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BQ25708: Behavior when BMS protection circuit opens charge path

Mike Palmer1
Prodigy 40 points
Part Number: BQ25708
Other Parts Discussed in Thread: BQ40Z50-R1

We are seeing odd behavior with battery discharge occurring when AC power is present and the battery BMS has opened the charge path.

Our 4S2P pack is protected by a built-in BMS that includes a charge FET that opens when the pack temperature goes below 10oC or above 45oC.

Our testing has shown odd behaviors at cold pack temperatures (<10oC):

a) Pack state of charge ~20%

- when AC Power is connected, we are notified by the system that the power status is "AC Power, Discharging"

- battery SoC will continue to drop until is reaches a reported ~4% at which point the system switches to "AC Power"

b) Pack state of charge ~8%

- when AC Power is connected, we are notified by the system that the power status is "AC Power, Discharging"

- battery SoC will continue to drop until is reaches a reported 0% at which point the system switches to "AC Power"

Our expectation is that under normal conditions when AC power is connected the system would always run on AC power and top-up the battery. While we understand that charge top-ups are not possible if the BMS CHG FET is open, we don't expect nor understand why the battery is discharging at this point.

What does the bq25708 do in response to the condition when a battery is present but the charge path is open?

Why is the bq25708 apparently not producing a VSYS equal to or greater (by 160mV) the battery voltage to prevent discharge when running on AC power? What role does the open charge FET play here?

  • 0
    TI__Expert 3320 points

    Hi Mike,

        You understanding is correct, when charge is disabled VSYS should be regulated at 160mV greator than battery voltage if minimum system voltage setting is smaller than battery voltage. In order to figure the root cause:

        1) I would like to know what is the minimum system voltage setting and  battery voltage under above scenario.

        2) Do you disable charge for above test?

        3) How much system load and is there any possibility that VINDPM or IINDPM are triggered for this test? I would like to know the charger status bits value during those weird scenarios. REG0x20 read back is helpful for our analysis.

    Regards

  • 0 in reply to Shishuo Zhao
    Prodigy 40 points

    Hi.

    1) Our MSV is 12.288V. The battery terminal voltage may be above or below this value.

    2) The battery protector in the pack (BQ40Z50-R1) is set to open the charge FET when the pack temperature is 10oC or less or 45oC or above. The scenario where we see this "AC Power, Discharging" problem occurs at pack temperatures low enough to have opened the charge FET. So, 'yes', charge is disabled.

    3) The problem appears to occur even at low system load (AC adapter current <1.5A; battery current -1A.) AC adapter voltage is ~19.8Vdc. We observed REG0x20 go from 0xA400 to 0xA000 during this abnormal condition.

    Because this only appears to occur when the battery pack is at a temperature where its charge FET is open we suspect the problem is related to how the bq25708 reacts to there being a battery present and its voltage indicating it needs charging but there being no charge current flow detected.

    Thanks.

  • 0 in reply to Mike Palmer1
    TI__Expert 3320 points

    Hi There,

         Thanks for your update.

         Just want to clarify that the disable charge battery FETs I am referring to is external battery FETs not battery protector FETs inside battery pack.  From REG0x20=A400H, seems that the charger is still in fast charge status, I am suspecting maybe charge is enabled. Please confirm that charge current is set to 0A and there is also dedicated charge disable bit REG0x12[0] to inhibit charge you can use to make sure charge is disabled.

         Please also need to provide battery voltage and system voltage during low temperature battery discharge process.

    Regards

  • 0 in reply to Shishuo Zhao
    Prodigy 40 points

    Hi,

    The FET that is opening is indeed the protection FET inside the battery pack. This happens because the temperature range over which we allow charging is 10oC to 45oC; when the pack temperature falls below 10oC the charge FET within the pack opens as part of the pack protection. This was kind of the reason for me asking: What is the behavior of the bq25708 when it is charging and the charge circuit is opened?

    Are you saying that our system needs to monitor the state of the protections in the pack and command the bq25708?

    So for example, if we read the ManufacturerAccess() 0x0051 SafetyStatus register in the pack controller and see, for example, that bit 26 ("Undertemperature During Charge") is showing '1' (detected) then we should write to the bq25708 reg0x12[0] to '1' to inhibit charging? If this is the general idea can you offer specific registers and bits from the pack we should be monitoring and the registers and bits in the 25708 we should be writing in response?

    The pack is a 4S2P so fully-charged is ~16.8V. During these episodes the pack voltage is nearer 12.3 (may be above or below, depending on the SoC; generally speaking we are seeing this at SoCs iof 25% or lower.)

    Thank you.

  • 0 in reply to Mike Palmer1
    TI__Expert 3320 points

    Hi Mike,

        When the internal FET is open, the charger will only charge the battery capacitor to the target charge voltage.Note the charger could not terminate by itself, host has to send terminating command to the charger to terminate.

        Your understanding is correct, but instead of battery pack you could just monitor our charger status to meet below two criteria to terminate charge.

         1,VBAT ADC(REG0x 26h[7:0]) reach maximum charge voltage setting REG0x15[]

         2,IBAT ADC[REG0x24h[15:8]] is reduced to below 10% of charge current setting REG0x14[]

    Regards