DS2781 Maxim Integrated Products, DS2781 Datasheet - Page 16

• Manufacturer Part Number: DS2781
• Description: 1-Cell or 2-Cell Standalone Fuel Gauge IC
• Manufacturer: Maxim Integrated Products

DS2781: Two-Cell Standalone Fuel Gauge IC
not overwritten with an incorrect value. Usually, writing AS by the host is not necessary because AS is
automatically saved to EEPROM on a periodic basis by the DS2781. (See the Memory section for details.) The
EEPROM stored value of AS is recalled on power-up.
CAPACITY ESTIMATION UTILITY FUNCTIONS
Aging Estimation
As discussed above, the AS register value is adjusted occasionally based on cumulative discharge. As the ACR
register decrements during each discharge cycle, an internal counter is incremented until equal to 32 times AC. AS
is then decremented by one, resulting in a decrease in the scaled full battery capacity of 0.78%. Refer to the AC
register description above for recommendations on customizing the age estimation rate.
Learn Function
Since Li+ cells exhibit charge efficiencies near unity, the charge delivered to a Li+ cell from a known empty point to
a known full point is a dependable measure of the cell capacity. A continuous charge from empty to full results in a
“learn cycle”. First, the Active Empty point must be detected. The Learn Flag (LEARNF) is set at this point. Then,
once charging starts, the charge must continue uninterrupted until the battery is charged to full. Upon detecting full,
LEARNF is cleared, the Charge to Full (CHGTF) flag is set and the Age Scalar (AS) is adjusted according to the
learned capacity of the cell stack.
ACR Housekeeping
The ACR register value is adjusted occasionally to maintain the coulomb count within the model curve boundaries.
When the battery is charged to full (CHGTF set), the ACR is set equal to the age scaled full lookup value at the
present temperature. If a learn cycle is in progress, correction of the ACR value occurs after the age scalar (AS) is
updated.
When an empty condition is detected (AEF or LEARNF set), the ACR adjustment is conditional. If AEF is set and
LEARNF is not, then the Active Empty Point was not detected and the battery is likely below the Active Empty
capacity of the model. The ACR is set to the Active Empty model value only if it is greater than the Active Empty
model value. If LEARNF is set, then the battery is at the Active Empty Point and the ACR is set to the Active Empty
model value.
Full Detect
Full detection occurs when the Voltage (VOLT) readings remain continuously above the VCHG threshold for the
period between two Average Current (IAVG) readings, where both IAVG readings are below IMIN. The two
consecutive IAVG readings must also be positive and non-zero. This ensures that removing the battery from the
charger does not result in a false detection of full. Full Detect sets the Charge to Full (CHGTF) bit in the Status
register.
Active Empty Point Detect
Active Empty Point detection occurs when the Voltage register drops below the VAE threshold and the two
previous Current readings are above IAE. This captures the event of the battery reaching the Active Empty point.
Note that the two previous Current readings must be negative and greater in magnitude than IAE, that is, a larger
discharge current than specified by the IAE threshold. Qualifying the Voltage level with the discharge rate ensures
that the Active Empty point is not detected at loads much lighter than those used to construct the model. Also,
Active Empty must not be detected when a deep discharge at a very light load is followed by a load greater than
IAE. Either case would cause a learn cycle on the following charge to full to include part of the Standby capacity in
the measurement of the Active capacity. Active Empty detection sets the Learn Flag (LEARNF) bit in the Status
register.
RESULT REGISTERS
The DS2781 processes measurement and cell characteristics on a 3.5s interval and yields seven result registers.
The result registers are sufficient for direct display to the user in most applications. The host system can produce
customized values for system use, or user display by combining measurement, result and User EEPROM values.
FULL(T) [ ]: The Full capacity of the battery at the present temperature is reported normalized to the 40°C Full
value. This 15-bit value reflects the cell stack model Full value at the given temperature. FULL(T) reports values
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