Battery Fuel Gauge Operates Without Learn Cycle

Mar 28, 2007 11:15 AM

Dallas Semiconductor’s DS2786 stand-alone, open-circuit-voltage (OCV) based fuel gauge estimates the available capacity of rechargeable Li-ion batteries using a combination of an OCV battery model and coulomb counting. This capability makes accurate capacity information available immediately after a battery pack is connected to the host, thus eliminating the need for a capacity-learn cycle.

The OCV model measures cell voltage in the open-circuit state and determines battery capacity based on a lookup table stored in the IC. During periods of moderate to high discharge rates, which preclude OCV measurements, the DS2786 uses coulomb counting as a secondary means of estimating relative capacity.

Using this combination of OCV data and coulomb count, the DS2786 is able to determine pack capacity simply from a partial charge or discharge cycle, thus eliminating the need for a full charge or discharge cycle as required by many fuel-gauge ICs. Measuring the change in coulomb count during partial charge or discharge, along with battery capacity values provided by the OCV lookup table, enables the total pack capacity to be calculated.

The accuracy of an OCV-based fuel gauge is dependent on the accuracy of the pack-voltage measurement. The DS2786's 12-bit voltage measurement is highly accurate to within ±10 mV with a resolution of 1.22 mV.

The OCV lookup table and other user parameters are stored in 32 bytes of on-chip EEPROM. All measurement and estimated-capacity data are accessed through an I²C interface. The device is available in a 3-mm x 3-mm, 10-pin TDFN package. Prices start at $1.22 for 10,000 pieces.