This drawing shows the basic elements of a so-called “smart battery” system. The actual cell, or battery, is enclosed within a battery pack assembly. The gas gauge circuit sits between the battery and the rest of the system. The fuel gauge monitors current in and out of the battery at all times using a low-value shunt resistance. By continuously monitoring  the voltage drop across this resistance, it is possible to know the milliamp-hours in and out of the battery, which allows the gauge to keep a running total of the present capacity of the battery. The gauge also can control two series switches that prevent the battery from being overcharged or excessively discharged. Under normal conditions, both of the switches are closed. However if a fault occurs, they can be opened to prevent damage to the cell. The gas gauge IC must have a highly accurate sensing circuit to measure the millivolts across the sense resistor. TI gauges also measure the voltage and temperature of the battery, as all of this information is required to make an accurate capacity estimate.