• State Resolved
  • Locked Locked
  • Replies 7 replies
  • Subscribers 63 subscribers
  • Views 168 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.

BQ24650: Will not start charging if solar panel ramps up too slow

Jesse van der Zouw
Expert 1045 points
Part Number: BQ24650

Tool/software:

We are facing issues with a BQ24650 circuit and were able to repeat the problem on the bench.

We are charging 12V 7Ah lead acid using a 8W 18V MPPT panel. 

In the morning, panel can be 14-17V for a while before the sun is fully up.

In this case, charger is enabled (STAT2 enabled), but not actually charging (voltage is below MPPT setpoint).

After 30 minutes, the charger disables (STAT2 disables) and does not enable again even if voltage becomes 18-21V.

Which timer/circuit/protection is this related to exactly? It seems like the pre-charge timer, but voltage on VFB is higher than VLOWV

We do not have control over the MPPSET pin, so cannot reset it with software.

Only solution seems POR after panel voltage is higher than the MPPT point, but lots of devices are already in the field.

Is this a known issue, what could be a fix?

  • 0
    TI__Mastermind 32531 points

    Hello Jesse,

    Can you provide your schematic?

    What is the measure volatage on VFB when this issue occurs?

    have you tried replicating this on an EVM?

    What are your exact test condition used to replicate in the lab?

    Best Regards,

    Christian 

  • 0 in reply to Jesse van der Zouw
    Expert 1045 points

    Schematic is posted above

    VFB is ~1.86V (12.6V battery voltage)

    To recreate this problem you need the BQ24650 with 18V MPPT resistor configuration and supply with 15V.

    This is high enough to enable the charger, but low enough to not meet the MPPT setpoint.

    After 30 minutes the charger will disable, and not re-enable even if you raise the voltage to 18V+

    Only solution seems POR

  • 0 in reply to Jesse van der Zouw
    TI__Mastermind 32531 points

    Hello Jesse,

    I've looked at this in the lab and ran the test for 30mins and 50mins. I do not see the issue of not charging after VIN rises above MPPT set voltage.

    I have a couple questions/ test conditions:

    What are you using to simulate the battery? Do you have a bidirectional power supply? it's possible their is an internal latch in the battery.

    If you remove D8, do you see this issue? I think I've seen a similar issue involving a diode near battery terminal. I know the diode is used to reduce leakage current, however this is a good test to help narrow down the issue. You may need to move VFB resistor divider after the diode and the add a 10uF cap after the resistor divder.

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    Expert 1045 points

    I'm using Agilent 66321B bidirectional PSU.

    The problem does not occur when I short D8, so good suggestion.

    However, I do not understand what the mechanism is what makes the difference.

    VFB voltage stays the same, except for slight difference due to forward voltage of diode.

    Do you know why this makes a difference?

  • +1 in reply to Jesse van der Zouw
    TI__Mastermind 32531 points

    Hello Jesse,

    Jesse van der Zouw said:
    The problem does not occur when I short D8, so good suggestion.

    That is good to hear.

    Jesse van der Zouw said:

    However, I do not understand what the mechanism is what makes the difference.

    VFB voltage stays the same, except for slight difference due to forward voltage of diode.

    Link to appnote with similar conditions:slva829.pdf.

    This appnote gives a brief description on why placement of VFB node and extra capacitor are needed. Basically the battery dectection is not operating correctly, and making these changes should resolve the issue.

    Best regards,

    Christian.

  • 0 in reply to Christian Moyer
    Expert 1045 points

    I understand now, VFB node is only ~1.86V when charging because D8 does not conduct in reverse when the charger is inactive.

    So this causes the issue with battery detection fault if the charger is powered but under MPPT setpoint for 30 minutes.

    We are looking to solve this for existing hardware to add UVLO between the solar panel with some hysteris.

    So it will only enable if >19V and disable again at <17V. 

    Thanks for your help.