BQ2060A_15 Datasheet, PDF (13/61 Pages) Texas Instruments – SBS v1.1-Compliant Gas Gauge IC

bq2060A
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SLUS500D – OCTOBER 2001 – REVISED OCTOBER 2011
Self-Discharge
The bq2060A estimates the self-discharge of the battery to maintain an accurate measure of the battery capacity
during periods of inactivity. The algorithm for self-discharge estimation takes a programmed estimate for the
expected self-discharge rate at 25°C stored in EEPROM and makes a fixed reduction to RM of an amount equal
to RemainingCapacity()/256. The bq2060A makes the fixed reduction at a varying time interval that is adjusted to
achieve the desired self-discharge rate. This method maintains a constant granularity of 0.39% for each
self-discharge adjustment, which may be performed multiple times per day, instead of once per day with a
potentially large reduction.
The self-discharge estimation rate for 25°C is doubled for each 10 degrees above 25°C or halved for each 10
degrees below 25°C. The following table shows the relation of the self-discharge estimation at a given
temperature to the rate programmed for 25°C (Y% per day):
TEMPERATURE( C)
Temp < 10
10 ≤ Temp < 20
20 ≤ Temp < 30
30 ≤ Temp < 40
40 ≤ Temp < 50
50 ≤ Temp < 60
60 ≤ Temp < 70
70 ≤ Temp
SELF-DISCHARGE RATE
1/4Y% per day
½ Y% per day
Y% per day
2Y% per day
4Y% per day
8Y% per day
16Y% per day
32Y% per day
The interval at which RM is reduced is given by the following equation, where n is the appropriate factor of 2 (n =
1/ 4, 1/ 2, 1, 2 . . . ):
Self-DischargeUpdateTime =
640 x 13500
seconds
256 x n x (Y% per day)
(2)
The timer that keeps track of the self-discharge update time is halted whenever charge activity is detected. The
timer is reset to zero if the bq2060A reaches the RemainingCapacity()=FullChargeCapacity() condition while
charging.
Example: If T = 35°C (n = 2) and programmed self-discharge rate Y is 2.5 (2.5% per day at 25°C), the bq2060A
reduces RM by RM/256 (0.39%) every
640
256 n
135000
(Y% per
day)
+
6750
seconds
(3)
This means that a 0.39% reduction of RM is made 12.8 times per day to achieve the desired 5% per day
reduction at 35°C.
Figure 7 illustrates how the self-discharge estimate algorithm adjusts RemainingCapacity() vs. temperature.
Light Discharge or Suspend Current Compensation
The bq2060A can be configured in two ways to compensate for small discharge currents that produce a signal
below the digital filter. First, the bq2060A can decrement RM and DCR at a rate determined by the value stored
in Light Discharge Current EE 0x2b when it detects no discharge activity and the SMBC and SMBD lines are
high. Light Discharge Current has a range of 44 µA to 11.2 mA.
Alternatively, the bq2060A can be configured to disable the digital filter for discharge when the SMBC and SMBD
lines are high. In this way, the digital filter does not mask the leakage current signal. The bq2060A is configured
in this mode by setting the NDF bit in Control Mode.
Copyright © 2001–2011, Texas Instruments Incorporated
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