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Discharge Learn Accumulator

FCC

Derating

bq78412

www.ti.com

SLUSAA0 – OCTOBER 2010

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.

where

•

Derating is the capacity derating fraction as a function of load current

(1)

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

method.

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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Product Folder Link(s): bq78412

Not Recommended For New Designs