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BQ24618: Disable/Override 30-second precharge timer?

Thomas Bouchard
Prodigy 230 points
Part Number: BQ24618
Other Parts Discussed in Thread: BQ25606

Hi, I'm designing a charging circuit for an 11P (11-parallel) battery pack.  I purchased the EVAL board which was originally configured for charging a 5S battery pack (21V battery charge voltage regulation).  So I first built a 5S battery pack and tested the EVAL board to make sure I understood its operation; all ok. 

I then modified the board to reduce the battery charge voltage from 21V to 4.2V, and also set the charge termination current to 550mA (my LG INR18650-MJ1 batteries have max charge voltage of 4.2V and a charge termination voltage of 50mA).  This also sets the pre-charge phase-1 battery charging current to the same 550mA.  These changes are required since the battery pack of 11 batteries in parallel has a voltage of 4.2V and a termination current of 50mA x 11 = 550mA.

I ran testing, and everything worked as I expected except that because the battery pack is 11 parallel batteries, when below the Vlowv precharge (phase-1) to constant-current (phase-2) threshold, which for this configuration is 3.1 volts, the 30-minute precharge timer expires, and then charging terminates (in error condition; STAT1=STAT2=0).  In order to resume charging (Phase-1 precharge), a POR must be done.

My system will terminate battery consumption at 2.5V.  For this charger and an 11P battery pack, it takes about 90 minutes to increase the battery voltage during Phase-1 precharge from 2.5V to 3.1V.  So before the charger gets to the precharge-CC threshold, the 30 minute timer expires and the charger shuts down.

I understand the Constant-Voltage safety timer can be set and disabled.  My question is, what about the 30 minute precharge timer?  Can it be increased to 2-3 hours or disabled entirely?  If not, is there some way I can "trick" the IC into thinking the time is not expired?  Or increase precharge current to different value from termination current (don't think so)?

Or maybe automatically trip a POR if error condition occurs (which in my case is precharge 30-min timer expiring before 3.1V Vlowv threshold is reached).

Ideally, I'd simply like to disable the 30-min precharge timer, or increase its length to 120 minutes or more.  Any way to do this? 

Tom

  • 0
    TI__Mastermind 20570 points
    Yes pulling TTC high or low can disable safety timer this includes the pre-charge timer.
  • 0 in reply to Jing Zou
    Prodigy 230 points
    Thank you for the response. According to the BQ24618 datasheet and the EVAL board manual, setting the TTC pin to HIGH, disables the "timer", but allows "termination". Setting the TTC pin to LOW disables the safety timer and "termination". I had set the TTC pin to HIGH, disabling the timer, but allowing charging to be terminated when charge current hits the current set by ISET2.

    I will of course change TTC from HIGH to LOW and test...but it will take me a little bit to discharge this 11P pack.

    So are you saying that the 30-minute timer is linked to "termination", such that if TTC is pulled to high ("allowing termination"), then the 30-second timer is active during the precharge Phase-1 of the charging cycle...but if TTC is LOW and "termination is disabled", then the 30-second timer for precharge is also disabled?

    It's not clear from any of the documentation when this precharge 30-second timer is active or not.
  • 0 in reply to Thomas Bouchard
    TI__Mastermind 20570 points
    The 30 min timer is the pre charge timer. It is also considered as safety timer. When you pull either TTC high or low should disable this 30 min timer.
  • 0 in reply to Jing Zou
    Prodigy 230 points
    Ok, thank you. However, I had tied the TTC jumper on the EVAL board to "HI", and measured it at 3.3V. So I can tell you for sure that TTC tied to HI does not disable the 30-second precharge timer.

    When the battery pack discharges sufficiently, I will change it to "LO" and let you know how it goes.

    v/r
    Tom
  • 0 in reply to Thomas Bouchard
    TI__Mastermind 20570 points
    Do you mean 30 min instead of 30 second? If it is 30 seconds, then it is not the pre charge timer.
  • 0 in reply to Jing Zou
    Prodigy 230 points
    No, sorry, I mis-spoke. I'm referring to the 30 minute pre charge timer. It is the 30-min pre charge timer that I'm trying to eliminate or extend. My 11P battery pack takes about 90 minutes to reach the Vlowv threshold (3.1V) from 2.5V, and the charger shuts off in an error state (STAT1=STAT2=0) after 30 minutes.

    And to recap; setting TTC to HI (3.3V) did not disable this timer.

    When my battery pack discharges I will change the TTC to LO and test and let you know.

    v/r
    Tom
  • 0 in reply to Thomas Bouchard
    TI__Mastermind 20570 points

    Tom,

    Sorry for the confusion. In the bq246xx, the precharge timer cannot be disabled. That was my mistake.

    Unfortunately, it cannot be extended either, you will need to re-initiate a new charge cycle each time it expires. You can achieved by toggling CE pin.

  • 0 in reply to Jing Zou
    Prodigy 230 points
    Ok, thank you. As suspected, also driving TTC to LO did not disable the 30-min precharge timer.

    Toggling the CE does re-initiate precharge.

    So given that the 30-second precharge timer cannot be disabled, this presents a problem for a battery pack that uses parallel batteries of more than 3 or 4 (precharge timer likely to shut down charging before Vlowv threshold can be achieved).

    Can you recommend an easy way to add a component or two such that when the timer expires the CE can be toggled LOW-HIGH and restart the precharge? Any other suggestion for how I might be able to use this chip without needing manual CE-pin intervention?

    Any other suggestions for how I might charge an 11P pack?

    Tom
  • 0 in reply to Thomas Bouchard
    TI__Mastermind 20570 points
    Unfortunately this is the limitation of this device. You will probably need to use a MCU to toggle the CE pin.
    We do have other chargers like the bq25606 which is a standalone 1s SW mode charger has 2 hours of precharge timer. However, the charge the max charge current is 3A and precharge current is also lower.
  • 0 in reply to Jing Zou
    Prodigy 230 points
    So I implemented a work-around for this.

    I increased the termination current/precharge current 3x and now the charging of the 11P pack reaches Phase-2 CC mode in 23 minutes, 7 minutes prior to the 30-minute error timeout. So the precharge reaching the 30-min error timeout is solved by increasing the termination current (which correspondingly increases the precharge current).

    Of course now, the termination for the battery is 3x higher than what the datasheet states. However, through multiple charge/discharge tests, this method still achieves a charge of 91.5%. Therefore, this will be good enough for my application and is a simple work-around.

    If want to achieve 100% charge, then the idea I came up with was to have the resister-to-ground of the ISET2 charge termination resister divider selectable with an analog switch. The switch will be controlled by a comparitor that compares the battery voltage to the 3.3V reference voltage. This would only add three small components (comparitor, switch and extra resister).

    When the battery voltage is below 3.3V (precharge phase), the ISET2 resister is set such that the termination/precharge current is 3x. When the battery voltage reaches 3.3V, the charger will be in Phase-2 CC rapid-charge mode (my battery regulation voltage is 4.2V, and therefore the charging goes into CC mode at 3.1V), and at this point, the ISET2 resister then changes so that the termination voltage is set back to the lower (correct) termination current, allowing the Phase-3 CV mode to charge the battery pack to 100%.

    This effectively allows the charger to have a precharge current different than the termination current.

    I haven't tested that, since the 91% charge is good enough, but do you foresee any issues doing that?

    v/r
    Tom
  • 0 in reply to Thomas Bouchard
    TI__Mastermind 20570 points

    Hello Tom,

    Regarding on the pre charge current, I thought the original setting is based on your battery spec. That is why I did not suggest you to increase the pre-charge current.

    If the pre-charge current does not exceed the battery spec, then your implementation would work.