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bq24450 for String of 12 Lead Acid Battery Charger in Dual Step Current Charger mode; calculus confusion

Mario Marcoccia
Prodigy 40 points
Other Parts Discussed in Thread: BQ24450, BQ2031

I'm designing a charger for a string of 12 (twelve) car batteries connected in series.
I'm using the bq24450 in Dual Step Current Charger mode.

- The maximum charging current (Imax) of the batteries is 10 Amps. (Battery capacity = 110 AH)

- The input voltage (Vin) is 170 Volts DC maximum. (unfortunately, this value can not be lowered because it comes from a generator)
- The minimum expected Voltage for all batteries in series (assuming each battery is discharged to 10.8 Volts) is 130 Volts (12 Volts x 10.8V = 129.6)

-Then, the maximum voltage through the Power FETs  will be 40 Volts.

- In order not exceed  the bq24450 supply operating voltage , the ground pin is connected to a tap point in the high side of the tap (second battery in the string, i.e, a  maximum approximate Voltage = 27.6 Volts ). 

- The pass transistors are two power FET channel-P, p / n: IXTH90P10P.

 I put some questions and doubts in the text below.

 - Then, I assume  that the Voltage Trip will be the voltage sum of the two high side batteries:  (13.8V + 13.8V = 27.6 Volts) (used as power supply of the bq24450),

 Is it Correct?

- The figure below (Fig-11 & Fig-10) shows a resistor RB connected in parallel to minimize the charging current offset: 

-I'm confused about the calculation of this resistance RB;

- (IB) & (ID)  currents:  (IB) is bq24450 Operating  Current? (approx. 2 mA)?;    

- (VBL) voltage: should I consider for calculating, the value of (VBL) when batteries are at 10.8 volts (discharged)? (i.e.: 10.8 V/batt x 10 batt. = 108 Volts)?

   

- Regarding  Reference to Ground (circulated in red color)  in Fig-10 (bellow):

- (1) I assume that, with a string of batteries, the ground reference for those points (derived from pins 14 and 15), is the same as pin 6. Is this correct?

Thanks in advance.

Regards.


  • 0
    TI__Mastermind 31120 points

    I would not recommend a linear battery charger for so many cells.

    I would look at a switcher charger like the bq2031, but we really do not have any application material for so many batteries in series so hopefully you have a experienced power supply designer to work out any issues that may be present with such a high power application.

  • 0 in reply to Charles Mauney
    Prodigy 40 points

    Thanks Charles. I appreciate your suggestion. 

    I'll see applications with bq2301 for large strings of batteries. 

    Any way, I'd like to know if you can resolve my questions regarding Dual Step Current Charger with bq24550, because I have another application for 3 batteries.

    Thanks in advance.

    Regards.

  • 0 in reply to Mario Marcoccia
    TI__Mastermind 31120 points

    For the multicell application above, it look like pin 6 should be tied to the top of the 10 cells as per your first figure.

    What is your question on the 3 cell dual current charger.

     

     

  • 0 in reply to Charles Mauney
    Prodigy 40 points

    Thank Charles.


    These are my questions regarding 3 batteries in series about components  calculations:

    NOTE 1: I use two batteries in serial (of the 3 batteries) as Input operating voltage for  bq24450.

    NOTE 2: (The Fig- 10 & 11  mentioned below,  are the same; only change the number of series connected batteries).


    Then: 

    (1) I assume  that the Voltage Trip will be the voltage sum of the two high side batteries:  (13.8V + 13.8V = 27.6 Volts) (used as power supply of the bq24450),

          Is it Correct?

    (2) The figure below (Fig-11 & Fig-10) shows a resistor RB connected in parallel to minimize the charging current offset: 

           I'm confused about the calculation of this resistance RB;

    (3)  (IB) & (ID)  currents:  

           (IB) is bq24450 Operating  Current? (approx. 2 mA)?.   

    (4) (VBL) voltage:

          Should I consider for calculating, the value of (VBL) when batteries are at 10.8 volts (discharged)? (i.e.: 10.8 V/batt x 1 batt. =10.8 Volts)?

    (5)  Regarding  Reference to Ground (circulated in red color)  in Fig-10:

          The ground reference for the components connected to pins 14 and 15 , 

          Is the same as the pin 6?.  Is it correct?

    Thanks.


  • 0 in reply to Charles Mauney
    Prodigy 40 points

    Charles,

    (1) The figure  (Fig-11 & Fig-10) that I posted shows a resistor RB connected in parallel to minimize the charging current offset: 

           I'm confused about the calculation of this resistance RB;

    (3)  (IB) & (ID)  currents:  

           (IB) is bq24450 Operating  Current? (approx. 2 mA)?.

    Thanks.