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Original question:

BQ27441-G1: Average current and SOC measured by BQ27441 is not accurate

BQ27441-G1: Average current and SOC measured by BQ27441 is not accurate

Sajin M1
Prodigy 120 points
Part Number: BQ27441-G1
Other Parts Discussed in Thread: GPCCHEM, BQ27Z561

Hi,

I have a power section which is using BQ27441 as the fuel gauge to measure the status of a LiPo battery that has 4400 mAh capacity in total. The 4400 mAh capacity is achieved by connecting two identical 2200 mAh batteries in parallel. By the datasheet of the battery, the Maximum voltage is about 4.2V, the nominal voltage is 3.7V and the discharge cutoff voltage is 2.7V. The system parameters are listed below:
Maximum charging current: 2.4A
Maximum discharge current: 2A
Minimum running voltage: 3.4V
Maximum battery voltage: 4.2V
Cutoff current: 160 mA
the current sense resistor is 10 mOhm
For this system we configured the following parameters in the fuel guage:
BATTERY_CAPACITY = 4400               //4400 mAh battery will be connected
TERMINATE_VOLTAGE = 3400            // 3400mV terminate voltage
DESIGN_ENERGY = 16280                  //16280 mWh design energy
TAPER_RATE =275
FC set = 92% full charge set
FC clear = 90% full charge clear
TAPER_VOLTAGE =4160
Fortunately, the fuel gauge is measuring battery voltage and temperature at much better accuracy but it is not for average current and capacity. I am listing my issues and questions below:
Issue 1:
The current (Both Discharging and charging) measured by the fuel gauge was having an error of more than 8% (more than 200 mA), but after manually changing the CC gain from0.237 to 0.205 it is working properly now. but in the case of the remaining capacity and percentage of charge the there is a huge margin of error is there.
Issue 2:
The fuel gauge is showing 100 % of charge while charging even it is only at 3881 mAh and 0% before it is reaching the Terminate voltage (3.4V)
Question1:
I need to measure the capacity of the battery with much accuracy(less 1% error), what is the step for setting up the proper configuration for the fuel gauge to get the proper remaining capacity and percentage of charge?
  • 0
    TI__Guru 52115 points

    Hi Sanjin,

    I guess you should have received a report from the GPCCHEM. Could you share it with me? Thanks.

    Andy

  • 0 in reply to Andy Liu (BMS)
    Prodigy 120 points

    Hi Andy,

    Thank you for the reply. I am attaching the report with this mail please have a look

    Fullscreen 4380.GPC_report.txt Download 
    Chemistry ID selection tool, rev=2.49		
		
Configuration used in present fit:		
ProcessingType = 2		
NumCellSeries = 1		
ElapsedTimeColumn=3		
VoltageColumn = 7		
CurrentColumn = 13		
TemperatureColumn = 6		
		
Best chemical ID : 2150	Best chemical ID max. deviation, % : 2.06	
		
		
		
Summary of all IDs with max. DOD deviation below 3%		
		
Chem ID	max DOD error, %	Max R deviation, ratio
2150	2.06	0.24
2152	2.19	0.41
1762	2.24	0.38
224	2.31	0.68
2155	2.31	0.44
2380	2.32	0.46
2348	2.38	0.35
1895	2.4	0.39
2104	2.47	0.38
2021	2.49	0.85
2445	2.58	0.55
2419	2.59	0.53
2002	2.61	0.73
2404	2.62	0.37
241	2.63	0.85
2588	2.74	0.56
2408	2.75	0.62
2317	2.76	0.53
2187	2.82	0.78
2429	2.94	0.41
		
Max. deviations for best ID is within recommended range. Chosen best chemical ID is suitable for programming the gauge.		
		
		
Selection of best generic ID for ROM based devices like bq274xx		
		
		
Device / Family #1		
Generic Chem ID	Device/ Voltage/ Chemistry	max DOD error, %
3142	bq27421-G1D: 4.4V LiCoO2	15.76
354	bq27411-G1C: 4.35V LiCoO2	16.01
128	bq27421-G1A: 4.2V LiCoO2	28.12
312	bq27421-G1B: 4.3V LiCoO2	29.73
Best generic ID 3142		
Warning: Generic ID Deviation is so high that it is most likely due to anomaly in the data. Please check that data files have recomended format, units and test schedule		
		
		
Device / Family #2		
Generic Chem ID	Device/ Voltage/ Chemistry	max DOD error, %
1210	bq27621:  (ALT_CHEM1) 4.3V LiCoO2	15.85
354	bq27621:  (ALT_CHEM2) 4.35V LiCoO2	16.01
1202	bq27621: (default) 4.2V LiCoO2	17.94
Best generic ID 1210		
Warning: Generic ID Deviation is so high that it is most likely due to anomaly in the data. Please check that data files have recomended format, units and test schedule		
		
		
Device / Family #3		
Generic Chem ID	Device/ Voltage/ Chemistry	max DOD error, %
3142	bq27426: (ALT-CHEM2) 4.4V LiCoO2	15.76
3230	bq27426: (default) 4.35V LiCoO2	16.7
1202	bq27426: (ALT_CHEM1) 4.2V LiCoO2	17.94
Best generic ID 3142		
Warning: Generic ID Deviation is so high that it is most likely due to anomaly in the data. Please check that data files have recomended format, units and test schedule		
		
		
Warning: Max R deviation ratio can not be checked because discharge hr-rate is above 12.  Hr rate = 22.9759911961134		
Warning: Max R deviation ratio can not be checked because discharge hr-rate is above 12.  Hr rate = 22.9759911961134		
Warning: Max R deviation ratio can not be checked because discharge hr-rate is above 12.  Hr rate = 22.9759911961134

  • 0 in reply to Sajin M1
    TI__Mastermind 24870 points

    Hello Sajin,

    This is a ROM based gauge. There is no method to change the chemID.

    Are you able to upgrade to a bq27z561 flash based gauge that supports chemID programming?

  • 0 in reply to Kang Kang
    Prodigy 120 points

    Hi kang,

    Thank you for your reply and sorry to say that we can't change the fuel gauge right now, we already manufactured some large quantities of boards. Is there any other way to optimize the SOC?

  • 0 in reply to Sajin M1
    Prodigy 120 points

    By the datasheet, the total capacity of the battery is 4400 mAh, 16280 mWh and the cut off voltage is 2.7V and we are setting the same for the configuration of BQ27441, but for our design, we can't go up to 2.7V, we are stoping the discharge when the voltage reaches only 3.4V thereby we are not using the total 4400mAh or 16280 mWh. Will it be the cause for the issue?

  • 0 in reply to Sajin M1
    TI__Mastermind 24870 points

    Hello Sajin,

    You will need to look at the Qpass register during the discharge. If it is not even 90% of the discharge vs. design capacity, I would recommend setting the terminate voltage to 3.4 volts if that's your system shutdown voltage.

  • 0 in reply to Kang Kang
    Prodigy 120 points

    Hi Kang,

    We will test that thing that you have said about the Qpass register and update it with you.

    I have one more doubt is there, It is about the CC_GAIN. We had a current measurement accuracy issue previously and it was solved by altering the value in CC_GAIN register. But by going through the datasheet I got to know that CC_GAIN is no for customer usage and it is only for debugging purposes, so is that mean we can't alter the CC_GAIN value? Is it not the proper way to tackle the error in current sensing? If so what is the proper method?

  • 0 in reply to Sajin M1
    TI__Guru 62006 points

    You can change CC_GAIN. It is not restricted to debugging only.

    About gauge accuracy: The gauge uses the built in ChemID to run discharge simulations. You must complete a successful learning cycle where the gauge can measure QMax and cell impedance. This requires accurate current measurements.

    A learning cycle is basically a series of controlled charge/relax/discharge/relax sequences. See https://training.ti.com/how-perform-successful-learning-cycle-gauges

    Once the gauge learned these properties, it will be as accurate as the built-in ChemID allows for the cell that you use.

    Make sure that you configure charge termination correctly (you use C/27.5 but a C/25 with a taper rate of 250 is more appropriate for your taper current - the guideline is 15% more current than your taper current to make sure that the gauge detects charge termination reliably).

    Also verify that the discharge/charge and quit current thresholds are compatible with your application. The gauge must be in the correct state (charge/discharge/relax) for this to work.