Emergency Phone Charger

Rob Bell & Daniel Koch

Issue 4, October 2017

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What started as a quest to debunk a "lemons charging phones" video, turned into something far more practical.

When life gives you lemons, you make a... phone charger? WAIT - that’s not how that goes..

These days, mobile phone charging is rarely a problem when we have dozens of USB chargers, computers with USB ports, and various other ways to access power. But when the weather turns wild and the mains power shuts off, there’s often a mad scramble for the nearest working chargers, to get the phones up and running again.

Recently we’ve seen a number of videos making their way around the internet regarding the theory of charging a phone using lemons. Some articles even landed on respected news sites, so they’re convincing enough for sure. However, to us electrically-minded folk, at face-value it’s difficult to believe this is possible; though it does beg the question - just how much “juice” can we get out of a lemon?

The theory here is fairly straight forward, but still warranted some investigation. The great thing about this exploration is that it yields some experiments that are easily undertaken at home, which are suitable for just about any age. We’ll admit too, we got a little ahead of ourselves; on the surface, using these lemons like batteries we should be able to stack a few together to get something useful.

It’s just feasible enough to be believable, right? Well... let’s take a look.

Note: If you want to skip to the end-project, where you'll find a useful emergency phone charger, we understand.


In reality, getting power from a piece of fruit isn’t all that difficult. The old potato clock experiment continues to be a staple in electronics theory and experimentation. However, just because you can run a small LCD from a single potato for a substantial amount of time, doesn’t mean you can power substantial electronics.

Parts Required:
Lemons (we used up to 10)
Zinc Galvanised Nails
Copper Nails or Rod
Alligator Leads


It can be difficult to find copper nails these days. Copper corrodes easily, and is relatively expensive, so mild steel is generally preferred for common nail use. However, it’s quite easy to find copper rod. It might be in the form of copper welding rod, or even copper plated steel. We have used copper-plated welding rod. If you’re using something similar, cut the rod into lengths of approximately 10cm long. The overall length doesn’t make a whole lot of difference, providing they can be pushed all the way into the lemons, and leave room to attach your alligator leads.


Using a single lemon, insert one of the copper electrodes, and one of the zinc-plated nails. Use your multimeter to test the voltage and short circuit current of your lemon battery.

Our tests indicated between 0.5V and 1V. From a single lemon, we managed to produce a current of just 0.17mA. These figures vary with the lemon, the quality of the electrodes, and other such factors, but really the difference is insignificant.

testing voltage

Consider a “standard” minimum power USB outlet, which is capped at 500mA of current. At 5V, that’s a mere 2.5W of power. Not really a lot, but our lemon is producing just 0.000085W of power.

When you put that in perspective it means our USB port produces around 30,000 times the power of a single lemon; and we’re not even talking about newer, high current USB ports!


Now we’ll use series wiring to try and boost voltage. This is really no different to wiring batteries in series.

When you test the voltage of the bank, you should see approximately 10 times the voltage you measured in Experiment 1. For our lemons, this meant just under 5V. The problem is, there is barely 0.5mA of current available. No, not 0.5A, 0.5mA... that’s about 1% of the current available on an old-school USB port (or 0.2% of a higher current USB port). This presents a massive problem; we simply don’t have the power available. The potential simply isn’t there.

boost voltage

With 10 lemons we can illuminate a single, standard 5mm LED. That’s hardly mind-blowing, working out to around 0.00085W of power.


As with all electronics and experiments, there are variables.

ELECTRODES: As with a battery, the surface area of the electrodes does make a difference. We saw a marked improvement in current available when using our oversized electrodes, with an increase in voltage per lemon, as well as an improvement in short circuit current.

ROLL-FIRST: Lemons aren’t just a bag of juice. They have internal membranes between each segment. Rolling them with a firm pressure helps disrupt these internal membranes and provides better consistency in the resulting voltage and current output.

TEMPERATURE: Temperature plays a role in all electrical circuits - even fruit-based ones.

SEASON: The season and ripeness of a fruit will change the electrolyte potency, and therefore the power available.

However... none of these variables are going to provide enough of a change to give us usable power to charge a battery with.


We could mess about with different configurations, track down better electrodes to improve performance, but ultimately this solution really just doesn’t provide enough electrical power to be useful for phone charging. Based on the results of this experiment, it would take thousands of lemons to create 500mA at 5V, which is around the power you would need to see sensible charging rates for your phone. The videos reportedly achieving this using only a handful of lemons are obviously fake; and that’s all there is to it.


What you’ll discover through this process is, while it’s true you can quite easily obtain the 5V required to theoretically provide power, there is a serious lack of current available. Despite the relatively convincing videos doing the rounds on social media, it’s fairly safe to say they are fake. They often, conveniently, don’t show the entirety of the cables being used in the circuits.

In order to get enough power to charge a phone, you would, need a truckload (literally) of lemons. Around 1000 lemons in a series or parallel combination would be required to produce the most basic of charging currents.

So no, lemons are not a feasible way to charge your phone.

HOWEVER; is there a simple way we can build an emergency phone charger? You bet!

testing voltage


Despite our experiment, we aren’t about to let you down in terms of being able to charge your phone when little-to-no power is available! An emergency power source for your mobile phone might just save your life, or - more likely - at least make life a little easier when the mains power goes out. Of course, there are plenty of rechargeable power banks that can be used for this purpose, but most use some sort of lithium cell which you’ll need to leave recharging, or else it will probably be flat when you need it. With the long shelf-life of lithium batteries (indeed some are sold as “guaranteed long life”), we have a sensible source of power that’s shelf-stable, and provides enough power to get us out of trouble in an emergency.


When first considering this purpose, a DC-DC boost module seems like a simple solution. It will take a 3V source from two batteries, and gives you 5V output right? Technically, that’s correct, and you can power an Arduino project from it with no problems, for instance. However, Apple - in their infinite wisdom - decided that applying power wasn’t good enough; so charging a phone purely from 5V isn’t possible. You connect the power, and the phone ignores you.

As mobile phones evolved, lithium batteries grew in capacity. The old standard of 500mA charge-current quickly wasn’t considered fast enough, and charge-rates quickly escalated. However, that meant the charge controllers within the devices needed to become a little more intelligent. While some phone manufacturers don’t make it too difficult, Apple notoriously had specific requirements on the data pins of the USB socket in order for the phone to charge. For this reason, a number of USB outlets that are still available on the market won’t actually charge your phone. So frustrating!

Fortunately, the secrets behind why this occurred became not-so-secret in a relatively short space of time, so the information is easy to find. Effectively a voltage-divider circuit is required on the data pins. The voltage divider tells the phone what power is available. While newer iPhones might be able to charge at a faster rate, our DC-DC boost module is only rated at 500mA, so we can appropriately configure the data pins accordingly. Regardless of how fast your phone can charge, this is designed to run at 500mA, which will still get any phone switched on and charging in a short space of time.

NOTE: We have tested this on various iOS devices and it works well. We also connected a few Samsung devices and it appeared to work fine with them too. You might find however, that your particular device is picky about these things; 500mA configuration appears to be relatively universal however, and should suit an overwhelming majority of devices.


Parts Required: Jaycar Altronics
1 x 5VDC Boost Module XC4512 Z6366
1 x 2xAA Battery Holder PH9202 S5025A
2 x AA Lithium Batteries SB2355 S4906
1 x 75kΩ 0.5W Resistors RR0617 R7603
2 x 51kΩ 0.5W Resistor RR0613 R7599
1 x 43kΩ 0.5W Resistor RR0611 R7597
1 x Slide Switch ST0300 S2010
2 x M2 Screws HP0390 H3100
1 x PCB Mount USB A Socket (optional) PS0916 P1300
1 x 3D Printable Case (optional)--


Before we go randomly soldering up to our DC-DC converter module, it’s prudent to prototype the data pin resistors just to check things are working as expected.

Using a breadboard, simply follow the diagram below. It’s quite straight forward and will only take a minute or two.

For clarity and convenience, the breadboard provides USB Ground, Data +, Data -, and Vcc, all in the same line as they appear in the USB connector. This simply makes life easier for ourselves than having them in random spots. Now apply a 5V power source to the breadboard, which can be from a spare Arduino or DC supply you have. Using your multimeter, test each of the four pins to ensure you have the correct voltages. Obviously pin 4 is your ground, while Data + should have approximately 2.1V relative to ground, and Data - should have approximately 2.7V relative to ground. We haven’t tested the limits of where this will fail to work on a phone, but it definitely allows for tolerances with the resistors used, and we didn’t encounter any problems.

IMPORTANT NOTE: It's critically important that you measure the voltages on all pins of the USB port. While the 5VDC module takes care of the heavy lifting, incorrect voltages on the data pins of the USB outlet could damage the device you're charging.

If you have a USB A socket, you can connect each of the pins to the pins on your breadboard, and connect a USB charging cable to your phone. It’s easy to flip the connector and get your wiring backwards so be sure to check that you have things around the correct way. If your phone doesn’t show charging, check your voltages again, to ensure you have the voltages on their respective data pins, and that you haven’t wired the USB connector upside down.


It’s entirely possible to use this unit without a case, but we want to make it into a finished unit that’s easy to use, and could sit packed into your emergency kit, glove compartment, or anywhere else. Therefore, we have designed a 3D printable case to house the entire project, and you can find the STL file in the downloadable resources. We basically build the circuit into the case, so it's prudent to print it first.

This case houses the DC module, two AA batteries, and provides a mount for a small switch. We have used a semi-translucent PLA filament to print our case too, so we can also see the small LED on the DC-DC module to indicate power on, without any additional electronics.


5VDC module

The prototype is great to test the theory, but isn’t terribly useful past that point. Now that we know it’ll charge the phone, let’s make a real unit! Since, at the heart of this unit there are only four resistors, we decided it’s best to just solder them to the base of the DC-DC converter, where the USB socket legs come through the PCB. This means we don’t need a PCB, don’t have wires everywhere, and it will keep it all neat and tidy. There is limited space under the USB socket, so we decided it’s best to spread the connection points, as this allows us to reduce the risk of resistor legs accidentally shorting each other out. Once it’s completed and tested, we’ll coat it in some superglue, liquid electrical tape, or some other method to encapsulate what we’ve done, to reduce the likelihood of any shorting or problems down the road. You can see the diagram of the connections above, as well as a photo of our prototype below.

battery insert

Once everything is soldered up, it’s a good time to test things. Provide 3V to your DC-DC module (alligator clips and your battery holder will usually work just fine). You should see the on-board LED illuminate to indicate power. Now, grab your multimeter and check your voltages read as previously noted. If your module features an LED but it doesn’t illuminate, it could be a sign of a shorted connection.

If your voltages check out to be reasonably accurate (we’d say, within about 0.1V of what we’ve described), then you should be safe to connect your phone and see if it charges. Grab your favourite USB charging cable. Connect it to your phone, and the USB socket. You should see your phone’s charging icon appear. If so, IT WORKS!

IMPORTANT NOTE: You're connecting power to an expensive, sensitive device. Be sure to check things first. Most phones are intelligent enough to prevent any major issues, but don't risk it. Double check the voltages on the USB socket twice, the check them again.

It’s important to mention that if you’re using an old phone (good idea if you have access to one) that has been sitting in an discharged state for some time, it can take some time for the battery to have sufficient charge that the phone will turn on. Many phones such as an iPhone show you that it’s charging at least, even while it’s switched off. But just keep this in mind, to save yourself from chasing a problem that doesn’t exist.

Mounting is fairly self-explanatory. Mount the battery holder with double-sided tape (though it’s not really going anywhere). In order to make use of the switch, wire the positive lead from the battery holder through the switch, to the DC-DC module. Because everything is in such close proximity, we didn’t have to use any additional wire, we just used the excess from the battery terminal’s flylead. Then wire the negative wire from the battery hold, directly to the DC-DC module.

NOTE: We realised after some frustrations that the silkscreen on the under-side of our DC-DC module is backwards to what is marked on the top-side. Do your own investigation (even just trial and error) to ensure your silkscreen is the correct way around, as we’re confident some of you will have the same module as we do.

battery insert
Note: You do need to use a charging cable, this image is just for scale.


Really, this is a complete unit, which is extremely simple to create. It would be possible to integrate some battery monitoring, a torch, or any other number of other features. For this purpose of providing emergency power however, we really wanted to keep the design as robust as possible. You could add a lemon-interface, but that’s just crazy...!

It’s also important to remember that this is designed as an emergency device. It’s simply not practical to try and charge your phone from AA batteries all the time. The battery in your phone will leave a trail of AA batteries in its wake if you constantly try and recharge from them. But it should be enough to get your phone going in an emergency situation.