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bq24630 pre-charge threshold problem

Howard Chang
Prodigy 40 points
Other Parts Discussed in Thread: BQ24630

Hello,

I am curious about the pre-charge threshold of bq24630. 

On the datasheet, it mentions when the voltage on the feedback pin is lower than VLOWV (0.35V typical), the charger will pre-charge. The problem is, for a 4 cell battery, the battery voltage(Vbat) needs to drop down to around 3.5V in order to achieve the condition ( Vfb < 0.35V). Normally, 9V is considered to be low on a 4-cell battery, is there any solutions on how to raise this threshold or LiFePo4 battery does not need this pre-charge function.

I have tried to change the resistor value at the output voltage divider, but it will also change the charging voltage. Also, I have tried to find other TI chargers for 4 cell LiFePo4  battery, but all of them are having the same threshold voltage (VLOWV = 0.35V typical) .

Thanks for your help!

  • 0
    Prodigy 40 points

    Hi

    It has been a day and there are no replies .

    Did I not state my question clear or is there any concerns?

    Please kindly help on this question, thanks!

  • 0 in reply to Howard Chang
    Expert 1085 points

    I'm confused by this also.

    In the Typical System Schematic, shown on page 2 of the data sheet, what is the corresponding Vbat trip voltage?

    I get a Vbat value that is way too low for LiFePO4 cells to be undamaged.

    Or is this value per cell and not the Vbat voltage? If this is the case, then your Functional Block Diagram is misleading.

    Thanks.

  • 0 in reply to TcPaden
    Expert 1085 points

    And while I'm at it, could someone clarify the Vrech voltage (Fig 17 Typical Charging Profile)? Is this the same as Vrechg (page 5, under RECHARGE COMPARATOR)? And the same as RCHRG comparator in Fig 16 Functional Block Diagram?

    If these are all the same, what value does it have? Page 5 says it's 125 mV with respect to Vreg, but Fig 16 says it's 1.675V.

    Fig 16 makes the most sense to me, but I'd like to verify it.

  • 0 in reply to TcPaden
    TI__Mastermind 31120 points

    The feedback reference is 1.8V and the typical regulation per cell for LiFePO4 is 3.6V, a factor of two for the reference.

    Thus one cell would use a 50% divider to achieve 3.6V regulation.  I will normalize everything for 1 cell:

    The LOWV is 2x0.35V = 0.7V per cell.  This is 19.44% of the programmed regulation voltage.

     

    The refresh or recharge, in the spec table is 125mV WRT the REF of 1.8V, thus is 250mV/cell.

    For a single cell regulating at 3.6V the refresh would be at 3.35V nominal.  This is 93% or the regulation voltage