BQ27505-J3 Datasheet, PDF (26/44 Pages) Texas Instruments – System-Side Impedance Track™ Fuel Gauge

bq27505-J3
Not Recommended for New Designs
SLUS986A – OCTOBER 2009 – REVISED FEBRUARY 2010
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After about 5 minutes in relaxation mode, the bq27505 attempts to take accurate OCV readings. An
additional requirement of dV/dt < 4 mV/s is required for the bq27505 to perform Qmax updates. These
updates are used in the Impedance Track™ algorithms. It is critical that the battery voltage be relaxed
during OCV readings to and that the current is not be 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.
5.2.8 Qmax 0 and Qmax 1
Generically called Qmax, these dynamic variables contain the respective maximum chemical capacity of
the active cell profiles, and are determined by comparing states of charge before and after applying the
load with the amount of charge passed. They also correspond to capacity at a very low rate of discharge,
such as the C/20 rate. For high accuracy, this value is periodically updated by the bq27505 during
operation. Based on the battery cell capacity information, the initial value of chemical capacity should be
entered in the Qmax n field for each default cell profile. The Impedance Track™ algorithm updates these
values and maintains them the associated actual cell profiles.
5.2.9 Update Status 0 and Update Status 1
Bit 0 (0x01) of the Update Status n registers indicates that the bq27505 has learned new Qmax
parameters and is accurate. The remaining bits are reserved. Bits 0 is user-configurable; however, it is
also a status flag that can be set by the bq27505. Bit 0 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 (SLUA334). Bit 0 is updated as needed by the bq27505.
5.2.10 Avg I Last Run
The bq27505 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 not be
modified. It is only updated by the bq27505 when required.
5.2.11 Avg P Last Run
The bq27505 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 bq27505 continuously multiplies instantaneous current times Voltage( ) to get power. It then
logs this data to derive the average power. This register should not be modified. It is only updated by the
bq27505 when required.
5.2.12 Delta Voltage
The bq27505 stores the maximum difference of Voltage( ) during short load spikes and normal load, so
the Impedance Track™ algorithm can calculate remaining capacity for pulsed loads. This behavior can be
modified by setting bits 0 and 1 of the Data flash register Debug Options as follows:
Bit1 Bit0
00
01
10
11
Delta V Selection
Standard
No Averaging
Minimum Delta V
–
Description
Average variance from steady state voltage used to determine end of
discharge voltage
Last instantaneous change in voltage from steady state used to
determine end of discharge voltage
Value in Min Delta Voltage used
Unused
26
FUNCTIONAL DESCRIPTION
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