Author Options:

Why won't my homemade phone charger work? Answered

I'm trying to make two projects. First I'm trying to charge my phone with 4 AA battery's connected in series. Second I have a solar panel I want to use to charge each of those AA's individually. Heres my plan as it stands. I hook up my solar panel to one rechargeable battery with a diode in-between so when its cloudy the battery doesn't put its juice into the solar panel. After I finish charging each battery I wire all four up in series and connect that directly with a charging chord for my phone. I defiantly do not understand this concept so help me were I go wrong. However I've run into some problems. When I cut the cord to my phone charger I found a string of wires under neath the first layer. after cutting deeper I found a red,black,green, and white wire. Assumeing the others didn't matter I attached the red and black wires to the positive and negative ends of my 5 volt emitting AA battery set up. My phone didn't register it was charging. After reconnecting all the wires to their original usb, and plugging that into the outlet the charger worked again. So my first question is why can't I just put voltage to the red and black wires and how do I make this work with my homemade battery pack. Secondly After hearing some awnsers I've become concerned that this AA set up will damage my phone if I don't have a device that cuts off the voltage when my phone is charged or when my batterys drain to low. How do I fix this easily and without having to buy special components. I have a beginners arduino set so I can spare any of the components in that. I,m sure I'll have more questions as I go along but this is it for now. Thanks so much for your input I need the help.



Best Answer 1 year ago

To charge your phone efficiently, figure out how much your phone draws from a real USB, using one of these handy things:


Then either find a 12V solar panel that can deliver at 30% more than the maximum power that the phone pulls, and hack a car battery charger into the circuit. You should add a reasonable capacitor to the output of the panels as well.

so lets see if I got this straight. So because my solar panel isn't plugged in to anything its saying on my multimeter it can do more than it really can. Good to know you saved me from a problem in the future thanks! Now here is the problem I'm having now. I have 4 fully charged AA's that I've wired in series so their putting out about 5 V I have directly connected this to my phone charger by conecting the batterys to the red and black wires. However even thought I have the correct amount of voltage my phone doesn't say its charging.

It's a VERY BAD IDEA to connect batteries directly to the USB of your phone. USB tolerance is 5%, 4.75V to 5.25V. Less than 4.75 you're phone might not charge at all. More than 5.25V, then you might burn out your phone!


Google "battery discharge curves" because as your batteries discharge, the voltage across them falls. Topped off NiMh cells will have as much as 1.45V output voltage (open circuit). 4 in series is enough to kill your phone. It then drops to around 1.3V pretty rapidly and slowly discharges down to 1.2V. Soon after it drops off like a brick wall to almost nothing.


Also the more current you pull from the battery, the lower the voltage will be for a given part in the discharge curve. (the whole curve shifts down, especially nearer the end of the graph) And this is the result of ESR, or Equivalent Series Resistance. It looks like a resistor in series with an ideal voltage source. The more current flows through this "resistance" the more the votlage sags.

is there a way I can make my current constent. I read that the more amps you have the faster the phone will charge so I would hope to keep it constant at around 2 amps.

Since you are working with sensitive electronics, I think you should learn the basics of electricity first. You don't want to fry your phone, hopefully you haven't yet. Don't worry, my 14 minute crash course videos will get you started!

Don't worry about being bad at math, focus on the concepts.


While there is such a thing as a Constant Current (CC) supply, which will deliver a certain current regardless of the load, this is NOT suitable for your phone. Your phone's circuitry is expecting a constant 5V voltage and will draw as much current as it needs. A CC supply will vary the output voltage to attempt to maintain a constant current. However all real world CC supplies will have a voltage limit. The technical name for this is "compliance voltage".


My recommendation:

Figure out how many watts the phone typically draws from a wall socket or car charger. (hint: it's NOT the maximum current rating the plug is rated for!) You can use a $10 USB test meter. (these are great to have around anyway!)

Gather all the car USB chargers you can find around the house, and test each one to find the one that outputs the pushes the most power into the phone. Your choice of USB cable can also affect current draw. Take note of the wattage with the best charger and cable.

[optional] If you want to select the most efficient one, you need to measure the input voltage and current going into the charger, calculate that power, and divide output power over input power. Do this while charging the phone. Or you could get a very rough idea by just feeling how warm the chargers get after several minutes of use if they are of simalar form factor / size. (warmer = more power loss.)

Then get a 12V solar panel capable of delivering about 20% to 50% more watts than that, or if your 4.5V solar panels are capable of that power, rewire them to output ~12V.

Connect that unregulated output to the terminals of a car charger. (center pin is positive, side contacts are ground or negative.) and your done! No need for a diode either. (note, I did not mention the batteries. You could improve this design by adding some, but that would greatly complicate the circuitry required. Put simply you would need some special charging IC's, and you would want to use a lithium ion cells for greatly better charging efficency.

The best solution to adding batteries as a buffer for when a cloud rolls over is to instead use a proper "power management IC" which can output a fixed voltage, charge a battery, and select which to use depending on the output from the solar panel. These are harder to find in breakout boards, and often are more costly. Certainly more costly than just buying a battery bank.

Ok thanks so how do I fix this and make my voltage constant at 5 v

"So because my solar panel isn't plugged in to anything its saying on my multimeter it can do more than it really can."

Ehh, sort of? Yes. The open circuit voltage is an almost useless parameter!

Imagine you have a mechanical windmill. If you have nothing connected to the shaft of the spinning blades, then the blades will rotate pretty fast. As soon as you "load down" the output (say you connect it to a corn grinder or something) then the blades will slow down due to the load. If you put too much load, then the blades might entirely stop, and the whole thing will stall.

Would different phones require different amounts of power and thus a bigger solar panel? Cuz it would be awesome to be able to use this for other peoples phones to.

Out of curiosity's sake whats happening in the car charger. Is it just taking the 12 V and making it 5 or is it smoothing out the voltage as well? Also what should i do with my little solar panel I want to use it for something.

That is so cool! Thanks for taking the time to awnser this question. I'm on instructables via personal hotspot so I can't stream the videos now or they would destroy my data but if I get some good wifi I'll watch them. Ok I'm going to try and condense what you said and you can tell me if I got it all.

1. I shouldn't use a battery pack unless I'm willing to tackle complex circuitry that will allow my voltage to remain constant so I don't fry my phone.

2. Instead I should invest in a 12 volt solar panel capable of delivering around 40% more power then I need so that even when its not the brightest day I can still charge my phone. Also it needs to be a 12 V so its compatible with the car charger

3. I then need to connect my solar panel to the car charger and plug my phone chord into the usb.

By the way thanks max for the help!

Also how would I truly figure out the real voltage my solar panel could maintain to charge my battery. I didn't buy this panel so I can't look up its stats and it doesnt say on the panel.

if I plugged my phone in and mesured the voltage going threw my usb chord would that give me the right reading cuz now it has a load?

I rewrote my question to clear thing up thanks for your input! : )


1 year ago

You need to understand how the unregulated output of a raw solar panel works. It will have 3 ratings typically, if not a graph.

Open-circuit voltage,

Short-circuit current,

ideal current / voltage (the right amount of loading that causes the panel to output the maximum amount of power.)


As you draw more and more current from your solar panel, the voltage will drop more and more. Take a look at those graphs. Notice when drawing no current, the voltage peaks out, but as you draw more current, the voltage gradually falls until you reach a maximum current? And anything higher than that, the voltage falls off a brick wall to 0v very sharply??



Your phone might initially see the 5.1V open circuit output, initiate charging at as much as 2A, and cause the voltage to plummet to 0V almost instantly. Then the phone thinks the USB was unplugged and the cycle might repeat for a few times. You might even get a notification that the charger is malfunctioning.

I think it is insufficient amps to charge because when you connect a load the voltage drops

Did you measure the battery voltage connected to the phone, or open circuit ?

Measure continuity from the ends of the wires to the pins of the plug.

Throw the cable in the bin, use a new cable of good quality and try again.
No use trying a cable that made problems before cut it...
For proper charger most phones also need the right voltage(s) on the data lines.
And just batteries are a bad idea, you need something to stabilise the output and make sure it cuts off once the batteries are going too low.

Ok so this is my entire idea. I found a solar panel and I hooked it up to my multimeter and it puts out up to 5 volts. I found an instructable that shows me how to use a solar panel to charge a AA and I found you could use 4 AA's to charge a phone. so I put the AA's in series after they have been individually charged and i chart my phone. When my rechargeable batterys are charging I can substitute normal batterys in and charge my phone that way. is their something I'm doing wrong?

Ok so I need to put my voltage to the other wires too and my AA's are the ones in series that are putting out 5.13 volts so sorry iceng for not making that clear but thanks!


1 year ago

I would wager that 5.13 voltage was open circuit...

If you plug a load into it the solar cell will drop in output voltage as soon as it is asked to deliver a current and you cannot charge...