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BQ24610 max Ah for battery

Karlis Tucs
Expert 1385 points
Other Parts Discussed in Thread: BQ24610

Hello everybody.

I have an issue with calculating right resistors for setting Vbat,I charge, I precharge/termination currents.


First of all, I am using 57 Ah 4 cell Li - ion battery, which has charging voltage of 16,8V.

This battery can be discharged to 10V.  Precharge current (0,1C) should be 5,7A.. Termination current should be 1,1 A ( from battery datasheet).

 If I have discharged my batteries to 10V, and want to recharge them, then I need to charge them up from 10 V to 12,4 V(LOWV threshold) in 30 minutes or else the charger turns off and a FAULT is indicated on the status pins.

Judging from datasheet this gap between 10 and 12,4V translates into about 12 Ah. It means i need at least 24 A current for 30 minutes.

BQ24610 allows 10A max charging current.

If I lower the LOWV threshold, then Vbat battery charging voltage lowers too, and I cant charge battery to its full capacity.

This seems hopeless, but maybe there is some kind of solution.

Thanks in advance.

  • 0
    TI__Expert 5565 points
    Could you share the datasheet of this battery pack? 12Ah for 10V (deeply discharged) to 12.4V (low battery threshold) sounds rather large. This would imply that ~21% of the battery capacity is below 3.1V/cell of the battery pack.
  • 0 in reply to Smriti B
    Expert 1385 points
    www.batteryspace.com/.../NCR18650B.pdf in 4s17p connection. Maybe I have made a mistake in calculating Ah between 10V and 12.4 V.
  • 0 in reply to Karlis Tucs
    TI__Expert 5565 points

    Based on the limited information on the datasheet and using the discharge curve as an estimate (which does't take into account your battery impedance as your battery voltage changes), I estimate ~17A, which is still above the recommended operation of the bq24610.

    One thing you can do is set your VFB voltage divider for a lower target battery voltage (maybe 15.1V). This will set your BAT LOWV threshold to 11.2V. From my estimate, you need ~6.8A to charge to 11.2V from 10V. Once you reach a certain battery voltage, you can change the VFB resistor divider by switching in a different resistor like below:

    You can use a comparator to monitor the battery voltage, and once the comparator crosses a certain threshold, it can drive the MOSFET to switch in a new resistor to change the VFB voltage.

  • 0 in reply to Smriti B
    Expert 1385 points

    Thanks for advice.

    So circuit like in picture below could work ?

    When battery voltage is below 12,5V it’s Vbat is set to 14,7V and Vlowv is 10,85 V.

    Iprecharge is 6,7 A. I hope that it will charge from 10 to 10,85 V in 30 minutes with 6,7A.

    After it reaches 10,85 it starts to charge with 10A. After 12,5 V is achieved, then comparator senses the 12/5=2,5 V from voltage divider and turns on MOSFETs, thus Vbat is set to 16,8V and Vlowv is 12,4V, Itermination = 1,1A.

    This circuit shouldn't mess up discharge of battery right?

    EDIT:

    I just noticed that, comparator's output will always be HIGH because battery charging voltage is 16,8V.

    How can I measure voltage of battery if there is 16,8V applied voltage to it while charging?

  • 0 in reply to Karlis Tucs
    TI__Expert 5565 points

    For the positive input to your comparator, you want to pick a resistor divider value that causes the comparator output to go high after you have crossed your desired battery threshold. For example, you will want to set up the comparator such that it goes high when VBAT >= 12.5V.

  • 0 in reply to Smriti B
    Expert 1385 points

    Yes I understand that, and I have already done that in my circuit.

    My concern is - what voltage is applied to battery while loading it in CC mode? Is it the calculated Vbat voltage (2,1*(1+r2/r1)) or something really close to actual battery voltage?

  • 0 in reply to Karlis Tucs
    TI__Expert 5565 points

    In CC mode, it's the current that is regulated, not voltage. So during CC mode, the battery voltage will slowly rise as it's being charged. The programmed voltage (2.1 * ((R2/R1)+1) sets the final battery voltage. The battery charger will start regulating battery voltage after entering CV mode.

    See Figure 13 in the datasheet for the charge profile.

  • 0 in reply to Smriti B
    Expert 1385 points

    Thanks for clarification. I guess my circuit should work just fine.