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bq2005 transitioning to Top-Off state too early!

Heath Raftery
Prodigy 180 points
Other Parts Discussed in Thread: BQ2005

After resolving the issue of the bq2005 skipping to Top-Off state and moving straight to Trickle Charge, I now discover that I've had a problem all along with the Fast Charge state terminating after 60 minutes, when it shouldn't terminate until 180 minutes.

I have confirmed that the Top-Off state executes for 180 minutes, so figure that my TM1/TM2 settings are right. I have disabled -dV termination because it caused many spurious terminations, and I have very closely tracked the temperature input to ensure it is not triggering dT/dt. The BAT input is also not doing anything unusual.

The Fast Charge state executes as normal for 60 minutes by which time the temperature and battery voltage has all but plateaued. With no obvious signs of disturbance, it then transitions to Top-Off. With the charge rate set at C/2, and the battery pack reporting half full, this is obviously too short a Fast Charge period!

Why is Fast Charge only conducted for 60 minutes?

  • 0
    TI__Guru*** 147250 points

    Most likely you have noise on the temperature input.  Especially since you say that the -dV caused many spurious terminations.  The bq2005 will only terminate on one of 5 criteria: maximum voltage (Vbat > Vmcv), maximum time, maximum temp (Vtemp <Vtco), -dV (which you have disabled), and -dT/dt.  Since you have disabled -dV termination and say that you are not hitting maximum temp or maximum voltage (during this debugging, you should set the resistors with plenty of margin on these trip points to make sure you don't terminate on these), that leaves maximum time or -dT/dt.

    I would make sure that the thermistor is connected to SNS (battery ground), not Vss.  It would be good to make that connection with a dedicated, twisted, 2 wire connection from the thermistor/pack to the IC.

    There could also be noise on the TM1 or TM2 pins causing them to not look floating.  Perhaps, try a different timer setting (one where both pins are terminated either high or low, instead of floating) and see if this issue is still present.

  • 0 in reply to Chris Glaser
    Prodigy 180 points

    I doubt it's to do with noise, since the temperature detection is otherwise very good, except at 60 minutes into the charge. Additionally I think the -dV terminations are actually due to -dV! This battery pack seems to have small lulls in voltage as it charges and the ambient temperature fluctuates - I certainly see a much larger -dV as it reaches full charge, but the minor lulls during charge are too close for comfort.

    Anyway, I haven't seen the 60 minute issue since I first posted. I wonder if there's an issue with making changes to the TM1, TM2 settings, and the bq2005 taking a cycle to reset itself? I'll continue to monitor it, but after a complete and successful discharge/charge cycle I'm now practicing reserved celebration!

    Thanks for all your help.