Other Parts Discussed in Post: BQ27426

You might have heard of phrases like “gauge made easy,” “easy-to-use gauge,” “value-gauge products” or even “gauge in a few clicks.” The truth is that there is no easy way to make a “good” battery fuel gauge, but engineers at Texas Instruments have managed to ease initial gauge configuration simply by using the information on the back of the battery pack. In this blog post, I’ll explain how to read the data on the label, what it means, and how to use it in the initial configuration process.

TI fuel gauges in a family equipped with the Impedance Track™ “light” algorithm support this kind of configuration. In reality, there is nothing light in the algorithm itself. We call it “light” because that’s how we want systems engineers to experience the process of configuring the fuel gauge.

Essentially, the minimum information the fuel gauge needs to know is how big your battery is, what constitutes full, and what constitutes empty (for your system).

The first parameter is called design capacity (DC). It is a measure of the nominal usable energy that the battery has and it is expressed in milliampere hours (mAh). The label typically includes the minimum and nominal values.

In Figure 1, 3300mAh is the typical DC value used for the gauge configuration explained in this blog.

Figure 1: Back label of a single-cell battery pack

Besides DC, TI gauges are also configured with design energy (DE). This is similar to DC, but the units are in  milliwatt hours (mWh). You can see in Figure 1 that the typical DE for this pack is 12.6Wh (12,600mWh). Notice that the relationship between DC in milliampere hours and DE in milliwatt hours uses the nominal voltage. In this case, 3.8V x 3300mAh = 12.6Wh, which is the other value listed on the pack.

The next parameter to look at is the maximum charge voltage. This is actually what is listed in Figure 1 as “limited charge voltage” and represents the maximum safe voltage to which you should charge the battery cell.

There is, in fact, a direct correspondence between this parameter and the battery-voltage profile (also called open circuit voltage [OCV]). When using a Impedance Track gas gauge like the bq27441, bq27411 or bq27421, pay attention to the exact general part number (GPN), making sure that the extension matches the correct charging voltage.

When using the bq27426, battery fuel gauge, OCV selection occurs by sending a one-time-only I2C command that enables the correct chemistry profile corresponding to the maximum charge voltage of the battery you plan to use. In Figure 1, this value is 4.35V. Thus, if you plan to use the bq27421 (as an example), make sure to select the bq27421-G1B, which uses a preset battery profile for batteries chargeable to 4.35V.

The next item you need to know relates to what is “full” for your battery in your system. This entails configuring the taper rate, a parameter that represents the current to which the charger will taper when approaching the end of the battery charge. As a rule of thumb, this value is one-tenth the original design capacity, sometimes denoted as C/10. The units of taper rate are in 0.1 hours, so C/10 would mean that you should set the taper rate equal to 100. Referring back to the 3300mAh battery example, this represents a taper current of 330mA.

Finally, to complete the fuel gauge’s initial configuration, you need to tell the gauge what voltage represents “empty” to your system. This is typically the minimum operating voltage for your system and is usually between 3V and 3.4V. Program this value into the terminate voltage of the fuel gauge in millivolt units.

Once you’ve set DC, DE, terminate voltage and taper rate, along with the right part number corresponding to the correct charging voltage, your gauge is ready to begin operating. Impedance Track technology will ensure the correct updates of Qmax during operation; many other parameters that depend on Qmax will scale accordingly.

For step-by-step instructions, see the quick start guide corresponding to the gauge you’ve chosen. Here’s a link to the bq27421-G1 quick start guide, for example. Or try the parameter Q&A wizard within our Battery Management Studio (bqStudio) GUI along with your favorite gauge.