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BQ24650: Working circuit now has FAULT, low charge current

Matt Staniszewski22
Prodigy 10 points
Other Parts Discussed in Thread: BQ24650

Hi everyone,

Attached is a schematic of our BQ24650 design for a lead-acid battery along with some debug notes.  Note that we have fixed the location of R98 (now connected to anode of D12 instead of gate of Q5).  We had seen this design working in lab (charge currents up to ~500mA).  However, when we took it in the field to test, we found that all of a sudden the charge current from the solar panel was only around 20mA and that the battery voltage was slowly dropping as it wasn't charging.

Bringing the board back into lab, I tested it exhaustively and found that we have a FAULT condition (both LEDs unlit) and that we immediately enter termination (Vfb > Vrech) when Vcc > 0V (solar charging is connected).  We aren't hitting the first 3 fault conditions listed in the spec: temperature suspension, over voltage or sleep.  We may be hitting battery absent, since it sounds like the test is similar to checking for termination, but that would just point back to the termination issue.

I've also checked for all the conditions outlined in the 'Enable and Disable Charging' section of the spec (p. 14), and we aren't meeting any of the conditions.  The only one I'm unable to check is the TSHUT IC temperature, which I'd need to find a way to check.

My primary questions are:

1) Given the same battery voltage at the output node of the charger, how could Vfb jump from 1.895V (not in termination) when no solar charging is applied (Vcc = 0V) to 2.05V (termination) when solar charging is active (Vcc > 0V)?

2) Are there any other additional conditions that cause a FAULT other than the 4 listed in the spec?

3) How can we test if we have hit the TSHUT IC temperature? 

We now have a second board running in the field flawlessly, so I'm a bit concerned why the first board should fail.  Before I call the IC dead, I wanted to ask your help.

Let me know your thoughts.

Thanks!

Matt

solar_charger_sch.pdf     solar_charging_issues.pdf

  • 0
    TI__Expert 5565 points
    Matt,

    1. If the battery voltage is the same, then the voltage on the FB should go DOWN when termination is reached. This is because the gate of Q5 would go low upon charge complete, thus changing the voltage divider on the FB pin.

    2. Charging could be suspended due to many reasons. The complete list is in page 14 of the datasheet in the section labeled "Enable and Disable Charging". Make sure that all of those conditions are met to resume charging.

    3. The only way to check the IC temperature is to use a thermal camera. With the thermal camera, you will be able to read the IC case temperature. The IC core temperature will be within a few degrees of the case temperature since this is a QFN package.