Battery Gas Gauge and Monitor ICs

Battery-based systems are sensitive to the amount of usable life left in the battery. This is particularly important for computers where a loss of power could mean a loss of stored data. In addition, most battery-based systems are portable, so their operating environment can vary. That environment can cover a wide range of temperatures, which affect a battery's efficiency, rate of charge and discharge, and therefore battery life.

One solution to this battery-sensitive situation is to include a means for providing a real time indication of remaining battery life to the system user. There are two approaches for indicating the remaining battery life: battery "gas gauge" and battery monitor. The term "gas gauge" is analogous to the automotive vernacular, this IC indicates battery life within its associated equipment. In contrast, battery monitors are actually data acquisition systems that accumulate data related to battery parameters and then transmit it to a host processor.

Battery monitors are mixed signal ICs that incorporate both analog and digital circuits. These monitors include one or more types of digital memory and special registers to hold battery data. Analog circuits include temperature sensors and amplifiers, as well as some interface circuits.

To measure battery current, the monitors usually include either an internal or external current sense resistor. Voltage and current measurements are usually via an on-chip A/D converter.

Among the monitored battery parameters are overcharge (overvoltage), overdischarge (undervoltage) and excessive charge and discharge currents (overcurrent, short circuit), information of particular importance in li-ion battery systems. In some ways a battery monitor assumes some of the functions of a protection circuit by protecting the battery from harmful overcharging and overcurrent conditions.

The gas gauge IC calculates the available charge of the battery and also compensates for battery temperature because the actual available charge is reduced at lower temperatures. For example, if the gas gauge IC indicates that the battery is 60% full at 25°C, then the IC indicates 40% full when cooled to 0°C, which is the predicted available charge at that temperature. When the temperature returns to 25°C, the displayed capacity returns to 60%. This ensures that the indicated capacity is always conservatively representative of the charge available for use under the given conditions.

Depending on the battery type, the gas gauge IC also adjusts the available charge for the approximate internal self-discharge of the battery. It adjusts self-discharge based on the selected rate, elapsed time, battery charge level, and temperature. This adjustment provides a conservative estimate of self-discharge that occurs naturally and that is a significant source of discharge in systems that are not charged often or are stored at elevated temperatures.

The gas gauge IC is usually packaged within the battery pack. Because specific inputs on the gas gauge IC connect directly to the battery, those inputs must consume very little power. Otherwise, battery life will be reduced during long storage periods.

The battery gas gauge continuously compensates for both temperature and charge/discharge rate. Typically, it displays the available charge on LEDs and also can send the charge data to an external processor via an I/O port. The LED presentation usually consists of five or six segments of a "thermometer" display. To conserve battery power, the display is only activated at the user's discretion.

Battery gas gauge ICs employ mixed signal, analog and digital circuits. One technique is to use analog circuits to monitor battery current by measuring the voltage drop across a low-value resistor (typically 20mW to 100mW) in series with the battery. This provides the charge input to the battery and the charge subs