BQ27421-G1: Unexpected Full Charge Capacity Behaviour inBQ27421

For some reason the image does not appear after I post the message. Strange.

FCC is held constant during discharge (when smoothing is enabled) by design but it will adjust to a new value when the cell relaxes and temperature changes. Do you have a log with temperature data?

I did not log temperature, but these tests were performed with the cell 2 inches away in an open air environment, so I doubt the temperature had an effect.

Does the FCC recover during subsequent charge/discharge cycles?

It's concerning that the BQ27421 would adjust to a new value when the cell relaxes. I just reviewed another log where I discharged my device to 42% and then stopped discharging. 5 seconds later it recalculated the Full Charge Capacity and misreports the SOC. See the following table.

	During Discharge	After Discharge
Full Charge Capacity (mAh)	491	262
Available Capacity (mAh)	205	230
SOC	42%	88%

It's alarming that the device would report such a large swing in the SOC after only 5 seconds and especially when the cell's capacity has not actually changed.

Can the BQ27421 be configured to behave differently? Do we have an improper setting?

To give a little background into our product, it is a wireless microphone transmitter. Non-removable battery. The device is rarely fully discharged. The device is frequently turned on and off during it's use, drawing from <1 mA when OFF and up to 200 mA when ON.

Is the BQ27421 designed to operate in an application such as this?

Thanks!

Yes, the FCC does recover, but the device must fully charged again. Unfortunately, since our devices are turned on and off many times before being recharged, this will not be an adequate solution.

The bq27421 should work in your application. What's the complete configuration of the bq27421 in your system? Please attach a datasheet of your battery. Can you provide a complete log from bqStudio (click on start log in the main registers view) during a complete discharge with a typical current for your system?

Here are the values that we modify from the default values.
- Max Voltage = 3083 mV
- Min Voltage = 3752 mV
- Battery Capacity = 500 mAh (but we also use other 1000 mAh in the same product)
- Design Voltage = 3700 mV
- Design Energy = 1850
- Terminate Voltage = 3000 mV
- Taper Rate = 43 ( 500 mAh / (0.1 x 115 mA)
Are these the configuration values that you're referring to?

As for your request for a log from bqStudio, do you mean simply attaching the battery to the BQ27421EVM with an appropriate load? If so, I will need to run that test.

Lastly, I will get the datasheet for you shortly.

Thanks.

Yes, these were the values that I'm interested in.
About the log: Connect your cell to the EVM, connect the EVM to bqStudio, start the log, charge the cell, let it relax for an hour, discharge the cell with a typical current to the terminate voltage and then let it relax for another hour. Please attach the log to this thread.

Hi Dominik,

Performing the test using the EVM with the proper charger circuit input might be a little challenging. We don't have the products charge circuit broken out to connect to the "Charger" input of the EVM (J6 & J7).

Our product has the ability output any data. Would it be helpful if I logged certain parameter as a CSV (Excel) file and sent that?

Just so that I understand the troubleshooting direction, will this logged data lead us towards a solution that will make the BQ27421 not relearn the Full Charge Capacity (and SOC) when our application doesn't expect it to?

Thanks.

Please log the unfiltered SOC, FCC and RemCap as well as the filtered versions during a full charge and full discharge every 5 seconds.
This data will help me understand what's going on.

Is there any more information that I can provide that would be useful?

Thanks!

>Is there any more information that I can provide that would be useful?
Please log the unfiltered SOC, FCC and RemCap as well as the filtered versions during a full charge and full discharge every 5 seconds.
This data will help me understand what's going on.