BQ27427: BQ27427 Gas Gauge SOC reports 1% for a long time

You'll have to run a learning cycle and configure the load prediction to be appropriate for your application.

Check out https://www.youtube.com/watch?v=WAcxrGDb68g and chapter 7.4.2.3.4 Load Select, Load Mode, www.ti.com/.../sluucd5.pdf

Hi Dominik,

In some of TI's documentation it mentions that if you set Capacity, Energy, term voltage, chemistry, taper rate, charge current threshold, discharge current threshold and quit current, then the gas gauging would be relatively accurate. Is there any way to get fairly accurate gauging without the learning cycle at all? Can we set a few things? do you have typical parameters to set?

Thanks

Abdul

Hi Dominik,

As far as I can see I have set all those parameters but I am not sure about Load Mode and Load Select. From the document I assume that the load mode stays in the default state (as I do not set it). So it should be the constant power model that is being used. That said, it is not clear to me what is the difference in terms of gauging and how I would decide what the Load Mode should be?

Also what do you mean by Load Select. I cannot seem find this parameter in the document.

Note that I am setting the parameters via the host microcontroller. I have the BIN pin permanently set to VSS through a 10K resistor. Is this correct for a non-removeable pack?

So it is important to set the CHEM ID before all other parameters are set? I am actually setting the parameters first and then setting the CHEM ID at the end. Could this be the problem with my gauging effectively reading ~1% for the last ~30% of the capacity left?

I understand also that the BQ27427 loses all the set parameters when the battery depletes completely. In that case should I be resetting all the parameters again when the battery charging re-starts and will it then continue gauging as normal?

Thanks a lot.

Abdul.

Load Mode: The gauge determines FCC, RM and therefore SOC with discharge simulations using the built-in ChemID for a model of the cell. These simulations assume a specific load current. If the application uses a typical average current that doesn't increase with lower voltage as the cell discharges, then you'd select Load Mode Constant Current. If the typical average current increases with lower cell voltage, then you'd select Load Mode Constant Power. The gauge will either hold the current constant or adjust it upwards during a discharge simulation, depending on Load Mode setting.

Load Select is used to choose an appropriate prediction. The options are explained in the TRM www.ti.com/.../sluucd5.pdf, 7.4.2.3.4 Load Select, Load Mode. By default, the gauge uses an average that it calculated in the past, which works for many but not all applications.

A completely depleted cell is an anomaly and doesn't usually happen in regular use because it can impact longevity of the battery so this should be avoided. The gauge is supposed to be configured only when ITPOR in Flags() is set. The cell should not be discharged below the UVLO- threshold, which will turn off the gauge. If this rare case happens, then you'd have to configure the gauge again when cell voltage recovers during charging, as ITPOR will be set. Note that the gauge must not be configured and the algorithm restarted if there is a significant current present (either charge or discharge) because the gauge needs to determine cell depth of discharge as a reference and that won't work well unless the cell is relaxed (true OCV).

BIN pulled to VSS with a 10k resistor is correct.

Please set the ChemID first and then configure the gauge. Especially if you ran a learning cycle (because setting the ChemID resets the Ra table to the ChemID's default values).

Hi Dominik,

So my main problem was that the gauge was showing 1% for the last 3rd of the total discharge time until 0%. 

So taking into consideration that I must set the CHEM ID before I set anything else, I have done the programming in this order...

Chemistry ID,

Capacity,

Energy, 

Term voltage,

Taper rate, 

CC Gain(change sign)

This seems to have resolved my problem and so far I am getting decent (proportional) gas gauging from the chip.

What I do not understand is why is it not clear from the documentation that the CHEM ID needs to be set before the other parameters? There is also little information regarding the CC Gain Value. I had to change this from negative to positive to enable the current reading to have the correct sign. What is this value and why is the default sign not correct/needs changing? Please elaborate on this.

Thanks for your help so far

Abdul.

The CC Gain Value is a result from the chip test program. It is intended to be calibrated for the end-application, which will reverse the sign. 

We'll update documentation to clarify the relation between ChemID commands and other parameters for this gauge.