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BQ27531-G1: BQ27531-G1

Gilad Friefeld
Intellectual 330 points
Part Number: BQ27531-G1
Other Parts Discussed in Thread: BQ24192, BQ27220, BQ25120

Hello.

I'm designing a product that includes a battery management system using the BQ24192 and the complementary BQ27531 - G1.

My problem is that I don’t have any access to the PACK- terminal (This is required for the use of BQ27531 - G1 SRP input), I only have access to the battery PACK+ terminal. The PACK- terminal connected directly to GND.

My question is:

can I use a different fuel gage other than the BQ27531 - G1 in pair with the BQ24192 charger?  one that doesn’t require a direct connection to PACK- terminal (only a connection  from GND to PACK-) and has an I2C interface to work seamlessly with the BQ24192 charger.

BR,

 

Gilad.

  • 0
    TI__Mastermind 49151 points

    Hi Gilad,

     Yes, there are many other gauge devices can be used for with the BQ24192 charger and use the same I2C bus to control the gauge and charger; but some features from this recommended pair have to be implemented by processor coding.   

     If you share me details of the battery PACK, I may give you a simple reference schematic for your design. 

    Thanks!

    Phil

  • 0 in reply to Phil Yi
    Intellectual 330 points
    Hi Phil.
    Thank you so much for answering!
    My battery pack is:
    www.orbtronic.com/protected-3400mah-18650-li-ion-battery-panasonic-ncr18650B-orbtronic.

    As I mentioned, I don't have access to the PACK- terminal, only the PACK +. PACK- is directley connected to GND.
    Can you please elaborate on the specific function I’ll have to implement via the processor?
    A schematic of your suggested design will be highly appreciated.

    Thanks again.
  • 0 in reply to Gilad Friefeld
    TI__Mastermind 49151 points
    Hi Gilad,

    The PACK does have a PACK- terminal, just treat its negative terminal as PACK-, what you need is to insert a current sense resistor between its PACK- (negative terminal) and your system GND.

    Please let me know if you have other questions.

    Thanks!
    Phil
  • 0 in reply to Phil Yi
    Intellectual 330 points
    Hi Phil.
    Thanks again for your patience.
    because i don't have a mechanical access to the negative terminal (that is how my product is desined) i can't place a sense resistor between the negative terminal and the system GND. the negative terminal is mechanicaly connected to the system GND, i cant chage that.
    i noticed some other fuel gages that don't need a sense resistor between the negative terminal and GND, such as: bq27441, bq27220, bq27421. do you reccomend to use one of them with the bq24192? would that be a suitable solution?
  • 0 in reply to Gilad Friefeld
    TI__Mastermind 49151 points

     Understood; if so, all high-side sense single cell gauge devices should be good to your design. based on your list, bq27220 is a CEDV gauge with lower accuracy when compare with other IT (Impedance Tracking) gauges; More detail comparison is linked below:

    Thanks!

    Phil

  • 0 in reply to Phil Yi
    Intellectual 330 points
    Hi.
    So, just to summarize our discussion, you do recommend pairing the bq29192 with bq27220. I won't get as accurate results with the 27220, as if I was using the 27531.
    Can you please provide a schematic for the final design?

    Thanks for helping.
  • 0 in reply to Gilad Friefeld
    TI__Mastermind 49151 points
    Linked below is the gauge EVM schematic for your design reference. I'll be happy to review your schematic before it goes to PCB layout.
    www.ti.com/.../sluubf5.pdf

    Thanks!
    Phil
  • 0 in reply to Gilad Friefeld
    TI__Mastermind 49151 points
    Yes, bq27220 is a CEDV gauge, bq27531 is a IT gauge which can deliver higher accurate results than any CEDV gauge.
    The link below is a reference design including bq27421 and bq25120
    www.ti.com/.../tidrir7.pdf
  • 0 in reply to Phil Yi
    Intellectual 330 points

    Hi Phil.

    Thanks for your answers. I have one more question....

    I’m going to send my schematic later on this evening for review. I have used the BQ24192 and paired it with BQ27220.

    As I mentioned before, I’m using a system-side connection to the fuel gage. I don't have a BIN output from my rechargeable Li-ion Battery (only positive and GND connection). The battery includes protection circuits.

    My battery is also meant to be taken out from the product (in order to be replaced with another battery) in addition to the option for charging when installed in the product. That’s a lot of information.....I know.

    My question is:
      if my battery doesn’t include a BIN output (only Positive & GND) and I take out a battery and install another, how can the fuel gage determine the capacity of the battery? Can you elaborate on that subject? I don't understand how accurate will the measurement be and how does the fuel gage actually calculate the battery capacity?

  • 0 in reply to Gilad Friefeld
    TI__Mastermind 49151 points

      In your case, use a 10-kΩ pull-down resistor from BIN to VSS, and set [BI_PU_EN] = 0, then your processor must inform the gauge of battery insertion and removal with the BAT_INSERT and BAT_REMOVE subcommands.

      The fuel gauge measures the charging and discharging of the battery by monitoring the voltage across a small value sense resistor. When a cell is attached to the fuel gauge, cell impedance is computed based on cell current, cell open-circuit voltage (OCV), and cell voltage under loading conditions.

       The fuel gauge uses an integrated temperature sensor for estimating cell temperature. Alternatively, the host
    processor can provide temperature data for the fuel gauge.

      The bq27220 fuel gauge accurately predicts the battery capacity and other operational characteristics of a single Li-based rechargeable cell. It can be interrogated by a system processor to provide cell information such as state-of-charge (SoC). By integrating charge passing through the battery, the battery’s SOC is adjusted during battery charge or discharge. The fuel gauging is derived from the Compensated End of Discharge Voltage (CEDV) method, which uses a mathematical model to correlate remaining state of charge (RSOC) and voltage near to the end of discharge state. This requires a full discharge cycle for a single point FCC update. The implementation models cell voltage (OCV) as a function of battery state of charge (SOC), temperature, and current. The impedance is also a function of SOC and temperature, all of which can be satisfied by using seven parameters: EMF, C0, R0, T0, R1, TC, C1.

    Phil Yi

     

  • 0 in reply to Phil Yi
    Intellectual 330 points
    Hi Phil.
    How can i send you the design for final review?

    Gilad.
  • 0 in reply to Gilad Friefeld
    TI__Mastermind 49151 points

    Send to my email: pyi@ti.com.