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RID on bq27501

Other Parts Discussed in Thread: BQ27501, TEST


 We are implementing your bq27501 battery monitor. We are having problems with the RID function... The monitor does not seem to react to the change in RID resistor. Could you please explain when this test is done and how the test is performed. ie.... Is it done only apon battery insertion or a reset? Or is there a continuous monitoring of the RID input?

How can we test whether the test is being performed or not? ie. If we scope the RID input pin... What values should we be reading on the scope and when will these values occur?

The pull up resistors whithin the ic.... if we have a 2 batterey pack types... one with a RID value of 220KOhms and one with 110KOhms, How sould the pullups be configured... weak, strong or NO pullup

We would appreciate any help you could offer

Best regards Roger Yttervik

  • Roger,

    Generally, you configure the RID selection as a part of the bqEASY flow.  This programs following Data Flash Locations:

    Pack 0 Voltage, Pack 1 Voltage

    Pack V% Range 0, Pack V% Range 1

    RID Pull-up Selection


    These values are described in the bq27501 datasheet. 

    Basically, on battery insertion detection or on the gauge reset, the gauge will look at the voltage at the RID pin to determine if RID is for cell 0 or cell 1.

    Although these values are configured in bqEASY, the default values discern between RID's of 8K and 500Ohm. 

    One thing to note: the measured voltages at the RID tend to have a fairly large range of error, so the Pack V% range value is critical.  If the measured voltage doesn't fall in the range defined by these parameters, then the RID detection does not work (the pack is determined as "unrecognized").  This is generally the problem that we observe.  In this case, we recommend changing the Pack V% Range value from the default 20% up to 50%.

    Let me know if this helps,


  • Thanks also for this reply Charles :-)

    I have also today experimented using various RID resistors. I managed to get the system working by changing from the 2 resistors (220K and 110K) to 7,5K and 500 Ohms.

    Is there any limit to what values one can use for the RID's? We have already produced and mounted quite a large number of these boards in our product and it would be a great help if we did not have to dissasemble our products to change these resistors.

    If this is not possible then atleast we do know how to get it to work:-)


    Thanks again

    Roger Yttervik


  • Roger,

    I haven't actually seen the output stage of the RID driver, but based on the example given in the datasheet, I'm pretty sure that the RID source is a current source with roughly 0.13mA.  My guess is that the output impedance isn't very high, so we don't give an exact formula.  With higher impedances like yours, It is a safe assumption that the output stage won't be in perfect saturation.

    As a result, I'd recommend that you just probe the voltage at the RID pin to determine the actual value.  Then, you can pick appropriate voltage ranges for the gauge to look for.  If possible, I'd do this for a few different gauges so you can make sure that the measurement is consistent.  I'm not sure how tightly the above source is specified.