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The accuracy of the CEDV algorithm strongly depends on the accuracy and validity of the open-circuit voltage
(OCV) measurement taken by fuel gauges that are based on CEDV technology. This appendix describes the
process of taking OCV measurements during different events.
7.1 Background
•
OCV Calculation: OCV (open-circuit voltage) is normally a calculated value because a true measurement of
OCV requires an unloaded and relaxed condition on the battery. Because such an unloaded and completely
relaxed condition is not always possible in a real system, the fuel gauge uses measured voltage, current, and
temperature (VIT) to compute the OCV and as a result of this calculation, the state of charge (SOC) of the
battery is established or reestablished.
•
OCV Qualification Time (QT): The time in which SOC_INT is asserted during an OCV measurement is
approximately 165 ms. This is the timeframe in which we test if the VIT measurement is qualified for an
OCV calculation. This is not the timeframe in which the actual VIT measurement is taken. During this time,
the instantaneous current (adci) is measured. If abs(adci) ≥ DesignCapacity/18, then the [OCVFail] bit is set.
Otherwise, the VIT just measured is qualified and the gauge proceeds with OCV calculation.
•
Current Measurement Time (CMT): The time of current is measured – 1 s.
•
Voltage Measurement Time (VMT): The time of voltage is measured – 125 ms.
•
Temperature Measurement Time (TMT): The time of temperature is measured – 125 ms.
7.1.1 OCV Qualification and Calculation
OCV qualification and calculation (QC) happens under two conditions:
•
OCV_CMD is sent by the host.
•
Battery Insert (BI) event is detected.
POR causes an immediate BI.
7.1.2 OCV Calculation Assumption
The current, voltage, and temperature must remain stable during QT, CMT, VMT, and TMT. In every case that
stable VIT is mentioned, the desired stable condition for current is < C/20. If this is not true, errors can be
introduced into the OCV Calculation.
7.1.3 OCV Timing
The timing of each step in the OCV sequence is shown in
1. After a POR, voltage, current, and temperature are measured before updating the fuel gauge parameters.
2. Quick voltage and current measurements are taken to qualify OCV VIT conditions.
3. Voltage, current, and temperature are measured for subsequent fuel gauge parameters updates.
SLUUBD4A – APRIL 2016 – REVISED NOVEMBER 2022
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Open-Circuit Voltage Measurement Background
Note
Copyright © 2022 Texas Instruments Incorporated
Open-Circuit Voltage Measurement Background
Figure
7-1.
Chapter 7
BQ27220
67
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