connect a 5v 5w solar panel a usb port and a 3.7v 2.5A lipo with pcb. If I connect a phone to usb port it doesn't charge, but if I remove the battery from the circuit, it does. I suppose it's because battery pcb downgrade the voltage to 4.2v or there is not enough energy to serve both. There is a simple way to prioritize the usb or make them share? Thanks in advance for your help

PS Sorry to eventual mistakes but I'm still learning electronics.

  • \$\begingroup\$ Current sharing needs dual current sense and arbiter logic to limit charge current with SMPS DC-DC so no simple solution \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jul 30 '17 at 15:28
  • \$\begingroup\$ Hi Brian Hi Sachin. Reason for duplicating was Chris said question is ananswarable and I thought this will let people think the question is close. But of course is not and prove is this new one receive several interesting input and the other no one. \$\endgroup\$ – Andrea Jul 31 '17 at 16:28
  • \$\begingroup\$ Hi Tony, thanks for the clarification, a resistor attached to usb pirrwon't attract current to it? And when phone is not connected current will charge battery \$\endgroup\$ – Andrea Jul 31 '17 at 17:08

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You can design your circuit this way. You also should limit the current your solar panel to the battery.

  • \$\begingroup\$ Hi Jason, my problem is everything is connected in usb block, not sure how separate. Do you why it happen? Is the battery pcb to lower the current to 4.2v as is voltage needed to charge or is the quantity of curreny available that is not enought for both and for a reason battery get the priority? Or they are conflicting tougher for a reason I could explain? Thanks for your help \$\endgroup\$ – Andrea Jul 31 '17 at 16:28
  • \$\begingroup\$ Lipo batteries need 4.2V to charge. It seems you're drawing too much current from the solar panel. Or did you connect wrongly? Do you have a picture to show? \$\endgroup\$ – Jason Han Aug 1 '17 at 1:19

You need a load sharing circuit that switches between PV-Cell and Battery when needed. The MCP73871 does this job pretty good. But anyway, to extract energy from a PV-Cell you should consider a MPPT IC.

  • \$\begingroup\$ Thanks S.G. But why then plain usb port is working when alone with solar panel? I though is possible to have on the same circuit a usb port and a battery. Still unclear to me why they are conflicting. If I add a mppt current won't flow from panel to phone, that is what I would like to achive, and what is working without the battery. If I add a resistor to the usb port will not help to get the priority? I apologies in advance if it's a stupid suggestion. \$\endgroup\$ – Andrea Jul 31 '17 at 16:42
  • \$\begingroup\$ It is hard to say without a schematic or measured values. Maybe the battery is pulled down by the mobile phone but i can't say without any information. Btw, why do you think current won't flow if you use a MPPT DC-Dc converter?Especially when the light condition changes the MPPT converter helps you to extract max energy from the solar panel. But as I said before: You need a load sharing circuit that does the prioritizing for you like the MCP73871. \$\endgroup\$ – S.G Jul 31 '17 at 16:56
  • \$\begingroup\$ I already try it, mpps is working but I would like to have just the usb, and I would like to understand why is not enough. \$\endgroup\$ – Andrea Aug 2 '17 at 15:11
  • \$\begingroup\$ how did you realize MPP? If it is working, why dont you want to use it? \$\endgroup\$ – S.G Aug 2 '17 at 15:43
  • \$\begingroup\$ Thanks again S.G. for your help, I mean I already usb pcb with mmpt and they are works fine but there is not a direct flow of the panel current to the phone, they change voltage to the fixed required. And also in this exercise I would like to keep it simple as possible, with just an usb port. What I'm looking is not an alternative but solve the mistery why without lipo system is working and how to fix it. \$\endgroup\$ – Andrea Aug 3 '17 at 10:00

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