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BQ24650: Setting V_refcharge for LiFePO4 Battery

Tomer Bruchiel
Intellectual 640 points
Part Number: BQ24650

Hello,

I need to build a solar charger for a LiFePO4 battery.

The requirements are to support 4,5,6 and 7 cells batteries.

Consulting the app note SLUA565 (Using the bq24650 to Charge LiFePO4 Battery), it is explained how to add a resistor in parallel to R23 (R15 in the app note) to reduce V_refcharge to 3.4V for example, however this app note address only 1 cell battery.

In order to support 4 to 7 cells, I need to do the following:

1. Change R22 (R13 in the app note)  for each battery type assuming R23 (R15 in the app note) is constant.
So in my design, if R23 is 100K than for 5 cell R22 will be 757K and for 6 cells 928K and so on right?

2. Set the parallel resistor accordingly, 3M ohm looks like the correct value.

3. MOSFET - in this solution the gate of the MOSFET is connected to STAT2. Vsys in my design can be 28V so it will burn the gate. I see no reference to that in the app note. Will a zener diode make sense or am I missing something here?

Thanks,

Tomer

  • 0
    TI__Guru**** 155146 points
    Hi Tomer,

    A Zener diode, with series resistor to drop the voltage, is one option. Another might be to change the pullup voltage to VREF. This will require you to resize your resistors and might need to change the LEDs, depending on their voltage drop.

    Regards,
    Jeff
  • 0 in reply to Jeff F
    Intellectual 640 points
    Thank you Jeff,
    This answers topic number 3.

    What do you think about 1 and 2? Are my assumptions and calculation right?

    Thanks again,
    Tomer
  • 0 in reply to Tomer Bruchiel
    TI__Mastermind 24075 points
    Hi Tomer,
    You can refer to the BQ246xx calculation tool V1.5 found at this link: www.ti.com/.../toolssoftware
    Here you can input your desired charge regulation voltages and see which resistors are required. For Question 1: the battery charge voltage section of the calculation tool will give you the right value for R22 based on R23. For Question 2: use the same section for your desired new charge regulation voltage (this new charge regulation voltage should be calculated according to your desired recharge threshold which would be 50mV less measured on VFB w.r.t charge regulation voltage), set R22 and calculate the new R23 required. Then you can find out which resistor is required to make a parallel combination with R23 to get the new R23, and that would be R1.
  • 0 in reply to Kedar Manishankar
    Intellectual 640 points

    Thanks Kedar,

    Actually the calculation tool is oriented for Li-ion batteries as this section does not take into account the Vcell value.

    This is why I posted these questions here.

    Tomer

  • 0 in reply to Tomer Bruchiel
    TI__Mastermind 24075 points
    Hi Tomer,
    Yes you are correct about the calculation tool being only for Li-ion, I forgot about that. Nevertheless, the procedure I mentioned above still holds true, and the correct resistor values should be calculated according to the formulas shown for VBAT.