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BQ34Z100 and high capacity lead acid battery pack

Thomas Lamant
Prodigy 40 points
Other Parts Discussed in Thread: BQ34Z100, BQ34Z110, BQ34Z100-G1

Hi,

Completely new to it, I thought electronics was pretty so far as I was just wiring a single wire to GPIO pins of my Raspberry Pi, but I got scared when I read the datasheet for the BQ34Z100!!

I would like to check with my RPi (via I2C) running on Linux the remaining capacity of my battery pack, made of 2 x 12V / 110Ah batteries in series (i.e. 24V / 110Ah).

1. While the former BQ34Z110 supported capacities higher than 65Ah, the datasheet of the BQ34Z100 says "Supports Battery Capacities up to 29Ahr with Standard Configuration Options". Can I still use it with my battery pack?

2. If I understood correctly, I need to step-down the voltage to something between 2.7 and 4.5V. The documentation talks about LDOs. Is that the best option? Could you give me an example of suitable product?

3. What is the minimum of extra parts I need if I just want to be able to read the gauge ? (I don't need LEDs etc.)

Many thanks for your help!

  • 0
    TI__Guru 73830 points
    The bq34z110 has a limit of 17Ah due to a firmware limitation, so the bq34z100-G1 has been improved. You will need to use capacity scaling, if you exceed 29Ah. We use a simple MOSFET shunt regulator on the EVM, so something like that will be adequate. If you refer to Figure 12 in the datasheet, you can eliminate SW1 and R15, if you do not need LEDs. You can also eliminate R2, R4, Q4 and Q5, if you do not care about reducing the load on the battery from the battery sense voltage divider.
  • 0 in reply to ThomasCosby
    Prodigy 40 points

    Thanks Thomas for your reply. However, may I ask you to be more specific?

    ThomasCosby said:
    You will need to use capacity scaling, if you exceed 29Ah. We use a simple MOSFET shunt regulator on the EVM, so something like that will be adequate.

    Do you mean that a MOSFET shunt regulator is used to do capacity scaling? Because in paragraph 8.3.12 of the datasheet, it says "The device supports [...] high capacity batteries above [...] 29Ah indiretly by scaling the actual sense resistor value compared with the calibrated value stored in the device". So is scaling done by MOSFET or by scaling the sense resistor?

    Back to my project, if I use Figure 12 as a reference to my own needs (28.8V max voltage out of the nominal 24V battery pack):

    1. As I don't need LEDs, I can eliminate the whole U3 block, as well as SW1 and R15 as you suggested. I also need to connect P1 and P2 to Vss as suggested in page 3 of the datasheet. Is that correct?

    2. If I don't need the ALERT function, I can elimitate everything that goes out of P5. Is that correct?

    3. Does Rseries in the datasheet refers to R1 ? In which case I can calculate R1 as suggested with the formula in the datasheet (result = 511kOhm).

    4. Then how do I calculate the right R2, R3, R4 and R30 ?

    Thanks!!

  • 0 in reply to Thomas Lamant
    TI__Guru 73830 points
    Thomas
    Sorry for the confusion. Capacity scaling and the shunt regulator are independent comments. I just released a new application note to the bq34z100-G1 product folder that covers capacity scaling. Please check that document.

    1. Your comments are correct.
    2. Yes, you can eliminate the HDQ components and leave pin 12 open.
    3. A 511k resistor will support a 28V stack voltage.
    4. R2 and R4 form a voltage divider to provide VGS to turn Q5 on when the voltage measurements are made. You can set them for the VGS requirement for FET that you use in the circuit. If you use the FET in our reference design, then you can leave them at the present values. You can leave R2 at 10k, if you use the zener that we used in the reference design. The value of R30 will depend on your charge and discharge current requirement. The maximum voltage drop across the sense resistor cannot exceed +/-125mV.

    Tom
  • 0 in reply to ThomasCosby
    Prodigy 40 points

    Thomas-

    Thank you for your reply and the application note. That's very helpful! I understand that capacity scaling is just a software setting (data flash configuration). Moreover, the upper left part of Figure 12 is now clear to me, I even checked the calculation. However: 

    1. Wouldn't it be more straight-forward to replace Q5 with an N-channel transistor with a Vgsth lower than 2.5, that could directly be triggered by VEN?

    ThomasCosby said:
    You can leave R2 at 10k, if you use the zener that we used in the reference design.

    I suppose you mean R3 instead of R2. Still:

    2. I don't understand what are the roles of R3, D7 and Q3. It seems that (appart from the very small Vds of the transistor diode, which is around 1V), V_BAT+ = V_REGIN, although V_REGIN must be lower than 5.5V according to the datasheet. Am I missing something?

    3. Could you help me in calculating the right R30? My max discharge current will be 8.3A in nominal (and 9.9A if transient time is taken into account i.e. if all motors start at the same time). My max charge current is theoretically 22.2A although it is unlikely that all charging devices run at the same time and at their max power.


    Thank you for your help and patience.

  • 0 in reply to Thomas Lamant
    TI__Guru 73830 points
    1. Using a low-side FET to switch the resistor ladder would require fewer components, but it would have more impact on the voltage divider and introduce error.
    2. Yes, I meant R3. REGIN is less than 5.5V. The regulator configuration sets it to abotu 3.7V.
    3. A 10 mohm sense resistor will support about 12A. If your maximum current is 22A, then I would use a 5 mohm sense resistor.
  • 0 in reply to ThomasCosby
    Prodigy 40 points

    1. OK!
    2. My bad, I was totally wrong about the way transistors worked. I got it now. It is indeed a very typical (transistor) series voltage regulator!
    3. OK. You simply divided 0.125mV by the max running current. Alright.

    I am all set now. Thank you for your help !