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BQ2004E: BQ2004E does not initiate top-off until battery temperature is less than Thtf

Bud Ward
Prodigy 30 points
Part Number: BQ2004E

20 NiMHcells charging at 0.5C (TM1 high, TM2 float). Charges at C/8 for hold-off time (546 sec.), followed by C/2 charge, at the end of which charge goes to zero until battery cools to Thtf (40°C). It then finishes the top-off charge at C/8 for the correct period (0.235∗154min., ~36 min.) and stays in trickle mode until reset.

My question concerns the gap between fast-charge and topping. While it is certainly may be a reasonable behavior, it does not seem to be documented in either the text or the state diagram. Am I missing something, or do I have a flaw in my implementation?

  • 0
    TI__Expert 8130 points
    Bud,

    From the information provided, seems as your Battery Temperature (V_Temp) is out of range in between the transition from fast charge to top-off mode. As indicated in the Data sheet, section "Maximum Votlage, Temperature, and Time" (Page 5), Maximum temperature termination occurs anytime V_Temp falls below V_TCO.
  • 0 in reply to iAlejandro
    Prodigy 30 points

    Hi Alejandro,

    Our Thtf is set to 38.4°C (Vhtf = 1.33V), while the Ttco is 55.8°C (Vtco = 2.0V). At termination, the voltage never decreased below 1.19V (t = 44.5°C). Fast charge was terminated by either temperature rise rate or Vtpd (they were very close). Topping initiated when the observed voltage rose to 1.32V. It appears that the transition between modes is predicated on the Vhtf < Vt < Vltf range. Can you verify or refute this, please?

    ---Bud

  • 0 in reply to Bud Ward
    TI__Expert 8130 points
    Bud,

    Yes. When V_TEMP ≤ V_HTF it is in a charge pending state. Therefore, Top-Off initiated when V_TEMP > V_HTF. (V_HTF & V_LTF have a tolerance of ±0.030 V) Figure 3. Charge Algorithm State Diagram in the datasheet provides a visual representation of the charging state transitions.
  • 0 in reply to iAlejandro
    Prodigy 30 points

    Alejandro,

    Page 4 discusses the charge-pending state at the beginning of the charge cycle. The state diagram on page 8 only shows charge-pending as a transition condition from the "Top-Off and Pulse-Trickle Charge" state. Nowhere else in the manual is charge-pending mentioned except in Table 2 on page 7 regarding the LED outputs.

    We are in agreement as to the actual operation. I still insist that such action is NOT documented in the manual. The expectation is that charging should continue once initiated; the temperature AS DOCUMENTED is only an entry condition to the Fast Charge state, and only Vtco is an exit criteria thereafter.

    ---Bud