My project is to take solar power from a 6w/6v panel, run it through a buck circuit tuned to 5v and charge one of the LiPo power banks. I perceive a buck circuit would do better at efficiency than a power regulator, though I could be wrong.

However, the power bank does not respond at all to the input. I've troubleshot the connectors, voltage all the way through up to the usb micro b male connector. You would be right to ask how can I be sure the micro plug is not the cause? When I hook this to a cell phone, it appears to be charging. (it may not be actually charging but at least it is receiving voltage).

Reading around on Google, I've tried tying the data wires together to short it out, and test it against known-good. In reading, the outer pins are for voltage but current is tied to the device. Since the power bank is not responding, current measurements are not shown but powering the cell phone shows 2.61 on the 20 scale. That's not a reasonable validation but this is to show that the current measurement does work.

So, what am I not understanding about the usb power requirements in reference to charging this device? thx, sam

  • \$\begingroup\$ "powering the cell phone shows 2.61 on the 20 scale" which unit? Volts? Amps? Measure the delivered power into a resistor and you'll be surprised how low it is. \$\endgroup\$ – pjc50 Mar 25 '15 at 15:57
  • \$\begingroup\$ Thx, using the Amps side of the multimeter. \$\endgroup\$ – sam452 Mar 25 '15 at 16:04
  • \$\begingroup\$ 2.61A or 2.61miliamps? \$\endgroup\$ – pjc50 Mar 25 '15 at 16:14
  • \$\begingroup\$ I would say 261mA but this is the reading itself. Happy to be corrected. flic.kr/p/qQTTGu \$\endgroup\$ – sam452 Mar 25 '15 at 17:09
  • \$\begingroup\$ That's definitely 2.61 miliamps. I suspect your problem is insufficient headroom for the buck converter and you might get better results with a buck/boost, but I'm not sure. \$\endgroup\$ – pjc50 Mar 26 '15 at 20:22

Usb battery packs have built in LIPO charging circuits, for 5V input to constant current charging the Lipo. They also have boost circuits for the 5v output.

Mainly, if you don't have a stable, 250 mA or higher, 5v +- 10% voltage source, then it won't charge properly.

Depending on the actual circuit it uses internally, it might have better tolerances, or even allow you to connect the solar panel directly.

That said your solar panel can only produce those 6W 6V at full sunlight.

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  • \$\begingroup\$ Thank you. Are you saying the buck circuit isn't going it at least 250mA? Am I wrong in reading Ohm's law that for full sunlight, 6w/6v = 1a = 1000ma? I must be wrong if I read your response correctly? \$\endgroup\$ – sam452 Mar 25 '15 at 15:00
  • \$\begingroup\$ 250 is a conservative low end figure. And you have to calculate for buck efficiency at 80 %. \$\endgroup\$ – Passerby Mar 25 '15 at 15:12
  • \$\begingroup\$ Thanks. At 80 % are you saying that 800mA is not enough? I would have thought 800mA is greater than 250mA. I hope you see why I'm puzzled? \$\endgroup\$ – sam452 Mar 25 '15 at 15:19
  • \$\begingroup\$ I'm saying the minimum input current to the battery pack should be 250 or higher. \$\endgroup\$ – Passerby Mar 25 '15 at 15:42
  • \$\begingroup\$ I accept your assertion. But please show me where my math is wrong? Isn't 800ma (nominal output of panel through buck) greater than 250mA input of the usb charger? \$\endgroup\$ – sam452 Mar 25 '15 at 19:04

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