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BQ21040: BQ21040 cannot charge Lithium Polymer with Protection Circuit

Leo Marrero89
Prodigy 70 points
Part Number: BQ21040

Hi,

We are using the BQ21040 with a Lithium Polymer cell that has a protection circuit already installed. I am currently testing performance at low voltage situations. Battery and charging circuit is connected to a constant 20mA load.

When the Lithium Polymer cell drops below 3V, it seems that its protection circuit disconnects (Open Circuit) for under voltage safety state. I try to charge with BQ21040 circuit by connecting external 5V but it seems that it cannnot charge battery any more. It cannot exit the safety state no matter how long the BQ21040 with external 5V circuit is connected.

To recharge my test battery, I disconnect from the test circuit and connect directly to a power supply (4.2V, current limit: 100mA), it seems to recharge no problem

Does the BQ21040 work well with existing protection circuits or must it connect directly to Lithium polymer tabs? I saw in the datasheet that the BQ21040 has a per-conditioning state, does this help get the protection circuit out of this safety state?

Thanks,

  • 0
    TI__Mastermind 22150 points

    Hi Leo,

    Thanks for reaching out on E2E. 

    Is the image you attempted to attach a schematic? Can you please try and post that again? 

    Is there an input current threshold mentioned on the protection circuit? The pre-conditioning state charges the battery at 20% of the programmed charge current. With a maximum current of 0.8A, this should be fine if you see 100mA working with an external power supply to charge. 

    What is your programmed charge current? If it is 100mA like what you've set on the power supply, then the pre-charge current is 20mA. 

    Are you able to measure the battery voltage with the protection circuit in the open circuit state?

  • 0 in reply to Anthony P Pham
    Prodigy 70 points

    Hi Anthony,

    Thank you for your response. I attachment was a photo of the battery. Attached again here for reference. Battery is 400mAh with max charge rate of 1C.

    Our current design is conservative and charge current set to Riset = 2.4Kohm so Charge Current is ~225mA, pre-charge current should be 45mA. I am able to measure cell voltage before the protection circuit and the protection circuit open circuit state triggers at around 2.6V. Before this state, I can connect the BQ21040 charge circuit and charges the battery no problem.

    I was checking with my multimeter again today and noticed that my crappy power supply delivers a 250mA "inrush" current to the battery before tapering off to the desired current limit I set. It seems to open the protection circuit and allow to charge. I am not sure if this is standard behaviour but it will cause problems in production as our products are sealed and replacing battery is very hard.

    I am considering bypassing the protection circuit to connect the BQ21040 directly to the battery tabs. Would this be advisable?

  • 0 in reply to Leo Marrero89
    TI__Mastermind 22150 points

    Hi Leo,

    Thanks for the battery and test information. Was the protection circuit part of the reason for choosing the battery?

    The BQ21040 has the following protections:

    • 6.6-V input overvoltage protection
    • Thermal shutdown protection
    • OUT short-circuit protection and ISET short detection
    • Overtemperature sensing protection through an NTC
    • Fixed 10-hour safety timer

    If this is satisfactory for the purpose and concerns of your project, then it is fine. If there are concerns that are not listed, we can try and look for other devices within the portfolio that better fit your needs.

  • 0 in reply to Anthony P Pham
    Prodigy 70 points

    Hi Anthony,

    The protection circuit comes with the battery which was purchased off-the-shelf. It is similar to this one: https://www.adafruit.com/product/258

    learn.adafruit.com/.../protection-circuitry

    Noted the safety specs as well and I believe that they are sufficient. I guess the main concern for now is this: Is the BQ21040 intended to connect directly to the battery? By this I mean connect directly to the tabs of the battery.

    Also, how should it be connected in correlation to protection circuitry? A or B (in the image below)? or something else?

    Thanks,

  • 0 in reply to Leo Marrero89
    TI__Mastermind 22150 points

    Hi Leo,

    The BQ21040 already covers most of the bullets listed in the second adafruit link regarding protection circuitry:

    • Regulation voltage is 4.2
    • If the battery is below 3.0V, the charger will apply a pre-charge current (preconditioning phase that we've discussed). This feature is to safely recharge batteries heavily depleted in voltage.
    • You've already mentioned that you have a 20mA load, and so this is well below the 1-2C range
    • The charge limit is below 1C
    • With the TS pin and an NTC to monitor battery temperature, the charge will only charge the device between 0-45C

    You can connect the BQ21040 directly to the battery.

  • 0 in reply to Anthony P Pham
    Prodigy 70 points

    Hi Anthony,

    I connected the charging circuit to the battery as advised and tests so far show that is working as expected, even for heavily depleted cells. I will update our designs for future work.

    Thanks,

    Leo