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My goal (edited)
My goal is to charge only one powerbank from separate sources:
A bicycle dynamo at 5.7V after the rectifier. Outputs 4W at best.
A polycristaline 10W solar panel that outputs around 6V.
A monocrystaline 30W solar panel that outputs around 23V in the same sun conditions as the former panel (approx. 620 W/m2 on Windy at the time of measurement)

The dynamo's output is converted to DC with Schottky diodes bridge, 4700uF capacitor and a Zener diode to protect from high voltages when not under load.

The rationale is that charging only one powerbank, with the power loss that comes with this setup, is better than carrying one powerbank per source. I had been looking for a solution for a while and now I got this idea to parallel DC boost converters so the one USB controller that gets all the inputs sees more or less the same voltage.

Test setup
I have done a test setup with two MT3608 fed by separate powerbanks (around 5V input), Schottky diodes on both plus and minus terminals and a buck converter with a QC USB port. Also, there is a 4700uF capacitor on the output.

Test results
The efficiency is around 79% when connected in parallel, considering the voltage drop on the diodes. When tested in single mode, only MT3608 and the QC USB controller, wihout diodes, the efficiency is around 84%. I am expecting higher efficiency for the solar panel because it operates at higher voltages and there is no need to boost. Most likely will boost the dynamo to the voltage of the solar panel. The results were measured with ammeters on the inputs of the MT3608 modules and the output of the USB controller.

(It would be difficult to test and get measurements for the dynamo and solar panel)

My concerns/questions for you

  • Is this a viable solution for energy harvesting?

  • When running in dual mode, a hissing sound is audible. Is this something to worry about? I've read on the forum that the boosters might fail with time when connected in parallel. I've also seen a fail on YouTube, although the video is about a high voltage application.

  • What would be the outcome of using this setup on a dynamo and solar panel? I've read dynamos are considered to be current sources. Both have fluctuating power output.

  • I've conducted tests with two loads, one requiring 3.5W and the other 6W, both significantly lower than the powerbanks' capacity. In scenarios involving a dynamo or solar power, the power demand of the loads (charging a powerbank) would exceed that of the power source. Will this cause problems?

Parts: USB QC3.0 QC2.0 DC-DC Buck Converter MT3608 Boost Converter

Schematics of my test setup

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    \$\begingroup\$ Supplies need to be specifically designed to be paralleled. Those cheap Aliexpress converters are not designed for that, so if it's "working", you're getting lucky. \$\endgroup\$
    – vir
    Commented Mar 21 at 23:48
  • \$\begingroup\$ You need to have a specific goal so you know what voltage and current you need. Also as @vir mentioned, they need to match very close, or one that is dominant could overheat. \$\endgroup\$
    – Sparky256
    Commented Mar 22 at 2:54
  • \$\begingroup\$ parallel DC boost converters Is the intended interpretation parallel outputs, but separate inputs? \$\endgroup\$
    – greybeard
    Commented Mar 22 at 7:24
  • \$\begingroup\$ Yes, it is for separate inputs. One bicycle dynamo 6V around 4W, two portable solar panels of 10W and 30W \$\endgroup\$
    – dobre dani
    Commented Mar 22 at 7:46

1 Answer 1

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Is this a viable solution for energy harvesting?

When running in dual mode, a hissing sound is audible. Is this something to worry about?

You normally don't need to diode-OR the grounds/negatives for paralleling so you can short them directly. Paralleling (diode-ORing, in your case) positive rails is enough. But, as others have mentioned in the Comments section, the outputs should match quite precisely otherwise one will provide most of the power (Even for the converters designed for direct parallel operation, the mismatch is usually no worse than 1%) or will be overloaded. And in this case, you may hear some audible noise which quite possibly will come from the inductor (excessive vibration or magnetostriction).

To be honest, I wouldn't trust any of those cheap AliExpress converter modules and their specs. I would be extra sceptical when it comes to using those.

What would be the outcome of using this setup on a dynamo and solar panel? I've read dynamos are considered to be current sources. Both have fluctuating power output.

You may need a bridge rectifier and a large bulk capacitor before feeding the output to the DC-DC converter module(s) you use.

... the power demand of the loads (charging a powerbank) would exceed that of the power source. Will this cause problems?

Depends. This may result in a sudden stop, overheating or maybe a catastrophic failure. Difficult to answer, at least for me.

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  • \$\begingroup\$ He should use more powerful converters able to handle all the load since there are multiple source and some of these sources could yield nothing at a given time. So each converter should be able to handle all the load. The goal is not to share the load among converters. \$\endgroup\$
    – Fredled
    Commented Mar 22 at 9:21

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