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BQ24171: Skipping precharge phase. Charge starts at Iset, not Iset/10 as expected.

KXCO
Prodigy 80 points
Part Number: BQ24171

Hello,

I am using BQ24171RGYT with a pack of 18650 LiPo cells in 3P2S configuration (7.2V).

Problem:  Precharge phase seems to get skipped.  Charge starts are programmed current of 4A.  Datasheet says precharge should limit initial current to Icharge/10 = 0.4A for 30 minutes but that does not seem to happen.

Test Steps: This happens on at least 2 sets of professionally built HW.  Plug in battery, plug in external power supply, measure battery current = 4A.

HW:

R1 = 300KΩ

R2 = 95.3KΩ

Rsr = 0.01Ω

Calculations:

Viset_calculated = Vref (R2 / (R1 + R2) = 3.3 (95300 / (95300 + 300000)) = 0.80V

Viset_measured = 0.78V.

Icharge = Viset / (20 x R_sr) = 0.78 / (20 x 0.01) = 4A

Results:

Since Viset_measured is similar to calculated value, I assume my divider resistors are correct.  Not sure what else to check.

Question:

Has anyone experienced this?  Any idea what could cause it?

Thanks for any help.

  • 0
    TI__Expert 8690 points
    Hi,

    What is the voltage at FB before plugging the external power supply and immediately after plugging the power supply?

    FB voltage is what determines the precharge state or fast charge state. FB lower than 1.45V is precharge and higher than 1.45V is fast charge.

    Regards,
    Steven
  • 0 in reply to Steven Bangdiwala
    Prodigy 80 points
    Thank you Steven. I think that solves it. FB will rarely if ever be below 1.45V in our application.

    FB will be 1.45V when Vbat is 5.8V. UVLO in battery's protection module is 5.4V. A battery that is 5.4V while loaded is probably over 5.8V while not loaded, just before plugging the external power supply and almost definitely over 5.8V just after plugging the power supply. So that probably explains why it skips precharge even when it was drained until UVLO tripped.

    Is it hard on the batteries to always jump straight to fast charge with no precharge?
  • 0 in reply to KXCO
    Prodigy 80 points
    Just to clarify. When I drained it to 5.4V it was with a 4A load which I reduced to 20mA load before plugging external power supply.
  • 0 in reply to KXCO
    TI__Expert 8690 points
    You also need to be careful of the line impedance from the charger to the battery. If there is too much impedance, you may jump from precharge to fastcharge quickly if you apply a high charging current. The same thing will happen if you apply a high load current to the battery, it will fall to a point where it may enter precharge.

    Definitely you will need to be in precharge if the battery is deeply depleted, to protect the battery from damaging.

    Regards,
    Steven
  • 0 in reply to Steven Bangdiwala
    Prodigy 80 points

    The battery has 2 x 10cm 18AWG wires and 2 x 10cm of 1-2cm wide 1oz traces between it and the charger.  Not a lot of impedance but some impedance.

    Can you quantify 'deeply depleted'?  We are using 18650 cells from Panasonic but might switch to cheaper ones later.

    What kind of of battery damage are you talking about'?  Shortened life or something more serious?

  • 0 in reply to KXCO
    TI__Expert 8690 points
    Hi,

    You might want to measure the resistance from the output of the converter to the cell and double check.

    I would look at the cell voltage to see where it is compared to the charger output voltage, then Rpath=(Vfb-Vcell)/Iout. Measure current through the sense resistor, do not use a DMM to measure the current, as it will increase the impedance of the path.

    Deeply depleted is if you are lower than 5.8V, precharge current is used to slowly get the battery back to a normal voltage of 2.9V per cell. Some other chargers have the threshold at 3V per cell, it depends on the design. After it is above 2.9V per cell, then it is good to initiate fast charge. If you charge high current during deeply discharged voltage, it will shorten the battery life and it may overheat.

    Regards,
    Steven
  • 0 in reply to Steven Bangdiwala
    Prodigy 80 points
    Ok, thanks again. I'll test this more when I have a chance. Am currently testing our 2 temperature sensors, and wishing I had a proper temperature chamber.
  • 0 in reply to KXCO
    TI__Expert 8690 points
    Okay, no problem. Glad I could help.

    Regards,
    Steven