First, users will need to understand the basic idea of fuel gauging as applied to battery systems. The so-called “fuel” inside a battery is actually stored chemical energy. When fuel is stored as a volume of liquid like in a car’s gas tank, it is relatively straightforward to measure since users can just look at the level of liquid in the tank. In the case of a battery however, getting an accurate estimation can be complex. The effective amount of energy that a battery can deliver to a system can vary significantly based on the operating temperature, age of the battery, and power demands from the load. A “fuel gauge” IC needs to measure a number of different parameters – voltage, current, and temperature, and combine this data with prior knowledge of the battery to make an estimate of the battery’s present state of charge, as well as estimate how long a system can continue to operate based on that state of charge. Many portable systems must retain the contents of their memory when the systems are shut down. Accurate fuel gauging enables users of portable devices to operate longer, because they know that there is enough capacity remaining to use a device, rather than shutting down early to be conservative and avoid data loss. Fuel gauges can also report back to the user that the battery needs to be replaced, or that some other fault condition has been detected.