I 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. I did not blow anything yet, thanks also to t


Summary of requirement:


  • 5V 5 Watt PV panel
  • Cellphone with "USB" "5V" charger input.
  • Single cell LiPo 3.7V nominal 2500 mAh.


  • Use PV to charge phone if connected.
  • Otherwise charge battery.
  • If possible share PV energy between phone and battery if both present.


Connecting the panel to the LiPo loads it to battery voltage which is too low a V to operate the phone charging circuit.

  • 2
    \$\begingroup\$ This is essentially unanswerable as doing so would require knowing the full details of each component. There is no such thing as a "2.5 A lipo", you are likely mistaking the capacity rating for a current rating. Nor is there really using a thing as a 5V solar panel, though there might be a device including both a solar panel and some sort of power converter. Generally, messing around with undocumented systems including a lithium battery is an extremely unwise idea. \$\endgroup\$ – Chris Stratton Jul 29 '17 at 18:32
  • \$\begingroup\$ Hi Chris for 2.5A I meant 2500mAh, I thought was the same. It's just a simple panel I purchase with tab wires, there is nothing in between and it seems work fine, charging the batt properly and charging the phone as well when separately connected. Unfortunately they are not working together, or I don't know how they could. The batt is protected by the pcb, if you are worry referring to the wire in flames accident, it was connecting the breadboard tiny cables to the panel, but they do not start flame, they just melt the external plastic, unable to take the current. Thanks anyway for your answer \$\endgroup\$ – Andrea Jul 29 '17 at 19:29
  • 3
    \$\begingroup\$ Hard to say what you have done without a schematic, but it sounds like a fire hazard. \$\endgroup\$ – Dmitry Grigoryev Aug 1 '17 at 9:32
  • \$\begingroup\$ Why fire azard if battery has a pcb and panrel could not generate more then 5v? \$\endgroup\$ – Andrea Aug 1 '17 at 17:29
  • \$\begingroup\$ re Fire hazard - using an in-battery protector as a charger is an invitation to trouble. They are meant to reduce the risk of major problems most of the time. Most may be less than 100% byt more than you ant. If the battery is meant to charge to 4.2VF the protector V_upper_limit MUST be > 4.2V to prevent interference with charging. 4.3V can be "too high ......" (smoke) and even 4.25V may be. Leaving a constant 4.2V on a charged cell will rapidly reduce its lifetime. eg 4.25V long enough MAY lead to plating out metallic Lithium. You really do not want to do that. \$\endgroup\$ – Russell McMahon Aug 6 '17 at 10:40

I connect a 5v 5w solar panel a usb port and a 3.7v 2.5Ah 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.

It is not so much a matter of energy availability (although that matters) as of voltage. The USB input phone charger requires voltage in about the 4.8 - 5.1V range to operate. The LiPo cell will load the PV panel to <= 4.2V OR be destroyed. If the battery has not caught fire or exploded yet it will prevent the phone charger working.

If you have a 5V 5W panel it will produce 1A in full sun. A 2500 mAh LiPo will usually have a safe maximum charging current of 2.5A so the PV panel will not provide excess current (probably). A single cell LiPo battery has an ABSOLUTE MAXIMUM voltage of 4.2V. As long as the battery is not fully charged the PV panel will probably niot damage it. Once the battery reaches 4.2V when charging it will need a progressively lower current to stop Vbat rising above 4.2V. If you connect a 5V and 1A capable source at this stage the battery will soon be damaged or destroyed.

Is there is a simple way to prioritize the USB or make them share?

Determine the lowest voltage at which the phone will still charge correctly.
Say it is 4.8V.
Choose some slightly higher minimum Vpv vpoltage - say 4.85V.
Use a comparator or opamp to control a switch (probably a MOSFET) between PV panel and LiPo such that the MOSFET is progressively turned off if the PV panel is loaded to beyond 4.85V.

Diagrammatic only. When Vpv is > 4.85V (and when scaled by R1 & R2 is greater than Vref) the FET Q1 is on and the LiPo (hopefully with a proper charger circuit) is supplied. If Vpv is loaded below 4.85 volts for amy reason (low sun, phone charger load, LiPo load) the FET is switched off. By adding finite gain to the comparator circuit the transitiin can be made linear rather than just on/off. If in linear mode Q1 will dissipate energy due to current and voltage difference. At maximum LiPo load, _FET = (Vpv - V_Lipo) ~= 2V worst case.
I_FET = 1A (PV max)
FET dissipation = V x I = 2 x 1 = 2 Watts.

Modest heatsinking needed.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Hi Russell, thanks for your detailed answer and sorry if I explained poorly. First let me say lipo has pcb, power control board, able to not over discharge, overcharge and keeping voltage to 4.2v max. Maybe is the reason the whole circuit seems running at that voltage. Mosfet is a great idea also because will stop reverse current (dark current) when panel is in shadow or is night. But I'm not sure it will help the phone getting enough current (in my unserstanding phone start charging even at 4.4v). Do you think a resistor connected to usb will be of any help? \$\endgroup\$ – Andrea Aug 1 '17 at 13:43
  • \$\begingroup\$ @Andrea The LiPo protection board is not a good way to control charging. It will eg not usually discinnect the battery when at 4.2V and charge current falls tro 25% or 10% or whatever of max. Floating a LiPo at 4.2V WILL destroy it rather rapidly. | IF the panel will charge the phone at say 4.8V then my method will allow sharing of the PVC energy as you requested. If the LiPo draws more current than the panel can support at 4.8V the voltage will fall under LiPo load and the MOSFET will reduce current to the LiPo to maintain 4.8V on panel. ... \$\endgroup\$ – Russell McMahon Aug 2 '17 at 11:15
  • \$\begingroup\$ @Andrea ... If the phone loads the panel to 4.7V oir 4.6V or 4.5V and still charges then in all these cases the LiPo will draw NO current as the FET cuts off at 4.8V as designed. So my system will supply energy to only the phone if phone load causes voltage to fall under 4.8V, and to phone and LiPo if combined load canm be supported at 4.8V or more. \$\endgroup\$ – Russell McMahon Aug 2 '17 at 11:16
  • \$\begingroup\$ Hi Russell, I will try your solution and let you know. Do you have in mind a specific type or shouldI chose one based on voltage and amperage listed in your analysis? \$\endgroup\$ – Andrea Aug 2 '17 at 13:38
  • \$\begingroup\$ @Andrea - Opamp - anything single supply and cheap will work. Old but cheap LM358 / LM324 OK. FET may need pullup from gate depending on overall voltages. | FET spec is low enough that many things will work. P Channel. Vds > 20V (20 30 40 50 ...). Ids > Imax. 2A oK but 5A 10A ... readiuly available at low cost. Vgsth well below available supplyvoltage. Say 2.5V max, lower better. Rdson lowerest that cheap allows. < 0.1 Ohm Ok but lower better. Many lowish cost 10-100 milliOhm Rdson FETs available. Suggest a FET and I/we will comment. \$\endgroup\$ – Russell McMahon Aug 2 '17 at 22:00

You need to use a module like this one :https://www.elecrow.com/wiki/index.php?title=Mini_solar_Lipo_Charger_v1.0 output for your phone.

which provide a boost in order to provide 5V

  • \$\begingroup\$ Hi Crdric, thanks for your help, I will have a look. Thanks again \$\endgroup\$ – Andrea Oct 28 '17 at 8:15

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.