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BQ27426: Question about OCV

user4369354
Expert 1220 points
Part Number: BQ27426
Other Parts Discussed in Thread: GPCCHEM

Hi

Please confirm about following question.

(Q1)
The document(slyt321) states that OCV will be measured when power is inserted.
Does this mean that OCV is measured after power is supplied to the IC?
In that case, the OCV can not be measured correctly because the voltage value is not in the no-load state?

(Q2)
The document(slyt321) stated that the value of Qmax will be updated by the second measurement of OCV.
Since the OCV is not measured unless the system enters the relax time, does the error of Qmax and SOC in the interval remain?
Or is the Qmax value updated during charging / discharging?

(Q3)
Does the IC have an OCV profile for each temperature?
Or is there an equation for calculating OCV that corrects the influence of temperature?

(Q4)
Does relax time mean the state where the voltage becomes constant with the load current I = 0?
In that case, since the current does not flow through the IC, the IC may not start up.
Or can it transition to relax time if the internal resistance is negligible micro current?
However, depending on the product, if the load current is always large, it is impossible to estimate the remaining amount because it can not transit to relax time?

Best regards,
Chiaki Endo

  • 0
    TI__Guru* 76230 points
    HI Chiaki
    1. Yes, upon battery insertion an OCV measurement will be taken. The battery should not be loaded otherwise the OCV measurement taken will have errors.
    2. Qmax needs two OCV (DOD) measurements taken at relaxation points and there must be atleast a 37% of design capacity passed charge interval for a qmax update to occur. This can occur either in the charging or discharging direction.
    3. It uses an equation to calculate the OCV table for different temperatures.
    4. The relax time is the time below the quit current value which you program in the gauge. That value should be less than or equal to c/20. As long as the current in low power state of the application is sufficiently small, that should be good enough for the gauge to take an OCV measurement.

    thanks
    Onyx
  • 0 in reply to Onyx Ahiakwo
    Expert 1220 points


    Hi Onyx
    Thank you for your reply.
    Please confirm additional following question.

    1:Does the first measurement of OCV be measured at the time of battery insertion and not when the system power supply is turned on?

    2:Is there a recognition that the termination voltage is set automatically if securing the necessary capacity for shutting down the system?
    Also, in that case, at what timing do you decide the end voltage?

    3:For example if we set Design capacity = 1000 mAh and we insert a 500 mAh battery and discharge,  will it discharge in poor precision unless it enters relax time?
    Because Qmax and SOC values are not updated.
    Also, what kind of problems will occur in this example?
    If this erroneous remaining amount estimation continues, will the capacity of the shutdown be inadequate and suddenly the system power supply go down.

    Best regards,
    Chiaki Endo

  • 0 in reply to user4369354
    TI__Guru* 76230 points
    Hi Chiaki
    1.OCV measurement is taken only for the battery so it is only when the battery is inserted that it is taken.
    2. Terminate voltage is not set automatically. you set that your self.
    3. Your SOC will be skewed as qmax and resistance willl have not updated. Also the gauge scales the resistance table using the design capacity so you cannot have a 1000mAh and insert a 500mAh cell

    You shouldn't do what you describe. The error will be terribe.

    thanks
    Onyx
  • 0 in reply to Onyx Ahiakwo
    Expert 1220 points
    Hi Onyx

    Please confirm following question.

    1:Is the first measurement of OCV the timing of inserting the battery (at this time the system is not running)?
    2:Is the condition of updating the OCV after the second time correct in the following three conditions?
    Also, are the following conditions a relax time transition condition?
    (1) I≦ C/20
    (2) I < "Dsg Current Threshold & Chg Current Threshold"
    (3) dV/dt < 4μV/s

    3:If the reserve capacity is set, will the shutdown voltage be updated automatically during discharge?
    * BTW, does the shutdown voltage match with the understanding that it is the same as the terminate voltage?

    4:For Question 3 I showed an extreme example.
    However, since the capacity of the deteriorated battery decreases, it is necessary to update Qmax.
    However, if discharging is started immediately after the system is started from battery insertion, will Qmax not be updated (does not shift to relax time), so SOC will be abnormal?

    Thanks
    Chiaki
  • 0 in reply to user4369354
    TI__Guru* 76230 points
    Hi Chiaki
    1. yes, An OCV measurement is taken and a rem cap simulation is run for the gauge to determine the SOC of the newly inserted battery.

    2. The gauge will take OCV measurements when in relax mode. This mode is determined by the quit mode. Whether or not that OCV taken is used to calculated a DOD0 which is needed for qmax updates is dependent on if the dv/dt condition is met. If is then that DOD0 will be stored. If it isn't then the gauge will keep taking OCV measurements and keep checking for to see if the dv/dt condition is met to get a DOD0 measurement for qmax.

    3. Shut down voltage is not terminate voltage. Shutdown is the voltage at which the chip will power off. Terminate voltage is the voltage that the chip will report 0% SOC if reserve capacity is not used. if reserve capacity is used, then the gauge will report 0% at a voltage higher than terminate.
    4. qmax is not something that needs to be updated at every ccycle. Not that it takes about 100 cycles for the capacity of a lithium ion battery to degrade by 5%. As long as you have leanred qmax during the golden file generation, SOC will be accurate during initial battery insertion as long as there is no current flow and then during usage when your units would be in relax mode, you will get DOD0 updates that would allow qmax updates. If there was current flow of your load or charger during battery insertion, then the gauge will resume accuracy only after charging to full or discharging to empty since the current flow will prevent accurate OCV measurement.

    thanks
    Onyx
  • 0 in reply to Onyx Ahiakwo
    Expert 1220 points
    Hi Onyx

    Thank you for your reply.
    Please confirm following questions (sorry for many questions).

    (Q1)
    Even with the same maximum charging voltage, if the chemical composition of the battery is different, will the discharge curve of OCV change?
    (Q2)
    When using BQ 27426, select the chemistry ID at the maximum charging voltage of the battery.
    However, in the case of a battery with a different chemical composition even at the same maximum charging voltage, is not there a possibility that the discharge curve deviates largely from the OCV profile provided by the chemistry ID?
    In that case, what is the error rate compared with the chemistry ID?
    (Q3)
    Which three chemical IDs provided by BQ27426 modeled batteries of which chemical composition?
    Regarding chemistry ID=1202 in particular, please tell us what scientific parts it is composed of.
    Because it is necessary for inquiring battery manufacturer.
    (Q4)
    When the battery is near the terminate voltage, what problem will occur when the system goes into high load mode and the load current sharply increases?
    Does a problem such as a sudden shutdown occur?
    (Q5)
    Although the temperature is measured inside the IC, is there an error in the measurement of the temperature when there is a component with high temperature near the IC?
    What is the maximum error rate in this case?
    (Q6)
    What is the error rate at the time of measurement?
    What is the maximum error rate when measuring error accumulates?
    (Q7)
    Please tell me the maximum error rate, minimum error rate, average error rate of "Impedance track method" and "BQ27426"
    (Q8)
    What is the accuracy right after inserting the battery?
    Also, what is the accuracy when empty batteries are inserted?
    (Q9)
    Updating of Qmax is conditional on “|SOC1-SOC2|” being 37% or more, but when “|SOC1-SOC2|” is less than 37% and the value of Qmax is not updated, what is the maximum error rate?

    Thanks
    Chiaki
  • 0 in reply to user4369354
    TI__Guru* 76230 points
    1. This gauge comes preprogrammed with three default chem ids. The OCV is fixed and doesn't get updated. You use our gpcchem tool to determine if the default chem ids are a match for your cell. the fact that the cell has the same max charging voltage a s the default chem doesn't mean that OCV for the default chem id would be a match for your cell.
    2. use gpcchem to determine the error and how cose the default chem id is to your cells.
    3. Use our tool. The chem composition is of no consequence. That can't be used. gpcchem allows you determine the error.
    4. No. Why will a shutdown occur?
    5. if you use the internal temp sense. it will measure the chip temperature which will be impacted by the components are it. IT will not exactly be the battery temperature.
    6. This is dependent on several factors like board parasitic, temperature, etc. There is no one way to quantify. you have to determine the error for your system.
    7. N/A. See 6 above. You can get up to 1% accuracy error if the chem id is an exact close match
    8. After inserting the battery, it depends on what part of the OCV curve you are located. As long as there is no current flow and if the chem id is a good match for cells and if the battery is relaxed. If you are not in the flat zone of the cell, the OCV taken would be ge good.
    9. It is not possible to quantify. It will depend on your sysstem

    thanks
    Onyx