Battery Fuel Gauge
What is a Battery Fuel Gauge?
Definition
A battery fuel gauge (also called a battery gas gauge) is a feature or device that measures the accumulated energy added to and removed from a battery, allowing accurate estimates of battery charge level.
How Do You Use a Battery Fuel Gauge?
Battery systems may be designed with a host-side fuel gauge, or a pack-side fuel gauge.
A host-side fuel gauge resides on the host system and is connected to the application processor, as shown above. Having the fuel gauge as part of the application circuit instead of being located inside the battery makes the battery cheaper and easier to replace.
A pack-side fuel gauge resides on the battery pack and is connected to the cell, as shown above. The proximity of the cell to the fuel gauge has advantages including improved accuracy of state of charge reporting and tiny solution size.
How Do Fuel Gauges Work?
There are two main methods of battery fuel gauging: coulomb counting and measuring open circuit voltage (OCV).
Coulomb counting is the process of estimating the amount of charge used by measuring the charge going into and coming out of the battery. While this works well in the short-term, a small drift error increases continuously over time and the battery needs to reach full and empty periodically in order to correct these errors.
Another way to monitor the state of charge (SOC) of a battery is to measure the open circuit voltage. OCV is estimated by measuring the immediate voltage and by running a real-time simulation of the battery model using battery characterization and past voltage history. Because OCV is reliably related to a battery’s SOC, the former can be used to determine the latter. This method is more accurate because it does not suffer from capacity drift over time and uses less power because it does not require a current sense resistor.
Coulomb counting offers better short-term accuracy, while OCV is better for long-term. To provide better results over a wider range of applications, these two methods can be combined with algorithms such as ModelGauge, which uses the OCV algorithm to periodically remove the offset error introduced by the coulomb counter while maintaining the stability of a voltage-based fuel gauge.
The type of battery fuel gauge you’ll want to use will depend on your application.