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BQ21040: 10 hour cut-off timer and load

Mike Y
Prodigy 40 points
Part Number: BQ21040
Other Parts Discussed in Thread: BQ24040

Hello Everyone,

I am using BQ21040DBVR for a project. Here are my connections:

TS (1)- 10k resistor - ground

CHG (3) - Not connected

VOUT (2) -  Load and battery

GND (5) - GND

ISET (4)- 680 ohm resistor - ground

Vin  (6) -  5V

Everything seems to work fine except for the safety timer. I am purposely draining my battery with my system load of around 1 milli amp, assuming the 10 hour safety timer kicks in the battery should be discharged to 4.0V in 7 days.  However, after several days, the battery voltage seemed to go up and down between 4.14V and 4.18V, which leads to my suspicion that the chip is still charging the battery. Would you be able to tell me what I am doing wrong? Does disturbance on the load cause the charger to reset the safety timer? I have capacitors on Vin and Vout, however they are not close to the chip itself, is that the cause of the issue?

Thank you 

  • 0
    TI__Guru* 75936 points

    Hello

    Yes charge should stop after 10 hr and a fault will be set, this is a hard fault and will require power cycle to clear.  But it sounds like you are completing charge.

    If Vin remains connected the BQ21040 will start a recharge cycle when the battery voltage drop to about 4.1V. This would appear as an up and down on battery voltage that you are seeing.

    Also see BQ24040 quick start guide, the two devices are very similar and most of the info will apply.

  • 0 in reply to Bill Johns
    Prodigy 40 points

    Hello Bill,

    Thanks for the quick reply,

    I believe I have a pretty good understanding of the IC and its operations. Are my connections, listed from the post before, correct? I am keeping a log and over the course of 3 days at 8 hour intervals, the battery voltage went down from 4.16V to 4.12V. However,  the voltage came back up to 4.14V during my reading. The IC is circuit is connected to a boost/buck converter, which is connected to a power supply, power was never cycled. The battery has internal protection circuit. Does that change anything?

  • 0 in reply to Mike Y
    TI__Guru* 75936 points

    Hello

    "Are my connections, listed from the post before, correct?" -- Bill J -Yes they are correct.

    TS (1)- 10k resistor - ground  

    CHG (3) - Not connected

    VOUT (2) -  Load and battery

    GND (5) - GND

    ISET (4)- 680 ohm resistor - ground

    Vin  (6) -  5V

    "The IC is circuit is connected to a boost/buck converter, which is connected to a power supply, power was never cycled." -- Bill J -- With power applied to the BQ21040 an a small load on the battery the voltage can be expected to move up and down between 4.16V and 4.12V.  Charge will restart at 4.1V and increase battery voltage to 4.2V then stop.  Load will draw battery voltage down to 4.1V and charge will restart.  Some variation in the exact values can be expected.

  • 0 in reply to Bill Johns
    TI__Guru* 75936 points

    Hello

    A little more info

    The charge current is set by "ISET (4)- 680 ohm resistor - ground" -- The Constant Current (CC)charge will be 800mA.

    Termination current is fixed at 10% of the CC or 80mA.

    Termination will occur when battery voltage has increased to 4.2V and charge current has dropped to 80mA.  This must happen before the 10hr timer expires or a hard fault is set. 

    After termination the battery voltage is monitored and when is drops to 4.1V then a recharge cycle is started.

    A good point to monitor is the ISET pin it will indicate current going to the battery, sample of output current for current regulation.

  • 0 in reply to Bill Johns
    Prodigy 40 points

    Bill,

    Thanks for the reply, everything you noted make sense except the charging is happening well after 10 hours after the initial power cycle on. The charger IC is still charging after it's plugged in for 50+ hours. I am under the impression the 10 hour timer will inhibit any charging after 10 hours, unless the charger supply  power is cycled, which it never did. Can you think of a reason why the chip is still charging/ providing power after 10 hours?

  • 0 in reply to Mike Y
    TI__Guru* 75936 points

    Hi

    How do you detect charge complete?

    You have no connections to the CHG pin that would indicate charge state for first cycle.

    Do you have indications that charge is not completing in 10 hours?   If input current goes to 0mA or voltage at ISET pin goes to 0V this would indicate charge has completed.