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BQ21040: Fails to restart discharged LiPoly

Part Number: BQ21040

I am developing a product powered by a LiPoly battery. If the battery is allowed to discharge too far, the protection circuit turns off access to the battery (@2.5V). It takes raising the battery terminals to >3.4V for the protection circuit to turn the battery back on and allow charging. The product has a small micro/radio/audio circuitry powered by the battery which draws no more than about 20mA during normal operation.

The BQ21040 is programmed to charge at 75mA. There is a load switch on the product that is enabled by the charger voltage when the product is plugged into the charger. This switch prevents the battery voltage in the product from being persistently applied to the battery charge terminal.

My problem seems to be that when the battery protection circuit is opened up and I plug the product into the charger, the charger can not/does not bring the battery terminal up enough in voltage to enable the connection in the protection circuit and thereby allow the battery to begin charging. About every 3 seconds, it looks like the BQ21040 tries 8 times in rapid succession to start up the battery charging.

Any suggestions? Is there something else you need to know in order to formulate a solution?

  • Hi John,

    Welcome to E2E! 

    This could possibly be that the BQ21040 is not providing enough current to close the protection circuit when the battery is discharged to voltages below 2.5V. Below this battery voltage, the BQ21040 is providing pre-charge current to precondition the battery. This would be 20% of the programmed current and so is 15mA. 

    Would it be possible to increase the charge current? 

  • Hi Anthony,

    Could be I quess. The testing I did with just the battery seemed to indicate that the battery "turn back on" was merely a function of the voltage, but I will test and see. The battery is only a 75mAHr battery so for the moment I am charging it during fast charge at 1C, hence the 75mA current limit.

    As I mentioned there is a circuit in parallel with the battery and I have been doing testing with just a battery and just the circuit, inserting each into the charging station. When just the circuit is being tested, most of the time the voltage on the power rail (VBAT) connects to the charger and follows the charger voltage. Occasionally, however, the VBAT rail does not follow the normal charger voltage but is "dragged down" to somewhere near 2 to 2.5V. This seems to happen more frequently when I put the two together. It never happens when I just try the battery.

    Tomorrow I will try a higher current.

  • Hi John,

    This could be a case where your circuit is pulling much more current than the charger is able to provide, causing the charger voltage to drop and can certainly be the case if the battery is also sharing the output current of the battery. In this case, a higher current would also most likely help solve the issue.

    Looking forward to hearing your results!

  • Hi all. Thanks very much for taking the time to review and respond to my inquiry.

    I did a divide and conquer approach. I started by trying to install just the battery into the charger and the battery always came out of protected mode. That is, the voltage always got up to the 3.4V and the battery woke up. Next I tried powering the circuit that normally parallels the battery, but without the battery, and it got locked up like before so I know it wasn't the battery causing my problems. After several other divide and conquer steps, I was led to the input switching supply that takes the battery and generates the system 3.3V. It appears that when voltage is applied by inserting it into the charger, the power supply begins to operate. There is no delay as the EN pin is tied directly to the battery voltage VBAT. I put a delay (RC) on the EN pin and now I have no problems with the battery startup.

    Consider this resolved, and thanks again for your help.

    Cheers,