select). When Load Mode is 0, the Constant Current Model is
used (default). When 1, the Constant Power Model is used. The [LDMD] bit of CONTROL_STATUS reflects the
status of Load Mode.
Load Select
Load Select defines the type of power or current model to be used to compute load-compensated capacity in the
Impedance Track™ algorithm. If Load Mode = 0 (Constant Current), then the options presented in Table 8 are
available.
LoadSelect
Value
0
1(default)
2
3
Table 8. Constant-Current Model Used When Load Mode = 0
Current Model Used
Average discharge current from previous cycle: There is an internal register that records the average discharge current
through each entire discharge cycle. The previous average is stored in this register.
Present average discharge current: This is the average discharge current from the beginning of this discharge cycle
until present time.
Average current: based off the AverageCurrent( )
Current: based off of a low-pass-filtered version of AverageCurrent( ) (τ=14s)
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