BQ26501_16 Datasheet, PDF (8/33 Pages) Texas Instruments – HANDHELD APPLICATIONS

bq26501
Not Recommended For New Designs
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SLUS586B − OCTOBER 2003 − REVISED MARCH 2004
APPLICATION INFORMATION
Gas Gauge Operation
Figure 4 illustrates an operational overview of the gas gauge function.
INPUTS
Charge Current
Discharge Current
Self-Discharge Timer
Temperature
Compensation
COMPUTATIONS
+_ _
Nominal Available
Charge (NAC)
OUTPUTS
Rate and
Temperature
Compensation
Compensated
Available Charge
Last Measured
Discharge (LMD)
Qualified
Transfer
_+ ₊
Learning Count
Register (LCR)
Temperature, Voltage,
Average Current,
Other Data
HDQ Interface
Figure 4. Operational Overview
UDG−03042
The bq26501 measures the capacity of the battery during actual use conditions and updates the last measured
discharge (LMD) register with the latest measured value. The bq26501 retains the learned LMD value unless
a full reset occurs. By measuring the capacity that the battery delivers as it is discharged from full to the EDV1
threshold without any disqualifying events, the bq26501 learns the capacity of the battery. During normal use
conditions, the bq26501 should learn a new capacity only after a full discharge. Learning cycles are disqualified
by several abnormal conditions (see list at end of section). In the event that a learning cycle occurs with a
significant reduction in learned capacity, the new LMD value is restricted to a maximum LMD reduction during
any single learning discharge of LMD/8. The capacity inaccurate (CI) bit in FLAGS is cleared after the first
learning cycle. This bit remains cleared unless a full reset occurs.
The battery-full condition is defined as nominal available capacity (NAC) = LMD. The valid discharge flag (VDQ)
in the FLAGS register is set when this condition occurs and remains set until the learning discharge cycle
completes or an event occurs that disqualifies the learning cycle.
The learning discharge cycle completes when the battery is discharged to the condition where VOLT ≤ EDV1
threshold. The EDV1 threshold should be set at a voltage that guarantees at least 6.25% of battery capacity
below that threshold. The EDVF threshold should be set at a voltage that the system sees as the zero capacity
battery voltage.
The bq26501 does not learn the capacity between EDV1 and EDVF thresholds, but assumes that the capacity
is 6.25% of LMD, so care should be taken to set EDV1 based on the characteristics of the battery. The measured
LMD value is determined by measuring the capacity delivered from the battery from NAC=LMD until
VOLT ≤ EDV1, plus LMD/16 to account for the 6.25% capacity remaining below the EDV1 threshold.
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