Datasheet

bq27541-G1
SLUSAL6C –NOVEMBER 2011–REVISED OCTOBER 2012 www.ti.com
FUNCTIONAL DESCRIPTION
FUEL GAUGING
The bq27541-G1 measures the cell voltage, temperature, and current to determine battery SOC based on
Impedance Track™ algorithm (Please refer to Application Report SLUA450 "Theory and Implementation of
Impedance Track Battery Fuel-Gauging Algorithm" for more information). The bq27541-G1 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 bq27541-G1 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 bq27541-G1 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 Clear Threshold. 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.
The bq27541-G1 has two additional flags accessed by the Flags() function that warns of internal battery
conditions. The fuel gauge monitors the cell voltage during relaxed conditions to determine if an internal short
has been detected. When this condiitons occurs, [ISD] will be set. The bq27541-G1 also has the capability of
detecting when a tab has been disconnected in a 2-cell parallel system by actively monitoring the SOH. When
this conditions occurs, [TDD] will be set.
IMPEDANCE TRACK™ VARIABLES
The bq27541-G1 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 Load Mode is 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 9 are
available.
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