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Original question:

BQ2004: NiMH charger

BQ2004: DV2004S1 Demo board

Vadym Vasylyshyn
Prodigy 130 points
Part Number: BQ2004
Hello TI Support team,
I'm using BQ2004 charger for my project to charge 12-cell NiMH battery block. I'm required to use discharge feature that discharges battery before recharge.
I'm studying Discharge-Before-Charge feature and I'm not sure how to control the depth of discharge or discharge threshold voltage.
Looking at circuit diagram Discharge-Before-Charge controlled by DCMD input. When DCMD commanded to ground, this activates DIS output (high) and keeps discharging until V_CELL < V_EDV.
Per equation on page 3 of datasheet V_EDV = 0.4*V_CC
V_CC = 5V
Which means V_EDV = 2V!!??
There's no way we wanna discharge 12-cell battery block down to 2V. I need to able to have discharge threshold voltage to be selectable down to 9V-12V. How can this be done?
Thank you,
Vadym

  • 0
    TI__Mastermind 24075 points

    Hi Vadym,

      VEDV corresponds to a threshold on the BAT pin. The BAT senses a divided down voltage of the battery, dependent on the resistor divider used to program the charge regulation voltage. MCV is a similar threshold which is defined as VMCV = 0.8 ∗ VCC ± 30mV. BAT does not sense the true battery voltage, as this is a charge controller.

  • 0 in reply to Kedar Manishankar
    Prodigy 130 points

    Hi Kedar,

    Yes, I see that V_CELL is not an actual battery voltage but V_BAT voltage that's defined by voltage divider between BAT and SNS pins on the BQ2004.

    So for 12-cell RB1 = 40k, which at nominal battery voltage(14.4V) an actual V_CELL = 12V. 

    The DCMD input is used to command discharge-before charge via the DIS output. Once activated, DIS becomes active (high) until VCELL falls below VEDV, at
    which time DIS goes low and a new fast charge cycle begins.

    The problem I'm running into is that I need to have ability to terminate discharge at any desired battery voltage threshold(V_PACK). But it seems that discharge voltage is fixed by V_EDV, where V_EDV = 0.4*V_CC = 0.4*5= 2V. 

    So that means that V_CELL needs to drop below 2V for discharge to be terminated. Which means that at V_CELL= 2V, an actual battery pack voltage will be V_PACK=2.4V! This is disastrous low discharge voltage for 12-cell NiMH battery. Is there a way to control depth of discharge with BQ2004?

    I greatly appreciate your help, Vadym.

  • 0 in reply to Vadym Vasylyshyn
    TI__Mastermind 24075 points

    Hi Vadym,

      This is an older part, and unfortunately has an inbuilt discharge threshold. There is no configurability here