BQ25798EVM: VSYS OVP FLAG and VBAT OVP FLAG

Hi,

What condition causes the VBAT_OVP? Are you enabling charge and then the OVP occurs? What is the voltage reading by multimeter on the pin TP29 - BAT?

Best,
Michael

Hi, 

Yes, the error occurs when I enable the charge (JP17 plugged).

When I remove JP17, the error disappears.

When the error is up I have 4.00V on TP29 (same value between B+ and B-), which is the value read by the ADC.

On TP28 (VSYS), I read the same thing than the EVM ADC : 4,6V

BTW I do not understand why VBAT and VSYS are not the same...

Also D13 STAT LED blinks at approx. 1Hz meaning there is an issue..

Hi,

Is there any battery protection device attached to the battery here?

Best,
Michael

Hi, good catch, there is indeed.

I removed it and retried but same result.

BTW I get the VBAT_OVP even when no battery is plugged (nothing is plugged at output..).

Hi,

With the BQ25798, any floating state on the battery terminals will result in the VBAT_OVP. This can be from a battery protection device in a HIZ state, a missing battery, or other similar state. When you removed the battery protection, did you still have the battery cell on the terminals?

Best,
Michael

Hi,

Yes, i tried with the battery without the protections.

I sucessefully made it work by shortening the EVM/ battery wire length (I had a amperemeter between before).

I do not understand why it doesn't work with long wire as there is not even much current (10mA...).

Hi Walter,

When you say that there is not much current being 10 mA, this is lower than the charge current I remember seeing set on the TIChargerGUI screenshots. Is this correct? I had a similar issue when I took the EVM to the lab and used wires similar to the setup in the first picture. The output current was less than I expected because the output voltage seen on the EVM was high and reached the regulation point. I think that the wires are not just long, but also thin, which is making them high impedance.

THOMAS WALTER said:

BTW I do not understand why VBAT and VSYS are not the same...

Also, I just realized that I did not answer this question. VBAT and VSYS are not the same because VSYS is typically regulated at a higher voltage than VBAT, except while charging. While charging, the BATFET is fully on, and any impedance between VSYS and VBAT becomes a voltage drop dependent on the charge current.

I am glad to hear that the battery is charging. Are there any other questions or issues I can assist with?

Best regards,
Michael

Hi Michael,

Thanks for your feedback.

One more question about VSYS charge termination voltage:

Are those following the voltages on VSYS wen the charge is terminated ? 

Does that mean that for instance, with a 1S cell, there is a small drop from 4.2V to (max 4.55V) on VSYS when the BATFET is turned OFF ?

Hi Michael,

Ok, well noted.

I still don't quite undertand why too long cables (even big ones) triggers an OVP warning, even when charge is almost terminated and there is little current so not much voltage drop..

In that case should I recommend my client (he'll decide the cable length) not to use too long cables ?

Also, it is not clear what value is VSYS when the charger is in OVP mode ?