BQ28400PWR Datasheet, PDF (17/24 Pages) Texas Instruments – Tablet PC and Netbook 2-Series Cell Li-Ion Battery Gas Gauge and Protection

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SLUSA61A – OCTOBER 2010 – REVISED DECEMBER 2010
APPLICATION INFORMATION
Run Time to Empty
To predict how much run time the battery pack can supply to the host system, a “Run Time To Empty” value can
be calculated.
The SBS host system needs to read, store, and update the following values during a discharging period and
average them over a user-determined period of time:
• DSG bit of the BatteryStatus register (ensure that it is in discharge mode)
• AverageCurrent (mA)
– Positive value = charge current
– Negative value = discharge current
– One minute rolling average of current (the user can accumulate this time for improved granularity)
• RemainingCapacity (mAh)
Then calculating:
RunTimeToEmpty = RemainingCapacity(avg mAh) ÷ AverageCurrent(avg mA) (The result will be in hours.
For minutes, the user can take the above results and divide by 60.)
Charging Time to Full
To predict how much charging time before the battery pack is fully charged, a “Run Time To Full” value can be
calculated.
The SBS host system needs to read, store, and update the following values during a charging period and
average them over a user-determined period of time:
• DSG bit of the BatteryStatus register (specify in charge mode)
• AverageCurrent (mA)
– Positive value = charge current
– Negative value = discharge current
– One minute rolling average of current (the user can accumulate this time for improved granularity)
• RemainingCapacity (mAh)
Then calculating:
RunTimeToFull = [FullChargeCapacity(avg mAh) – RemainingCapacity(avg mAh)] ÷ AverageCurrent(avg
mA)
Remaining Capacity Alert
To provide enough time for action to be taken when the battery is below a pre-determined capacity, the user may
implement a remaining capacity alarm alert in the SMBus host system. To do this, an SMBus read of the
RemainingCapacity value should be completed then compared by the SMBus host to a user-selected value. If
the read RemainingCapacity value is < the user's Remaining Capacity, then the host system should instruct the
user of what action is needed.
Remaining Time Alert
Similar to the Remaining Capacity notification, the system operation may need an alarm notification based on
time rather than remaining capacity. To do this, a determination of the EndTimeToEmpty (as discussed below)
and compared by SMBus host to a user-selected remaining time limit value. If the RemainingTimeLimit value is <
EndTimeToEmpty, then the host system should instruct the user of the action to be taken.
Copyright © 2010, Texas Instruments Incorporated
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