BQ27350 Datasheet, PDF (21/33 Pages) Texas Instruments – Single Cell Li-Ion Battery Manager With Impedance Track Fuel Gauge Technology

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bq27350
Single Cell Li-Ion Battery Manager
With Impedance Track Fuel Gauge Technology
SLUS754 – MARCH 2007
Dsg Current Threshold
This register is used as a threshold by many functions in the bq27350 to determine if actual discharge current is flowing into or
out of the cell. The default for this register is 100mA which should be sufficient for most applications. This threshold should be
set low enough to be below any normal application load current but high enough to prevent noise or drift from affecting the
measurement.
Chg Current Threshold
This register is used as a threshold by many functions in the bq27350 to determine if actual charge current is flowing into or out
of the cell. The default for this register is 50mA which should be sufficient for most applications. This threshold should be set low
enough to be below any normal charge current but high enough to prevent noise or drift from affecting the measurement.
Quit Current, Dsg Relax Time, Chg Relax Time, and Quit Relax Time
The Quit Current is used as part of the Impedance Track™ algorithm to determine when the bq27350 enters relaxation mode
from a current flowing mode in either the charge direction or the discharge direction. The value of Quit Current is set to a default
value of 10mA and should be above the standby current of the host system, but not exceed the C/20 rate..
Either of the following criteria must be met to enter relaxation mode:
1. | AverageCurrent( ) | < | Quit Current | for Dsg Relax Time.
2. | AverageCurrent( ) | < | Quit Current | for Chg Relax Time.
After about 30 minutes in relaxation mode, and when voltage change rate satisfies the criteria dV/dt < 4uV/sec, the bq27350
attempts to take accurate OCV readings. If state of charge (SOC) change from the previous OCV reading exceeds 37% when a
new reading is taken, the total cell capacity, Qmax, is updated. These updates are used in the Impedance Track™ algorithms. It
is critical that the battery voltage be relaxed during OCV readings and that the current is not higher than C/20 when attempting
to go into relaxation mode.
Quit Relax Time specifies the minimum time required for AverageCurrent( ) to remain above the QuitCurrent threshold before
exiting relaxation mode.
Qmax
Qmax contains the maximum chemical capacity of a cell and is determined by comparing states of charge, before and after
applying a load, with the amount of charge that passed. It also corresponds to capacity at very low rate of discharge such as
C/20 rate. For high accuracy, this value is periodically updated by the bq27350 during operation. The initial value of Qmax
should be set to the battery cell capacity specified in its datasheet.
Update Status
There are 2 bits in this register that are important
– Bit 1 (0x02) indicates that the bq27350 has learned new Qmax parameters and is accurate.
– Bit 2 (0x04) indicates whether Impedance Track™ algorithm is enabled.
The remaining bits are reserved. Bits 1 and 2 are user-configurable; however, bit 1 is also a status flag that can be set by the
bq27350. These bits should never be modified except when creating a golden image file as explained in the application note
“Preparing Optimized Default Flash Constants for specific Battery Types” (see SLUA334.pdf). Bit 1 is updated as needed by the
bq27350 and Bit 2 is set with Control( ) command 0x0021.
Avg I Last Run
The bq27350 logs the current averaged from the beginning to the end of each discharge cycle. It stores this average current
from the previous discharge cycle in this register. This register should never need to be modified. It is only updated by the
bq27350 when required.
Avg P Last Run
The bq27350 logs the power averaged from the beginning to the end of each discharge cycle. It stores this average power from
the previous discharge cycle in this register. To get a correct average power reading the bq27350 continuously multiplies
instantaneous current times Voltage( ) to get power. It then logs this data to derive the average power. This register should
never need to be modified. It is only updated by the bq27350 when the required.
The Ra Tables
This data is automatically updated during device operation. No user changes should be made except for reading the values from
another pre-learned pack for creating “Golden Image Files”. Profiles have format Cell0 R_a M, where M is the number indicating
state of charge to which the value corresponds.
5.3 TEMPERATURE MEASUREMENT
The bq27350 can measure temperature via the on-chip temperature sensor or via the TS input depending
on the setting of the [TEMPS] bit PackConfiguration( ). The bit is set by using the PackConfiguration( )
function, described in the section entitled EXTENDED COMMAND SUMMARY.
Temperature measurements are made by calling the Temperature( ) function (see STANDARD
COMMANDS for specific information).
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FUNCTIONAL DESCRIPTION
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