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BQ2031 Open circuit behavior

Other Parts Discussed in Thread: BQ2031, BQ2054

I'm comparing the DV2031S2, Rev B evaluation module to a circuit we manufactured in house copying the schematic given in the DV2031S2 datasheet. Our circuit seems to work just like the evaluation module with one exception.

On the EVM, even when open circuit, the LED's indicate that the system is in "Pre-charge qualification".  With our version of the circuit, the LED's are indicating "Battery absent or over-voltage fault". While I think that our version of the circuit is behaving "correctly", we were expecting the behavior to be identical to the evaluation module. 

We are using the charger to charge an A123 battery, This battery is a lithium battery designed as a "drop in" replacement for a lead acid battery.  However, when the A123 is depleted beyond a certain point, its on board electronics cause it to go open circuit.  This open circuit is reset when charging voltage is present.  The evaluation board charges the battery in this condition without difficulty. 

Any help in understanding this behavior would be greatly appreciated.

Mike

  • Mike,

    I'm not very familiar with this part, and I'm not able to find an expert over the holidays. I know you guys are on a tight timeline, so let's see if there is anything I can help with.

    I agree that it sounds like your board's behavior (battery absent when A123 is open circuit) does sound like what I would expect. In this case, if battery is absent you don't get your voltage to wake up the A123.

    Please double check that you have the exact same jumper settings as the working EVM. I don't have access to an EVM right now, but since this is an older product maybe there was a BOM change that isn't properly reflected in the documentation--can you measure the critical component values (bias resistors) and check pin voltages when the EVM is able to start-up in the pre-charge qualification mode?

    Looking through the bq2031 datasheet, pre-charge qualification is discussed in the "Battery Insertion and Removal" section on page 7:

    "The bq2031 determines that a battery is present when VBAT is between the High-Voltage Cutoff (VHCO = 0.6 * VCC) and the Low-Voltage Cutoff (VLCO = 0.8V). When VBAT is outside this range, the bq2031 determines that no battery is present and transitions to the Fault state. Transitions into and out of the range between VLCO and VHCO are treated as battery insertions and removals, respectively. "

    And according to Figure 1. Cycle Start/Battery Qualification Sate Diagram (pg 3) this also indicates that your board is failing the VLCO and VHCO threshold tests, but I don't know it would pass this test on the EVM.  I would recommend focusing your comparative search on this part of the state diagram. 

      

    These two app notes might also provide more details:

    http://www.ti.com/lit/an/slua017/slua017.pdf - General application note for the bq2031 

    http://www.ti.com/lit/an/slva378/slva378.pdf - Page 3 has a clearer schematic (minus the additional MPPT control circuit)

     

    Thanks,

    Rachel 

     

  • I am trying to do a similar mod, except is it possible to swap in the bq2054 in place of the bq2031? Comparing the data sheets it seems they are near identical in pin out, maybe a change or two in the resistors?
    Michael's issue may be that the A123 on board protection circuit may not let the batt sense circuit recognize the batt.
    His solution could be the bq2054 as well.