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Texas Instruments Incorporated
Power Management
Fuel-gauging considerations in battery
backup storage systems
By Keith James Keller
Analog Field Applications
Accurate fuel gauging in battery backup systems requires
special considerations. Using Texas Instruments (TI)
battery fuel gauges with Impedance Track™ technology
offers the distinct advantage of not requiring a full dis-
charge of the pack for learning as the cells age. This article
discusses different implementations and techniques for
completing a proper learning cycle in backup applications.
Additionally, a case study of an aged battery pack’s chang-
es in capacity and impedance is reviewed.
TI’s Impedance Track algorithm uses voltage, current,
and impedance measurements of the cells to accurately
calculate a battery pack’s remaining capacity and run time.
Proper selection of a cell’s specific chemistry is required
for the most accurate gauging. As of this writing, there are
six distinct classes of chemistries, with several options
within each class.
In determining a battery backup system’s cell aging over
time, the major concerns are (1) the maximum chemical
capacity (Q
max
) of the cell, specified in milliampere-hours
(mAh), and (2) the actual measured impedance of the
cells (R_a table values), which will determine true run
time based on loading and temperature.
Most notably, high temperatures will adversely impact
Q
max
and the internal cell impedances. Charging and stor-
ing the cells at a lower voltage (between 3.9 and 4.1 V for
standard 4.2-V cells) will increase their lifetime at the
expense of shorter run times.
Older gas-gauging technologies require a complete
discharge of the cells to update capacity information.
Impedance Track technology eliminates this full-discharge
requirement and instead uses two relaxed-voltage measure-
ment points to update Q
max
. In the default firmware, these
voltage measurements are typically performed before and
after the battery state of charge (SOC) has changed by
about 40%. With modified firmware from TI, this SOC
range can be decreased to as low as 10% for a “shallow”
discharge. Decreasing the SOC range for the Q
max
update
will affect gauging accuracy; the more SOC range used,
the better.
The two relaxed-voltage mea-
surements need to be taken in a
qualified voltage range based on
the cell chemistry. For more infor-
mation, please review Reference 1.
To see an Excel
®
file with disquali-
fied Q
max
-update voltage ranges
based on cell chemistry, go to
http://www.ti.com/lit/zip/slua372
and click Open to view the
WinZip
®
directory online (or click Save to download the
WinZip file for offline use). Then open the file:
chemistry_specific_Qmax_disqv_voltages_table.xls
Table 1 shows an excerpt from this file. As the table
shows, if the chemical ID is 0100, then Q
max
-update voltage
measurements are not allowed between 3737 and 3800 mV
due to the flatness of the voltage profile at this SOC. This
disqualified voltage range is based on measuring the cell’s
relaxed voltage after a rest period of at least an hour.
Impedance measurements and updates will happen during
discharge with a load of greater than C/10. (A “C rate” is
based on the cell’s capacity. If a 3s2p pack has a design
capacity of 4400 mAh, then the C/10 discharge rate is
440 mA. In this case, a safe discharge rate would be
500 mA.)
To store varying resistances at different SOC values,
15 grid points are used. Once one grid point has been
recalculated, all subsequent grid points may be modified
accordingly. A discharge needs to exceed 500 seconds to
avoid transient effects and distortion of resistance values.
How to initiate a Q
max
learning cycle
TI provides evaluation software that shows the status
and allows controlling parameters of an Impedance Track
gas gauge (see Related Web Sites). After confirming that
the battery voltage is outside the disqualified range, a
RESET command can be sent to the gauge that will set
the R_DIS bit and clear the VOK bit. When a proper OCV
measurement has been completed by the gauge, the R_DIS
bit will be cleared. Now battery charging or discharging
can be started which will set the VOK bit in a few seconds.
With the firmware set for a shallow SOC change of 10%,
allow the charge/discharge to change the SOC by at least
15%. After stopping the charge/discharge cycle, allow the
cells to relax (up to 5 hours in a deeply depleted state)
outside the disqualified voltage range. The VOK bit should
clear, which is the indication that a second valid OCV
measurement has been taken and a Q
max
update has been
completed successfully.
Table 1. Disqualified Q
max
-update voltage ranges based on cell chemistry
Description
Chemical ID
Vqdis_min
Vqdis_max
SOC_min, %
SOC_max, %
LiCoO2/graphitized carbon
(default)
0100
3737
3800
26
54
Mixed Co/Ni/Mn cathode
0101
3749
3796
28
51
Mixed Co/Mn cathode
0102
3672
3696
6
14
LiCoO2/carbon 2
0103
3737
3800
26
54
Mixed Co/Mn cathode 2
0104
4031
4062
77
88
5
Analog Applications Journal
1Q 2010
www.ti.com/aaj
High-Performance Analog Products
Power Management
Texas Instruments Incorporated
The following two examples describe different system
implementations for battery backup systems.
Example 1: Passive discharge of cells
In this configuration, the active current of the gas-gauge
chipset (~375 µA) can be used to discharge the batter-
ies over an extended period of time. Depending on the
capacity of the pack, this could be several months.
Keeping the gauge continuously in active mode is pro-
grammable by setting the SLEEP bit in the “Operation
Cfg A” register to 0. Another option is to assert the
/PRES GPI with the non-removable bit (NR = 0) set in the
“Operation Cfg B” dataflash register.
With firmware modified for a shallow discharge such
as 20% for a Q
max
update, the pack can be allowed to
discharge down to 75% of its capacity over time and can
then be charged back up to full capacity. The Q
max
param-
eter will be updated accordingly. Note that only the Q
max
values, not the cell impedances (R_a table values), will
be updated during this type of cycling. It is assumed that
a rest period of several hours is allowed at the end of
charge for the second relaxed-voltage measurement.
Example 2: Active discharge of cells
In this configuration, a discharge resistor in the system
can be used to actively discharge the cells. This could be
controlled by a host processor inside the battery packs or
externally in the system. As discussed earlier, a discharge
current of greater than C/10 for 500 seconds is required
for impedance grid-point updates.
Even though the 10% minimum discharge requirement
applies for a Q
max
update, ideally the pack should be dis-
charged through two impedance grid-point updates. These
occur during discharge at SOC intervals of approximately
11% (i.e., at 89%, 78%, 63%, 52%, etc.). In this case, dis-
charge from 100% to 75% capacity would be sufficient. If
the battery is being stored with the SOC at 80% for lon-
gevity reasons, two impedance grid-point updates would
happen within a 25% discharge.
A proper Q
max
update will happen only after two con-
secutive relaxed-voltage measurements separated by a
charge or discharge are taken (assuming that both mea-
surements are outside the disqualified voltage range of the
specific chemical ID). Therefore, after the pack is actively
discharged to 75% of its capacity, a rest period of several
hours is required, depending on the SOC. (Based on cell
chemistry, up to 3.5 hours is required for a semicharged
state, and up to 5 hours for a fully discharged state.)
Case study
The effects of long-term storage were studied by using a
Microsun Technologies 3s4p 8.8-Ah battery pack that had
LGDS218650 cells with the bq20z80 chipset produced in
June of 2006. The pack was stored at about 45% capacity
at room temperature for two years without being cycled.
The parameters of interest were changes to Q
max
and to
the cell impedances, as well as the accuracy of remaining-
capacity and time-to-empty calculations. The estimated
Figure 1. Changes in cell impedance over time
1600
1400
Cell0 Impedance
Measured Before
Discharge
1200
1000
Cell0 Impedance
Measured After
Discharge
800
600
400
200
0
0
20
40
60
80
100
Cell State of Charge, SOC (%)
self-discharge of these cells is less than 4% per year.
A constant resistive load of 3 Ω was used for discharging
the packs (equating to a discharge rate of approximately
3.5 A). Changes in Q
max
and in the impedance values are
respectively shown in Table 2 (on the next page) and
Figure 1. On average, Q
max
decreased by 3% and the
impedances of the cells increased by 35%. Even with these
changes in the cells, the accuracy of the initial discharge
cycle following the two-year rest period was greater than
99%; specifically, a capacity of 67 mAh was reported when
the terminate voltage was reached (67 mAh/8819 Q
max
=
0.00761, or an error of 0.761%).
Conclusion
TI’s battery fuel gauges with Impedance Track technology
provide an extremely accurate estimation of remaining
battery capacity. Understanding how the technology works
is especially important in designing storage and backup sys-
tems with long periods of rest. Examples were presented
of using passive and active discharge of the pack to update
Q
max
and cell impedance values. Additionally, discharge
results from an aged battery pack were shared to illustrate
the concepts and overall accuracy of this technology.
Reference
For more information related to this article, you can down-
load an Acrobat
®
Reader
®
file at www.ti.com/lit/
litnumber
and replace “
litnumber
” with the
TI Lit. #
listed below.
Document Title
TI Lit. #
1. Yevgen Barsukov, “Support of Multiple Li-Ion
Chemistries With Impedance Track™ Gas
Gauges,” Application Report................
slua372
Related Web sites
power.ti.com
www.ti.com/sc/device/bq20z95
To download bq evaluation software:
www.ti.com/litv/zip/sluc107b
6
High-Performance Analog Products
www.ti.com/aaj
1Q 2010
Analog Applications Journal
Texas Instruments Incorporated
Power Management
Table 2. Q
max
and cell impedance values before and after discharge of a sample pack
Cell Impedance Measurements
Before Discharge
Cell Impedance Measurements
After Discharge
Q
max
(mAh) Before
Q
max
(mAh) After
xCell0 R_a 0 = 93
Cell0 R_a 0 = 124
xCell0 R_a 1 = 102
Cell0 R_a 1 = 136
xCell0 R_a 2 = 112
Cell0 R_a 2 = 149
xCell0 R_a 3 = 117
Cell0 R_a 3 = 156
xCell0 R_a 4 = 103
Cell0 R_a 4 = 137
xCell0 R_a 5 = 102
Cell0 R_a 5 = 136
xCell0 R_a 6 = 112
Cell0 R_a 6 = 149
CELL0
xCell0 R_a 7 = 112
Cell0 R_a 7 = 148
9096
8819
xCell0 R_a 8 = 117
Cell0 R_a 8 = 165
xCell0 R_a 9 = 128
Cell0 R_a 9 = 179
xCell0 R_a 10 = 138
Cell0 R_a 10 = 195
xCell0 R_a 11 = 146
Cell0 R_a 11 = 259
xCell0 R_a 12 = 204
Cell0 R_a 12 = 479
xCell0 R_a 13 = 393
Cell0 R_a 13 = 927
xCell0 R_a 14 = 573
Cell0 R_a 14 = 1355
xCell1 R_a 0 = 71
Cell1 R_a 0 = 98
xCell1 R_a 1 = 79
Cell1 R_a 1 = 109
xCell1 R_a 2 = 88
Cell1 R_a 2 = 122
xCell1 R_a 3 = 95
Cell1 R_a 3 = 131
xCell1 R_a 4 = 79
Cell1 R_a 4 = 109
xCell1 R_a 5 = 80
Cell1 R_a 5 = 111
xCell1 R_a 6 = 89
Cell1 R_a 6 = 123
CELL1
xCell1 R_a 7 = 87
Cell1 R_a 7 = 125
9102
8833
xCell1 R_a 8 = 90
Cell1 R_a 8 = 139
xCell1 R_a 9 = 98
Cell1 R_a 9 = 147
xCell1 R_a 10 = 108
Cell1 R_a 10 = 164
xCell1 R_a 11 = 114
Cell1 R_a 11 = 223
xCell1 R_a 12 = 159
Cell1 R_a 12 = 453
xCell1 R_a 13 = 338
Cell1 R_a 13 = 960
xCell1 R_a 14 = 491
Cell1 R_a 14 = 1397
xCell2 R_a 0 = 56
xCell2 R_a 0 = 83
xCell2 R_a 1 = 63
xCell2 R_a 1 = 93
xCell2 R_a 2 = 71
xCell2 R_a 2 = 105
xCell2 R_a 3 = 79
xCell2 R_a 3 = 117
xCell2 R_a 4 = 65
xCell2 R_a 4 = 96
xCell2 R_a 5 = 62
xCell2 R_a 5 = 92
xCell2 R_a 6 = 73
xCell2 R_a 6 = 108
CELL2
xCell2 R_a 7 = 69
xCell2 R_a 7 = 108
9096
8823
xCell2 R_a 8 = 73
xCell2 R_a 8 = 118
xCell2 R_a 9 = 82
xCell2 R_a 9 = 127
xCell2 R_a 10 = 89
xCell2 R_a 10 = 145
xCell2 R_a 11 = 93
xCell2 R_a 11 = 211
xCell2 R_a 12 = 134
xCell2 R_a 12 = 304
xCell2 R_a 13 = 323
xCell2 R_a 13 = 734
xCell2 R_a 14 = 475
xCell2 R_a 14 = 1079
7
Analog Applications Journal
1Q 2010
www.ti.com/aaj
High-Performance Analog Products
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SLYT364
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