• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Subscribers 62 subscribers
  • Views 383 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

BQ25792EVM: Vsysmin setting keeps resetting to default of 9.0V

Charles Jenkins59
Intellectual 360 points
Part Number: BQ25792EVM
Other Parts Discussed in Thread: USB2ANY

We are evaluating the 25792 as a Lead Acid AGM battery charger for 12V batteries. The 3s default settings are close enough to a workable charge setup and have enough program room to change the charge voltage to 13.99V.  Charging works well.

One feature that made this part perfect for our solar application is that it regulates the VSYS output to Vsysmin in the absence of the battery. Competitive parts simply pass the input voltage to the output. The default value of Vsysmin is 9.0V. Using the USB2ANY and the web based GUI to monitor the EVM, the only registers we changed were the battery charge voltage and the Vsysmin voltage.

When the system reached end of charge the charger holds its 11.5V setting of Vsysmin but seconds later resets this to 9.0V.  Why does the register want to revert to its default settings? we used the register display screen to set Vsysmin to 0x24h but it changes back to 9.0v (0x1Ah) randomly.

Here is a plot that shows the battery disconnect (white) with a Vsys of 13.5V and then the register change to 9.0v (red) and finally battery reconnect (magenta) where you can see the battery discharge pulses looking for state of charge on the battery.

  • 0
    TI__Guru**** 169236 points

    Hi Charles,

    The VSYSMIN and charge current and EN_ILIM pin registers reset if the WD timer expires.  Is your host writing to the watchdog timer bit before it expires (default of every 40s) or have you disabled the watchdog timer?  I just tried on my test bench and don't see SYSMIN change after termination if WD is disabled.

    Regards,

    Jeff

  • 0
    Intellectual 360 points

    Thanks Jeff. Still newbie to that GUI.  I was wondering about that so I set the auto read time to fast as possible but it still seemed to revert.  Guess you have to kick the dog on purpose. The GUI does not do that for you.  Found the disable. My 11.5V stays as desired.

    One other question. The max cell voltage for a 3s config is 13.99v for the lithium chemistries.  Can the voltage go above this if needed by the temperature compensation or would we have to switch to 4s to get that temperature induced adjustment?  AGM batteries might need the extra push at cold temps and I was trying to figure out the probable voltage range over temperature for the 3s setting.

    Observation ... The ADC_Ibat reading is only capable of positive values (+ into battery)?  We tried a 1A load on Vsys with VIN disconnected and this register displayed 0A.

  • 0 in reply to Charles Jenkins59
    Intellectual 360 points

    I may have answered the temperature question myself. Looking over the temp charts in the data sheet the Li temp profiles are designed for cell protection and decrease current/voltage outside of the T2-T3 range. The AGM profile is more linear and wants an increase in T1-T2 range.  Looks like we will have to use the uP to manage the charger settings for cold temps.

    When is the lead acid charger profile coming out? ;-)

  • +1 in reply to Charles Jenkins59
    TI__Guru**** 169236 points

    Hi Charles,

    You will have to change to 4S in order to go higher than the 3S max battery regulation voltage.  SYSMIN changes as well.  If you set EN_BAT pin =1 (REG0x14b5), the ADC reads negative current.

    The charger has some JEITA settings flexibility in REG0x17-0x18.  Unfortunately, we have no plans of releasing a lead acid charge for this device.

    Regards,

    Jeff