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Issues using BQ27501

Other Parts Discussed in Thread: BQ27501, BQ27510



I have implemented a BQ27501 to monitor a multi pack Li-poly battery. Based on SLUA496, an opamp scales down the battery voltage and supplies the BAT pin. The #BAT_GD pin is never pulled to GND, and is always floating on application of power to the system. I have a FET that is supposed to turn the power system of the PCB on, when the #BAT_GD pin is pulled low. Since the pin is not pulled low, the PCB does not power on. Note that the BQ27501 is supplied by a 2.5V supply that is directly regulated from the battery pack, and therefore the BQ27501 is powered on.

I also purchased the EV2300 module and the I2C lines from the BQ27501 on the target PCB is connected to the module. The module is not recognized by any of the software I have downloaded from TI's website. The drivers are installed correctly and windows recognizes the EV2300 correctly.

My questions are,

1. Do you have suggestions as to why the #BAT_GD pin is not pulled low?

2. Do I need to calibrate BQ27501 or change any parameters?

3. What software do I use with the EV2300 to communicate with the BQ27501 on my target PCB?

4. If I do need to calibrate or change any parameters, can I do that with the EV2300 and some software?

5. How can I use the EV2300 to verify that the implementation of the BQ27501 is correct?




  • Hi,


    I have been able to answer the following questions, so I wanted to update this post.

    I have identified the bqEasy software to be used with the EV2300 and I am able to communicate over I2C to the target device on the custom PCB. I am able to access registers and modify them.

    I also flashed the default .dfi file onto the target device. However, the #BAT_GD pin continues to float and does not seem to activate. I do not see any hooks to modify any registers that may affect the pin.


    Any suggestions on things to look for that would explain why this pin continues to float? Keep in mind that I am unable to apply system power using the FET controlled by the #BAT_GD, so I am hesitant to go through the calibration procedures before ensuring that the implementation is correct.


    Any ideas are welcome. Thank you.

  • hi Girish,

    if you have external pull up on /BAT_GD pin, once the gauge go through initialization process, it will pull down the /BAT_GD pin (after 2-3sec). you can chnage /BTA_GD polarity in op config register.




  • Ming,


    I have a 10K external pull up on the #BAT_GD pin to 2.5VDC. I also have a 1.8Mohm pull up on the BI/TOUT pin to 2.5VDC. I also have a 33nF capacitor from the BI/TOUT pin to GND as per the reference schematics in the datasheet. I do NOT have an external thermistor and also do not have a battery pack with resistor IDs in them. So, the RID pin and the TS pin are left unconnected. I am using a TLV2401CDR opamp to buffer the scaled down battery voltage to the BAT pin. I am using a 3S2P 4000mAh custom LiPo battery pack. The battery pack is made of three (3) 3.1V cells in series capable of 2000mAh to form the 3S part, and then two of these 3S's are combined in parallel to create a 3S2P 4000mAh battery pack. The only inputs I have to the BQ27501 are the Pack+ and the Pack-. I have attached the battery monitor schematics to this post. Please note the pull up on the #BAT_GD pin is NOT shown on this schematic.

    Now, with all the default settings, when the battery is inserted to the system and the [TEMPS} bit changed to 0 to use the internal temp sensor, the BI/TOUT pin starts at 2.5VDC and then falls down to 2.1VDC and then periodically jumps up to the 2.5VDC and then falls back to 2.1VDC. This seems to be consistent with the datasheet that says the this pin is polled every second. However, the BI/TOUT pin is NEVER pulled to GND.

    Meanwhile, the #BAT_GD pin (with default settings of BQ27501) stays at 2.5VDC. It never changes state.

    However, when I change the PFC_CFG1/PFC_CFG0 to 0/0 from the default 0/1, the #BAT_GD pin jumps down to 1.9VDC, when the battery is inserted. After I changed the PFC once, now even when I change it back to 0/1 (the default), it stays at 1.9VDC.

    I am not sure what else to try at this point, so any help is appreciated!

    Thank you.

  • BI/TOUT pin connection should follow the ref design. If no external thermistor  is used, connect TS pin through 10k resistor to the ground for battery detection purpose. BAT_GD will not go low without BI detcted.



  • Ming,

    Were you able to verify that the schematic is as expected?

    BI/TOUT is connected as per the reference schematic with the 1.8Mohm pull up resistor and 33nF capacitor  to GND. Is there anything else that needs to be connected to that pin?

    On connecting the TS pin through a 11kohm resistor to GND, the temperature being read by the BQ27501 is correct and I can verify that now through the bqEasy software. Previously, the temperature being reported by the bqEasy SW was not correct.

    However, the #BAT_GD pin remains at 1.9VDC and does not go below that. I have played with setting the PFC to 0/0 and 0/1 and it does not change the behavior of the #BAT_GD pin.


    Any further ideas?


  • if ref design is followed, you should also have 18.2k connected from TS to BI pin and TS is also connected to ground through 10k. Is this correct? if you don't have 18.2k then BI pin will never be pulled to low when battery is inserted so BI is not detected then BAT_GD will not function.



  • Ming,


    Connecting a 20Kohm resistor from TS to BI/OUT pin solved the problem. The #BAT_GD pin is pulled low and my power supply turns on.Thank you for your inputs.


    However, the datasheet and the app notes that I have for the BQ27501 DO NOT STATE that TS pin is to be pullled low through a resistor to use the internal temperature sensor. In fact, from section 5.4 page 24 of datasheet -  "Alternatively, it can also measure internal temperature via its on-chip temperature sensor, but only if the [TEMPS] bit of the Operation Configuration register is cleared." This is the ONLY mention about using the internal temperature sensor in the datasheet.

    Also, there is NO MENTION of using the 18.2Kohm resistor between TS to BI pin when using the INTERNAL TEMP sensor, anywhere on the datasheet and app not. Again, from section 5.4 and section 4 of datasheet - "The bq27501 can also be configured to use its internal temperature sensor. When an external thermistor is used, a 18.2k pull up resistor between BT/TOUT and TS pins is also required." This clearly states that the resistor is required only for the external thermistor and not for internal temperature sensor.

    Am I missing something? I believe the datasheet needs to updated to correct the above two statements.


    Thank you.


  • i am glad the issue is solved. Yes, the DS did not specify the 18.2k for internal temp sensor. The battery detection function is needed for both cases.


    I will contact the person to get it updated.



  • Hello,

    Just one question: When I use a BQ27510 without external sensor and an non-removable batterie do I need to pull up the TS-pin or can I leave it just open ? I pulled down the BI/TOUT-Pin with a 10kOhm resistor. I attached my schematic here:


    What does the WAIT_ID mean in the Flags register?

    Thanks, Greatings,



  • Matthias,


    I faced a similar problem recently and even though the datasheet does not specify this, you need to ground the TS pin through a pull-down resistor - I used 10kohm - for the BQ27501 to recognize that you want it to use the internal temperature sensor. Without doing this, even if you clear the [TEMPS] bit in the Operation Config register - what the datasheet asks you to do - the BQ27501 still tries to use an external temperature sensor and since the line is floating will read garbage.

    You need to have a 1.8Mohm pull up to 2.5VDCon the BI/TOUT along with the 33nF capacitor to GND as per the reference schematic. In addition, even though the datasheet specifically states that you need to have a 18.2Kohm resistor between BI/TOUT and TS pin, you still need to have that resistor even if you are using the internal temperature sensor.

    These are two issues I found after a few days of debugging due to datasheet inconsistencies.


    Hope this helps.


  • Hi Girish,

    thanks for your remarks. The temperature measurement works well even in my configuration with the TS pin open. The "only" problem I have is that fuel gauging does not work well. Temperature measurement works well with the integrated sensor. The fuel gauge reports 10 % at the end of a discharge cycle for the last 1/3rd of the cycle until the battery shuts down (at 3.0V) :-(.

    I dont know if this is related.

    You mean I need always a schematic like this:



    I hope I will get an answer from TI.