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BQ76920: false current detected while balancing

Nikhil Mahajan
Prodigy 140 points
Part Number: BQ76920

hello everyone, 

im working on bms for which I'm using bq76920. which awsome ic. thank to ti .

ov,uv ,scd,ocd  is working fine.

now I'm trying to implement balancing on this ic. but when I start balancing around 890 mA current is displayed. but when I checked across shunt there is no drop across the shunt. what may be the issue.?

also after detecting the short circuit or overcurrent fault, what should be next logic. I'm thinking after short circuit or overcurrent is detected until we disconnect the load MCU won't turn on the MOSFET. is there any other method or logic for this ?. because if we connect this BMS to some circuit and accidentally we short or overcurrent occurs we have to disconnect and connect again.

thank you in advance.

  • 0
    TI__Guru 54715 points
    Hi Nikhil,
    Glad you like the bq76920.
    Balancing affecting the current reading but with no differential voltage across the sense resistor likely indicates a shift in the reference. Check that the SRP and SRN pins are within the recommended operating range. If the sense resistor and the bq76920 VSS are not connected sufficiently to battery- the common mode can shift significantly during balance. Another common cause of unexpected current is connecting equipment refernces (grounds) to different sides of the sense resistor. This can usually be observed by voltage across the sense resistor however.

    The recovery process after a short circuit or overload depends on the system design. Some systems will wait some amount of time and try again. With this process if the short remains the FETs will be turning on into a short circuit. The FETs and heatsinking must be designed to take this, and the MCU must allow sufficient time for cooling between re-tries. Some systems will wait for a signal that it is OK to re-try. This might be a battery installation status line or charger present signal. Some battery systems will attempt to recover with a test signal. The bq76920 has the load detection comparator on the CHG pin which can be used with low side switched systems. If the small CHG pull down current does not bring the CHG pin below the threshold, the LOAD_PRESENT bit remains set. If the attached system has a residual load, the CHG pull down current may not be able to bring the line low, the MCU may need to add a controlled test current or other technique. For high side switched systems the bq76920 load detection is not suitable and the MCU will need to implement a comparator or measurement to determine when recovery is acceptable.
  • 0 in reply to WM5295
    Prodigy 140 points

    hello sir,

    thank you for your reply. I checked the circuit and yes there was a small issue with dry soldering. i soldered that connection properly. but now it constantly showing -54 mA reading. and when i apply actual load still it showing false value. I guess the issue is with a 0x32 register. it not showing any value when we connect the load. but when we disconnect the load it displays some value.

    also, i checked resistance across r3 and r4. across r3 is it showing properly 100 ohms and when I checked across r4 it is showing 76 ohms.  but when i remove the ic across both resistor r3 and r4 it is showing 100ohm.

  • 0 in reply to Nikhil Mahajan
    TI__Guru 54715 points
    Hi Nikhil,
    It sounds like you may have had some contamination or leakage on the board with the part installed, or a damaged part. Or potentially the meter could bias something in the IC. When I measure on an EVM both SRP and SRN measure 100 ohm.
  • 0 in reply to WM5295
    Prodigy 140 points
    thank you for reply ,
    i did some checking in my circuit. i found that my ic is damaged. i changed the ic and now its working fine.
    thank you.