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bq27421-G1A not reporting 100% SOC after a full charge

Louise Chan Jr.
Prodigy 30 points
Other Parts Discussed in Thread: BQ24040, BQ27421EVM-G1A, BQ24040EVM, EV2400

Hi,

I'm currently working on a project that uses the fuel gauge IC mentioned in the subject for our battery management circuit.

The problem that I noticed in our current setup is that every time the bq24040 battery charger IC terminates charging (CHG pin goes to HI-Z), the fuel gauge seem to always fall short in terms of reporting a 100% SOC on the battery and only reports 93% 95% on a full charge.

I made sure that I have properly configured in firmware the bq27421-G1A's basic battery parameters according to the battery that I am using which has a capacity of 180mAh:

Design Capacity = 180mAh

Design Energy = 666

Terminate Voltage = 3V

Taper Rate = 202 (based on a taper current of 8.9mA)

The weird thing though is that if I use the same firmware that handles the configuration of the bq27421-G1A using the BQ27421EVM-G1A and BQ24040EVM evaluation boards and duplicate the battery management circuit of our system by adjusting the trimmer potentiometers and configuring jumpers of the evaluation boards to match our setup, the bq27421-G1A chip of the BQ27421EVM-G1A will always report a correct 100% SOC (using the same re-chargeable battery) between the time when the bq24040 chip of the BQ24040EVM is tapering up to the time it terminates charging which is what I was hoping for our setup to also behave.

The only difference that I noticed with our setup is that the bq27421-G1A's VDD pin was left floating (no 0.47uF capacitor connected between it and the VSS).

I don't have a good background in hardware design (i.e. I am only mostly involved in embedded software design) so I don't know if the missing decoupling capacitor in the fuel gauge's VDD pin has something to do with this problem or if there is some other factor causing the bq27421-G1A's SOC in our setup to always fall short of reporting a 100% charge during a full charge condition.

I would appreciate it if anyone will be able to help me figure out what's causing this problem.

Thanks.

--Louise

  • 0
    Genius 3970 points

    The capacitor between Vdd and Vss is really important; it is connected to the 1.8-volt LDO regulator built into the bq27421 and stabilizes the chip's internal power supply. I've found that the chip can be highly temperamental if the capacitor is not located close to the chip (or unconnected altogether). I've had spontaneous resets occur but haven't had any issues with the device not registering a full charge.

    When the bq24040 charger terminates charging, is the FC bit set in the Control Status register? If not, then the bq27421 hasn't recognized that the battery is in a fully charged state.

    Try adjusting your taper rate to accommodate a slightly higher taper current (since yours is configured for 8.9 mA, try bumping it up to 10 and see if it improves anything).

    Also, do you have an EV2300/EV2400 USB interface and the GaugeStudio software? If you do, can you send the .gg file that contains the configuration data for your chip?

  • 0 in reply to Jason Gin
    Prodigy 30 points

    Hi Jason,

    Thanks for the information that you provided regarding the effects that you have observed relating to the capacitor connected to the bq27421's VDD pin. Will try to modify our setup to include this capacitor and see if any improvements can be observed to the SOC at full charge. On a side note, I think this might also explain the occasional bq27421 reset that I encounter when applying/removing power to the charger circuit.

    FC bit is not set when the bq24040 charger terminates and bq27421 reports a 93% to 95% SOC.

    As for the taper rate adjustment, I have also tried modifying the taper rate that I am using before (although I previously based it on a 15mA taper current instead of 10mA) but was still not able see any improvements on the SOC at full charge (i.e. SOC still reports around 93%-95% at full charge).

    I have the EV2300 and GaugeStudio software and have also used it but have not yet created a .gg file from the configuration data that I am using. I will try to get this info and post it here when I get back to work next week.

    --Louise

     

  • 0 in reply to Louise Chan Jr.
    Prodigy 30 points

    Didn't get the chance to create a .gg file from the GaugeStudio as the I2C communication block of the bq27421 IC may have gotten damaged (EVM now always sends a NACK reply when sending an I2C command) after experimenting with the removal of the 0.47uF cap connected between the VDD and the VSS pins.


    Anyway, your reply about the effects of not having this cap installed may be the culprit of the problem that I'm experiencing (though I will not have a chance of verifying it this time aroundç) so I'm considering your reply as the answer.

    Thanks for the help.