Gas gauge algorithm

slua364 and slua450  app notes give some insight

Both app notes are about impedance track. I want to know the theory of pure voltage based and pure coulomb counting gas gauge.

There isn't much to say about pure voltage or coulomb-counting gas gauges. The voltage-based method correlates a battery's terminal voltage to a certain State of Charge percentage (basically X volts = Y% SoC). Coulomb-counting is done by integrating the current passed through a sense resistor over time, and it is synchronized at a defined "empty" and "full" voltage (so X mAh until Y volt threshold). This Battery University article describes this in a bit more detail but is pretty much what I've summed up here.

thanks,

Hi Liting,

Is your goal just general education about how gauges might work using those algorithms, or do you want to create your own gauging solution using one of those techniques?

First, you might want to watch the Single Cell Gauging 101 videos (three in total) at our TI's Battery University.  Go to ti.com/battery then click on the Support & Community tab.  In the Fuel Gauging section you will see three videos (as of today).  Here is a direct link to the first one.  The first video sets up some background about battery characteristics to help you understand the gauging techniques.  The second video talks about voltage-only gauging and coulomb-counting gauging.  The third video talks about Impedance Track gauging.

If you want to learn how some of our coulomb-counting gauges work you might study the bq27210 documentation as an example.  At TI we usually call our gauges that primarily use coulomb counting EDV or CEDV (for compensated end of discharge voltage).

If you want to roll your own coulomb counting gauge then this very old app note (SLVA102) explains some basics:

Gas Gauging Basics Using TI's Battery Monitor ICs

The ICs mentioned in that app note are what we call "battery monitors" since they generally provide basic info such as current/coulombs, voltage, and temperature.  It is up to you to use that information to figure out battery state of charge. 

This is in contrast to "battery gauges" which provide a more sophisticated complete solution by using the VIT information in an algorithm and using past history and learned characteristics to predict SOC, capacity, and other useful items.  Here's another old app note about the distinction: Choosing Between Battery Gas Gauges & Monitors to Track Charge Availabilility

In general there is no reason to choose a "battery monitor" over a "battery gauge" unless you have a very specific need.  Even then, the best IC you could choose to get a raw coulomb count would be a bqJunior gauge (such as bq27000/bq27010/bq27200/bq27210) since its hardware is much better than the monitors mentioned in SLVA102.  Here's an app note on how to use bqJunior for simple coulomb counting: Building a Superior Battery Monitor with the bq27x00

Happy gauging!