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BQ25570, harvesting problem

Olexandr Gudozhnik
Prodigy 30 points
Other Parts Discussed in Thread: BQ25570

Hello all TI users!

I have a little problem that I hope to resolve with the help of local gurus :)

I have a board with BQ25570 and solar cells. My schematics is very close to the datasheet's typical solar application circuit.

A Li-Poly 240mAh battery is connected to Vbat. Three solar cells from IXYS (KXOB22-04X3L, 1.5V, 15mA) are connected in series to VIN_DC. Output is set to 3V. Overvoltage and Vbat_OK resistor dividers are pretty much the same as in datasheet. V_ov is set to 4.1V.

When I connect the battery (3.7V approx), system draws 2.7 mA from it. When I go outside, this current goes down to 2.1 mA. So the colar cells give me as low as 0.6-0.7 mA of current. I was hoping that this solar cell config would provide me with at least 20-30 mW when well lit (which it is!).

So I measured some signals to find out what went wrong. Apparently the boost charger is switching for a very short periods of time, most of the time it's idle. And I do not understand why as my battery voltage is well below OV threshold.

Here you see the boost switching node voltage:

And a zoom on an activity period:

 

Here is a solar panel voltage, showing also MPPT sampling:

Vbat and Vstor voltages:

VRDIV voltage:

VBAT_OV voltage:

And, finally, Vout:

What bothers me is that a voltage level on VBAT_OV pin goes up to 1.216 V. Does this mean that over voltage is detected? I've double-checked (on the board) the divider, it's calculated to trip at 4.1V, and i have 3.72 on Vbat. Any ideas?

Anyway, if somebody could explain that super low duty-cycle of the boost converter, please share!)

Thanks in advance for any input!

Best wishes,

Olex

  • 0
    Prodigy 65 points
    I am facing a similar issue Olexandr. Were you able to resolve the low charging current?
  • 0 in reply to Ajit Singh
    TI__Genius 12485 points

    Olexander, 

    Have you tried connecting the VINDC to a DC supply?

    Regards,

    Gautham

  • 0 in reply to Ajit Singh
    Prodigy 30 points

    Hello Ajit Singh,

    I think I've over-estimated the light intensity in my conditions. Since, I've tested the setup outside, with good weather, and the charging current is acceptable.

    Look at the datasheet of you solar cell and measure the real light flux. Just to be sure )

    Cheers,

    Olex

  • 0 in reply to Gautham
    Prodigy 30 points
    Hello Gautham,

    very good point!) Didn't think of that! Will try as soon as I'm back on this project.

    Thanks!

    Olex
  • 0 in reply to Gautham
    Prodigy 65 points

    Gautam,

    We have tried that as well. But it didnt work. My question is of a similar nature and details are at e2e.ti.com/.../502060:1230:0]

    Let me know anything else that can be tried. We have been able to achieve a max charging current of 3.2mA and tried all options - DC power supplies, solar cells and batteries.

  • 0
    Expert 1790 points
    In full, direct sunlight your three solar panels in series ( 12 cells at say 0.6V each) operates at say 4.5 volts, which is greater than your design over voltage threshold for your battery. So you don't need boost on the input side. In fact the BQ may be cutting off current from the solar cells because the voltage is too high? Isn't there a note about an internal shunt in the data sheet?

    More typically the input side boosts low voltage from solar cells (say in parallel) to a higher voltage on the battery. And the output side (the buck converter) boosts to an even higher voltage for your application.

    I could be wrong.
  • 0 in reply to lloyd konneker
    Mastermind 7560 points

    Hi,

    did you find the issue? I am having the same problem.

    I am using one solarpanel and it puts 100uA only but the battery gets charged with 10uA.

  • 0 in reply to Michael Liesenberg85
    Expert 1790 points
    ML: who are you asking? (This is not my problem, I was just offering a suggestion. I am not an EE.)

    For your situation, I think you need to compare power instead of current. If the solar cell is producing 100uA @ 0.6V, and you are charging the battery at 10uA at 6V, that seems reasonable (power in equals power out, ignoring conversion losses.) But you need real numbers, the harvester chip will hold the solar panel at MPPT. You can look up the MPPT voltage and current for given light conditions on its data sheet.

    (There might even be special procedures to measure the power of the solar panel while it is in circuit. Again, I am not a EE, this is just an idea for further research.)

    For the original post, I was suggesting that for many solar cells in series producing a high voltage, the harvester chip has some high voltage protection mechanism that might be kicking in.
  • 0 in reply to lloyd konneker
    Mastermind 7560 points
    Hi lloyd,

    you are rigth. So if i charge the battery with 4.1V and my input is 0.6 and 100uA, then my battery will be charged with 0.6/4.1 *100uA = 14.63uA
    That make sense.

    Thank you,

    Michael