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BQ2000: Issue with Ni MH batteries

Mayur Kachhadiya
Prodigy 80 points
Part Number: BQ2000

Hello TI,

I am currently testing BQ2000 Evalution kit for charging 3 Ni MH cells in series. As described in testing procedure, Top-off mode is selected and jumper on "Y" for 160 timeout. 200 K-Ohm resistor was soldered on board for 3 Ni-MH cells and jumper was put on that. We supplied board by 12 V DC input. All set up was done as described in datasheet page 3 (Section 2.3). While testing problem i found is that: LED on board is always blinking. How come? When batteries are being charged by Eval kit, LED should be on Low or? am i  missing any important configuration on Board?

What could be your suggestion or solution for charging 3 Ni-mH cells in series? 

Regards

  • 0
    TI__Expert 8130 points

    While in the fast charge state, the LED pin should be pulled Low due to the LED pin being an open-drain output. When the LED pin is pulled Low, the LED is ON. Page 15, Table 6. Charge Status Display, in the datasheet specifies the LED display output.

  • 0 in reply to iAlejandro
    Prodigy 80 points

    Hi,

    That i already analysed. But in my case, LED is always blinking. Never goes to LOW or HIGH, even batteries are connected. When batteries are detected by Eval kit, it supplies fast charge to batteries and LED should go LOW automatically. Nothing happenend here. Only blinking always. Note that batteries are empty. 

  • 0 in reply to Mayur Kachhadiya
    TI__Expert 8130 points
    It seems that the charger is entering the battery condition state and not exiting. This condition indicates the possibility of a defective or shorted battery pack. Can you please check the following:
    -Voltage at BAT and TS Pin?
    -Resistance value of R6?
    Also, make sure that the 200kΩ has been properly soldered.
  • 0 in reply to iAlejandro
    Prodigy 80 points
    Yes, you're right. charger always enters in condition state. Voltage at BAT pin is 417 mV, which should be higher than 950 mV (VLBAT). I tried with trickle charge and set vaule of resisitor 6.8 K Ohm between RC and VSS pin. On Eval kit i soldered that resisitor at place of R23 (spare). But i had same problem. Is there any solution to get BAT voltage above than VLBAT (950 mV: Internal threshold)?
    R6 vaule is also OK. it is 750K ohm.
    pls feedback
  • 0 in reply to Mayur Kachhadiya
    TI__Expert 8130 points

    A  resistor (R6/R7) voltage-divider between the battery pack's positive terminal and Vss scales the voltage at BAT pin. As you mentioned previously, a 3 NiMH cell battery  and R6 = 750kΩ are in use. Therefore, we recommend setting R7 = 523kΩ. As a result, the voltage at BAT pin should be equal to about 1.479V, which leads to the following condition of VLBAT < VBAT < VMCV to be true. Figure 3. in the datasheet demonstrates the resistor-voltage divider network.

    Also, in regards to soldering a 6.8kΩ on R23 will only set a different trickle pulse-width of about 3ms and disable top-off. IF top-off is wanted with that following trickle pulse-width of 3ms, then solder the 6.8kΩ on R26 (spare).

  • 0 in reply to iAlejandro
    Prodigy 80 points

    Yeah, now BAT pin voltage is sufficient. I'm using top off charge mode. In that case what could be RC behaviour like sawtooth wave or ? I got always DC level with thick line (High). How can i see if battery is being charged or not? 

  • 0 in reply to Mayur Kachhadiya
    TI__Expert 8130 points
    The best way to tell if the IC is in top-off or trickle charge is to look at the RC pin when the temperature (TS pin) is between the LTF and HTF. In top-off, the RC pin will have a saw-tooth waveform for it. In trickle charge, the RC pin will be at a DC value.
    Also, Table 6. summarizes the charge status display output of BQ2000.