DN471 - Simple Calibration Circuit Maximizes Accuracy in Li-Ion Battery Management Systems

Simple Calibration Circuit Maximizes Accuracy in Li-Ion Battery
Management Systems
Design Note 471
Jon Munson
Introduction
In Li-Ion battery systems it is important to match the
charge condition of each cell to maximize pack performance and longevity. Cell life improves by avoiding both
deep discharge and overcharge, so typical systems strive
for operation between 20% and 80% states of charge
(SOC). Detection and correction of charge imbalances
assures that all cells track within the desired SOC window,
preventing premature aging of some cells that could
compromise the entire pack capacity. Highly accurate
measurements are required to determine SOC with Li-Ion
cells due to their exceptionally flat discharge characteristics, particularly with the lower voltage chemistries (see
example in Figure 1).
4.0
1
2
LTC6802IG-2
35
GPIO1
VREG
VTEMP1
V–
1M
3
4
31
LT1461AIS8-3.3
7
VIN
SHDN
GND
8
VOUT
6
5
28
26
2.2μF
1μF
OTHER PINS NOT
SHOWN FOR CLARITY
DN471 F02
Figure 2. LT1461 as an External Calibration
Source for an LTC6802 Li-Ion Battery Monitor
Accounting for the Error Sources
Fundamentally, there are several key characteristics that
comprise an overall accuracy specification:
3.5
3.0
VOLTAGE (V)
Si2351DS
2.5
• Quantization error of the ADC
2.0
• Initial accuracy of the ADC (or calibration reference)
1.5
• Variation from channel to channel
1.0
25°C, 2.3A
0°C, 2.3A
–20°C, 2.3A
0.5
0
0
20
40
60
CAPACITY (%)
80
100
DN471 F01
Figure 1. Discharge Characteristics of 3.3V Li-Ion Cell
Although the popular LTC ®6802 Battery Stack Monitor
offers high accuracy analog-to-digital conversion, some
applications demand accuracy that is only attainable with
a dedicated voltage reference IC. The LT®1461 is especially suited as a high performance calibration source,
available in the small SO-8 package. Figure 2 illustrates
this configuration. The calibration reference is measured
with an ADC channel normally intended for temperature
measurement. A programmable I/O bit controls power
to the reference.
10/09/471
• Variation with temperature
• Hysteresis effects, primarily that of the soldering
process
• Variation with operating time (long-term drift)
The maximum specified error in the data sheet for the
LTC6802IG-2 includes the first four items and is ±0.22%;
about ±7mV when measuring 3.3V, the most demanding
region of the discharge curve. The spec budgets ±3.3mV
(±0.1%) as the maximum variation over the –40°C to
85°C operating temperature. Since the differential nonlinearity (DNL) of the ADC is about ±0.3 LSB, the quantization error contribution is about ±0.8 LSB, or ±1.2mV.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of
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owners.
and normalizes all ADC readings with the same computed
correction factor. The tolerance and drift of the reference
and channel-to-channel variations are left uncorrected
but the net uncertainty would be improved by almost a
factor of 2, to ±6.2mV.
Typical channel-to-channel variation is minimal, under
±1mV, leaving about ±1.5mV for trim resolution and accuracy in the IC manufacturing process. Thermal hysteresis
is specified as 100ppm, and an additional approximately
±0.1% error may develop from the shift of the printedcircuit soldering process.
A slightly more complex technique (method 2) involves
storage of a single correction factor that accounts for the
true reference voltage as measured with high accuracy
test-fixture instrumentation. This then eliminates the
initial error of the LT1461, improving the overall accuracy
to ±4.1mV, nearly a 3× total improvement.
Projected typical long-term drift is under 60ppm/√khr.
If the practical vehicle battery system active life cycle is
targeted at 5khr (about 15 years or 150,000 miles), an
uncertainty of around ±0.5mV could develop. This is a
relatively small contribution to total error.
While small, there is still some channel-to-channel
variation that can be calibrated out with a method that
uses more initial test-fixture measurements (method 3).
This is similar to method 2, but with high accuracy measurements of every channel taken (including the reference)
and the saving of individual correction factors for each.
This further reduces the error to ±3.1mV (almost a 4×
total improvement).
The LT1461AIS8-3.3 voltage reference IC has an output
tolerance of ±0.04% and less than ±1.2mV of change
over temperature with its exemplary 3ppm/°C worst-case
stability. The LT1461 exhibits a long-term drift of under
60ppm/√kHr and thermal hysteresis of 75ppm. Solder
reflow shift is expected to be under 250ppm (±0.8mV).
Since a significant portion of the LTC6802 ADC error accumulates after the initial delivery of the IC, an external
calibration technique improves accuracy in a finished
product.
Conclusion
A precision voltage reference, such as the LT1461, can
improve the accuracy of an LTC6802-based battery management system to about ±3mV worst-case. The reference
is a simple addition to the highly integrated LTC6802 Li-Ion
monitoring solution, thanks to the spare general-purpose
ADC channels available. The low operating current of the
LT1461 voltage reference also makes it ideal for this and
other battery-powered applications.
Examining Calibration Strategies
There are a number of options to improve system accuracy,
at the expense of additional complexity. With the simple
circuit of Figure 2, several options are available that take
advantage of the external calibration reference. Accuracy
projections of several methods are tabulated in Table 1
and described below.
References
“Battery Stack Monitor Extends Life of Li-Ion Batteries in
Hybrid Electric Vehicles,” Linear Technology Magazine,
Volume 19, Number 1, March 2009, page 1.
The simplest scheme (method 1) involves no local memory
or measurements at production. This method takes readings of the nominal 3.300V calibration voltage periodically
Table 1. Accuracy of Calibration Methods Described for 3.3V Measurements
EXTERNAL CALIBRATION METHOD QUANTI- FACTORY SOLDERING CHANNEL THERMAL
THERMAL
(ALL TOLERANCES SHOWN IN ±mV) ZATION
TRIM
SHIFT
MATCH VARIATION HYSTERESIS
LONGTERM TOTAL
DRIFT ERROR
LTC6802 Without External Calibration
1.2
1.5
3.3
1.0
3.3
0.3
0.5
11.1
1: Calibration with LT1461, No Stored
Information
1.2
1.3
0.8
1.0
1.2
0.2
0.5
6.2
2: Calibration with LT1461, Store
Calibration Values for Reference
Voltage
1.2
-
-
1.0
1.2
0.2
0.5
4.1
3: Calibration with LT1461, Store
Calibration Values for Reference
Voltage and Each Input
1.2
-
-
-
1.2
0.2
0.5
3.1
Data Sheet Download
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