If you have just designed in an Impedance Track™ gas gauge, this post will help you get started with the essential learning cycle you’ll need to achieve the best accuracy. A learning cycle is when you cycle your battery for the first time using an Impedance Track technology-based gas gauge. The learning cycle optimizes certain parameters that track the battery’s chemical capacity and internal impedance. A successful learning cycle can prevent inaccurate gauging in many real-world environments.
You’ll need some basic equipment to start the learning cycle:
First, we need a gas–gauge evaluation module (EVM) with a gas-gauge IC. The user’s guide on the EVM page will help you set it up with the proper load and power supply. Next, we need a load/source to charge and discharge the battery. Applications engineers at TI often use a Keithley 2400 source meter. This meter is capable of supplying and sinking current. The Keithley can also accurately charge the battery to a specified voltage with four-wire sense functionality.
If Keithley source meters have more capabilities and performance than you need or can afford, the next best thing would be to use a gauge development kit (GDK), such as the bq27GDK000EVM. It has an on-board charger (bq24192) and an on-board programmable load. The GDK board also has an integrated gas gauge, but it defaults to an external EVM configuration. To use the on-board gas gauge, see the user’s guide for the GDK.
Figure 1 shows the connection from the GDK to the external EVM. On the left, we use a Keithley source meter to compare and contrast the GDK and the Keithley.
Figure 1: Keithly Source Meter and GDK setup
Before diving into detail about the learning cycle, here are some common terms which will be used in the next section.
- C-rate – The discharge or charge rate in fractional form, with the numerator being the nominal capacity of the battery in mAh and the denominator being the number of hours it takes to discharge the cell to empty at a nominal rate. For example, if a battery is 3000mAh, a C/5 discharge rate will be 600mA.
- Relaxation – A period of time where no (or negligible) current flows into or out of the battery.
- Ra tables – A set of resistance tables which characterize the impedance profile of the battery.
- Qmax – A number or mAh measure that determines the chemical capacity of the battery as a function of design capacity. For new batteries, after a learning cycleyou might expect Qmax to be 5-10% larger than the nominal design capacity printed on the label.
I’ve divided the procedure for a learning cycle into flash- and Read Only Memory (ROM)-based gas gauges. The initial setup is slightly different for the two classes.
- Program the correct chemistry ID with the TI bqStudio Chemistry Tab plug-in. To determine the chemistry ID, use the online GPC tool.
- Configure the basic parameters (design capacity, design voltage, terminate voltage, design energy, and taper current or taper rate) to values that correspond to your battery and system specifications. You can change these values in data memory using TI’s bqStudio software, in many cases via the Q/A plug-in.
The battery should now be discharged to empty (terminate voltage). Your setup should allow the battery to relax for at least five hours after reaching terminate voltage. After at least five hours have elapsed:
- Check the Control Status register in bqStudio to make sure that the RUP_DIS bit is cleared and the VOK bit set. At this point, send an IT_ENABLE command (0x0021) via the bqStudio command bar.
- After sending the command, begin charging the battery.
Once the battery is charged to full, a relaxation of at least two hours is required. After at least two hours have elapsed:
- Discharge the battery at a rate higher than C/10 but less than C/5 until empty (terminate voltage).
- Relax the battery for at least five more hours.
ROM-based gas gauges (such as the bq274x21-G1 will not need the IT_ENABLE command or chemistry ID selection. ROM-based gas gauges are meant for quick configuration and ease of use, but their accuracy is not as good as flash-based gas gauges.
I recommend using the bqStudio Data Memory plug-in to export a gg.csv file both before and after the learning cycle to compare Qmax and Ra table values. After a successful learning cycle, Qmax should be 5-10% higher than design capacity and Ra tables should have changed from the beginning of the learning cycle. I also recommended running another charge/relax/discharge cycle in order to analyze the gas gauge’s state-of-charge accuracy.
There are many resources to help you with the learning cycle and accuracy optimization of gas gauges. We include more details about the learning cycle in the technical reference manuals of our latest fuel gauges, and the Additional Resources section lists several application notes and blogs to help you with the process. Feel free to post your questions on E2E™ Community forums as well.