New Industry Products

Dallas Semi’s 2-Wire Stand-Alone Fuel Gauge Reports Remaining Battery Capacity Without Learn Cycle

February 20, 2007 by Jeff Shepard

Dallas Semiconductor, a wholly owned subsidiary of Maxim Integrated Products, introduced the DS2786 stand-alone, open-circuit-voltage (OCV) based fuel gauge. The DS2786 estimates the available capacity of rechargeable Li+ batteries using a combination of an OCV battery model and coulomb counting. The OCV model measures cell voltage in the open-circuit state and determines battery capacity based on a lookup table stored in the IC. 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.

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 ±10mV with a resolution of 1.22mV. In addition, the DS2786 contains two auxiliary voltage inputs that are designed to measure resistor ratios, making them suitable for measuring thermistor or pack-identification resistors. Current is measured bidirectionally with a resolution of 11 bits and an LSB value of 25µV. Current measurements, plus a programmable offset bias, are internally accumulated at the completion of each current-measurement conversion period. The results can be used to increment or decrement the relative-capacity value to maintain an accurate fuel gauge when conditions do not permit an OCV capacity measurement. Internal temperature measurements are available with an accuracy of ±3°C and a resolution of 0.125°C. These measurements can be taken in place of an external thermistor measurement through one of the auxiliary voltage inputs.

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 inclusion of the I2C interface enables simple implementation on the host side of the portable product, although the DS2786 can also be mounted inside the battery pack. Host-side mounting simplifies pack design and reduces the cost of battery-pack procurement. Typical applications for the DS2786 include 3G multimedia handsets, digital still cameras, and digital audio (MP3) players.

The device is available in a tiny, 3mm x 3mm, 10-pin TDFN package. Prices start at $1.22 (10,000-up, FOB USA).