I am new to electronics, please don't mind mistakesenter image description here Right now i have a 5v solar panel, and a 3.7v battery,and i plan on buying step up converter from 0-9-5v to 5v. I have an iphone 6 with the new lightning usb, and i read that you need a special usb step up converter that also provides something to the data pins on the usb. i'm also not sure how a 5v solar panel should charge a 3.7v battery. Also, an iphone needs 1.2A i think, and i dont know how to check what amperage my solarcell/batteries/converter have.

battery: https://www.123accu.nl/123accu-18650-batterij-1-stuk-3-7V-2600mAh-i26531.html but with 2000mAh instead of 2600

i could use a battery pack with rechargeable aa's, but those don't hold a lot of power and i don't own any. But if i bought some, how does it work with solar panel voltage charging the combined battery voltage?

EDIT: fixed battery link


2 Answers 2


What you need to do is to buy both step up and step down converters. During the day, you use a step down converter to charge battery from your 5V solar panel. When you need to charge your phone, you use the step up converter to convert 3.7V up to 5V.

There are couple issues you need to watch out. First, you need to find proper step down converter. Most step down converters are regulator (fixed output). They are OK, but not ideal because the voltage of battery changes after ages and you have no control of how much power goes in battery. It may cause damage of the battery and inefficient charge. Then, when you buy your step up converter, making sure they are capable of 2 amp. You don't need to measure the current since you have a fixed load (basic physics 101, fix voltage and fixed load gives you fixed current) Also, using AA battery not gonna fly because they output power of them are too low and the capacity of them is very small. You need something having output power 10 to 15W (3.7V with 3A at least).

There are also other aspects I didn't cover. You need to do a lot more research.

  • \$\begingroup\$ so i have find a 5v to 3.7 regulator? \$\endgroup\$ Jan 16, 2016 at 22:53
  • \$\begingroup\$ Not necessarly the boost converter needs to supply 2A if it is made to latest standards as phone charging device. The resistors on USB data bus have the function of coding the capability of the charger, so the device would know the maximal supply current. \$\endgroup\$ Jan 16, 2016 at 22:54
  • \$\begingroup\$ i know this sounds really dumb, but could you make a shematic of what you mean Anke? i'm new to electronics and its hard to follow \$\endgroup\$ Jan 16, 2016 at 22:57
  • \$\begingroup\$ Schematic is for actual circuits, I guess you want a block diagram. The one Marko provide is good. The battery charger is essentially a buck (stepdown) converter. \$\endgroup\$
    – Anke Ge
    Jan 17, 2016 at 4:22
  • \$\begingroup\$ Also, you don't need a diode to prevent back flow because the way converter designed only allow power flow one direction(unless it is a buck-boost). And, I think you and Marko both are confused on some terminology. Buck is step down converter which convert high voltage to low voltage. Boost is step up converter which convert low voltage to higher voltage. Your solar panel has higher voltage than battery, so you need to convert it to lower voltage, then you use a buck(step down) converter. \$\endgroup\$
    – Anke Ge
    Jan 17, 2016 at 4:32

The picture you provided is wrong in terns of diode polarity and the connection of battery (parallel), also the use of step up converter isn't clear. The best solution is to use a panel with higher voltage (or connecting two of them in parallel) and use of buck (step down) converter to 5V -> USB charger, then your iPhone does the rest, as it has more sophisticated battery charger built in. You can also look for a fixed output 5V with overvoltage protection circuit. One example: http://www.ebay.com/itm/USB-DC-Converter-5V-3A-Charger-Non-isolated-Buck-Module-w-Indicator-For-Phone-/181888568212?hash=item2a59674b94:g:qnkAAOSwFnFWC0gu
Keep in mind that newer phones use 5V/2A charger, that's pretty big solar panel needed.

EDIT_1: I see now. The link from batteries doesn work. The diode is inverted, then you need as you thought a step up converter usb phone charger, possibly with a switch, so it will not drain battery while charging. Also a overcharging protection circuit is needed for the battery, as it can explode. Preferably it would be a nice solution to have all in one PCB. Something like this: https://www.adafruit.com/products/1944 unfortunately there is no valuable data to understand if it meats your requirement.

Altrough Adafruit does fancy things, you can search the ebay or whatevr to find low cost stuff: http://www.ebay.com/sch/i.html?_from=R40&_trksid=p2047675.m570.l1313.TR0.TRC0.H0.XTP5100+2+Lithium+Charger+Module+.TRS0&_nkw=TP5100+2+Lithium+Charger+Module+&_sacat=0 this is battery charger, still you will need higher voltage that 5v to charge

http://www.ebay.com/sch/i.html?_odkw=3.7V++converter+usb+phone+charger&_osacat=0&_from=R40&_trksid=p2045573.m570.l1313.TR0.TRC0.H0.Xli+po+boost++converter+usb+phone+charger.TRS0&_nkw=li+po+boost++converter+usb+phone+charger&_sacat=0 step up converter


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Thanks a lot!!! what is meant (on the battery chargers ebay page) with 2 cells? oh and how much volts would i need from the panels? edit: the ebay page says 5-18v input range, wouldnt my 5v panel work? also wouldn't the 4.2v charge be too much for my 3.7v battery? \$\endgroup\$ Jan 16, 2016 at 23:03
  • \$\begingroup\$ You have to try it, I don't know what panel you use, so at higher latitude maybe you won't never get 5V. As for the chemistry of your battery, I don't know because the link you provided is broken. \$\endgroup\$ Jan 16, 2016 at 23:27
  • \$\begingroup\$ link is fixed, also, shouldnt there be a diode to prevent backflow from the battery to the battery charger? \$\endgroup\$ Jan 16, 2016 at 23:40
  • \$\begingroup\$ also which step up that you linked should i use? the buck one seems better \$\endgroup\$ Jan 16, 2016 at 23:54

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