data/text/DLP2/890598

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

Part Number: BQ27441-G1 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?

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

Hi Andy, Thank you for the reply. I am attaching the report with this mail please have a look4380.GPC_report.txt

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?

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?

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?

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.

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?

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 <secret URL>/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.