DS2780 Maxim Integrated Products, DS2780 Datasheet - Page 14

• Manufacturer Part Number: DS2780
• Description: Stand-Alone Fuel Gauge IC
• Manufacturer: Maxim Integrated Products

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DS2780 Stand-Alone Fuel Gauge IC
Active Empty Voltage (VAE): VAE stores the voltage threshold used to detect the Active Empty point. The value
is stored in 1-byte with units of 19.52mV and can range from 0V to 4.978V. VAE is located in the Parameter
EEPROM block. See the Cell Characteristics section for more information.
Active Empty Current (IAE): IAE stores the discharge current threshold used to detect the Active Empty point.
The unsigned value represents the magnitude of the discharge current and is stored in 1-byte with units of 200mV
and can range from 0 to 51.2mV. Assuming RSNS = 20mW, IAE can be programmed from 0mA to 2550mA in
10mA steps. IAE is located in the Parameter EEPROM block. See the Cell Characteristics section for more
information.
Aging Capacity (AC): AC stores the rated battery capacity used in estimating the decrease in battery capacity that
occurs in normal use. The value is stored in 2-bytes in the same units as the ACR (6.25mVh). Setting AC to the
manufacturer’s rated capacity sets the aging rate to approximately 2.4% per 100 cycles of equivalent full capacity
discharges. Partial discharge cycles are added to form equivalent full capacity discharges. The default estimation
results in 88% capacity after 500 equivalent cycles. The estimated aging rate can be adjusted by setting AC to a
different value than the cell manufacturer’s rating. Setting AC to a lower value, accelerates the estimated aging.
Setting AC to a higher value, retards the estimated aging. AC is located in the Parameter EEPROM block.
Age Scalar (AS): AS adjusts the capacity estimation results downward to compensate for cell aging. AS is a 1-byte
-7
value that represents values between 49.2% and 100%. The lsb is weighted at 0.78% (precisely 2
). A value of
100% (128 decimal or 80h) represents an un-aged battery. A value of 95% is recommended as the starting AS
value at the time of pack manufacture to allow learning a larger capacity on batteries that have an initial capacity
greater than the nominal capacity programmed in the cell characteristic table. AS is modified by the cycle count
based age estimation introduced above and by the capacity Learn function. The host system has read and write
access to AS, however caution should exercised when writing AS to ensure that the cumulative aging estimate is
not over written 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 DS2780. (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.
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.
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