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BQ78412 Fiches technique(PDF) 7 Page - Texas Instruments

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No de pièce BQ78412
Description  Pb-Acid Battery State-of-Charge Indicator With Run-Time Display
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BQ78412 Fiches technique(HTML) 7 Page - Texas Instruments

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Discharge Learn Accumulator
Gas Gauging After a Reset
During normal operation, the last learned full charge capacity (FCC), elapsed time and other important variables
are stored in permanent memory. If, for some reason the battery discharges to the point where there is no longer
sufficient voltage for the bq78412 to operate, it shuts down. Under such conditions, when the device powers up,
these variables are restored and battery is assumed to be at 50% relative SoC. If a charge current is present, the
device begins to measure the accumulated charge and time. Charging proceeds as normal with the appropriate
end-of-charge detection criteria. If the bq78412 powers up and there is no current, the device goes into idle state
followed by sleep state until a current is detected.
Battery Capacity Update
The bq78412 has two mechanisms for updating the battery capacity as the battery ages. (Note that the initial
capacity programmed into the bq78412 could be in error due to manufacturing tolerances or formation
procedures. This translates to a gas gauging error until the battery capacity is accurately learned.)
Both the Learned Capacity method and Age-Based Capacity method operate independently and both may be
enabled or disabled separately in order to maintain the correct measure of capacity of the battery over a variety
of operating conditions, but it is suggested that both be enabled for optimal performance.
Learned Capacity Method
When DevConfig1[14] is set to "1", the bq78412 opportunistically learns the full charge capacity (FCC) of the
battery based on a qualified discharge. A complete discharge from fully charged to fully discharged with no
charging events raising the remaining state of charge (SoC) above 80% is considered qualified. An internal state
variable qualified discharge (QD) is used for maintaining the status of discharge qualification. QD is initially
disabled. When the battery has reached the fully charged state, QD is set to enabled and discharge learn
accumulator is cleared to zero. When a discharge begins, QD is set to active. While QD is active, all passed
charge (positive or negative) is accumulated in the discharge learn accumulator. If at any time (while QD is in an
active state) a charging event raises SoC above 80%, QD is set to disabled and the discharge learn accumulator
is ignored. If the battery reaches the fully discharged state and QD is still active, the algorithm learns FCC based
on the discharge learn accumulator and the current load de-rating using Equation 1.
Derating is the capacity derating fraction as a function of load current
Age-Based Capacity Method
The counter for the elapsed time starts when the device is activated.
When DevConfig1[15] is set to "1" (non-default), the bq78412 updates the FCC based on elapsed time and an
aging algorithm with manufacturer defined parameters. The bq78412 decrements the FCC by 0.100 Ah every
CapDerateL days until DerateChange days have elapsed, after which the FCC decrements by the same amount
every CapDerateH days.
In this way the FCC is regularly de-rated (decremented) at regular intervals independently of the learned capacity
The values for CapDerateL, DerateChange, and CapDerateH must be carefully chosen to implement an
appropriate age-based capacity decrease formula.
For example: Assuming a 100Ah battery (when new) and a 3%/year capacity fade for the first 3 years and a
4%/year fade afterwards, the parameters might be set as follows:
3% of 100Ah = 3Ah decrease in one year
3Ah decrease in 0.1 Ah steps = 30 separate steps over 365 days
365 days / 30 decrement steps = one decrement step every 12.1 days
So CapDerateL = 12
4% of 100Ah = 4Ah decrease in one year
Copyright © 2010, Texas Instruments Incorporated
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