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BQ2002T VBat threshold question

Other Parts Discussed in Thread: LM317, BQ2002T

Hi,
I'm using a BQ2002T for charging a NiMh battery pack made of 6 cells (7.2V). I made a charging circuit similar to the evaluation board DV2002TL2 with a LM317 regulator.
I cannot understand why the Vbat threshold is 2V while a singular NiMh cell are 1.2V at full charge. I designed a voltage divider as specified in the datasheet RB1/RB2 = N - 1, so I choose 499K and 100K resistors.
Now my circuit never stops charging and I'm worried about damaging the battery.
Can someone explain me why Vbat threshold is set to 2V ?

Thank you in advance,

Carlo Ferraresi

  • Typically a NiMH just before termination has a voltage of ~1.53V.

    In order to get a good termination signal, the charger should remain in constant current mode until termination occurs.

    Thus I like to use about 1.7V/cell. 

    Typically the pack voltage will not get to the max charge voltage unless something is wrong...either a bad cell/pack or a improper design.

    If for some reason the charger does not terminate, the safety timer should kick in after the pack should have had time to charge.

    Are you charging at >=C/3?  The cells need a certain amount of current to cause termination.

  • Now I'm charging at 1C in fast charging mode because I connected the TM pin to GND, but I'm still missing something. In the BQ2002T datasheet at page 4, when it is described the starting of charge cycle, the point 2 says "Voltage at the BAT pin falling through the maximum cell voltage where Vmcv = 2V ±5% " .  

    The Vbat will be always below Vmcv,  I think the charge is controlled by the temperature sensor, but I'm not sure about that.

    I would ask you another question about the charging current. As reported in the DV2002TL2 datasheet, the charging current is imposed by R7 resistor, so  I have to select  R7 value in order to match the battery capacity ( C = 480mAh ) ? I think I made a mistake choosing a R7 value of 1 ohm .