Let's say I somehow got transported to medieval times. I have my phone, laptop or other devices with rechargeable batteries, but I forgot to take my solar chargers. Luckily, my every device is powered by standard micro USB cable, which I happen to have with me. I know that USB uses 5V direct current, from 0.5A to 2A.

To create current, I can use magnets and copper wires, which are luckily sold by local merchants (wires) and magicians (magnets). This will, however, give me alternating current. Using some diodes I could convert it, but semiconductors aren't readily available in my local medieval shop.

Next problem is, I need very specific and stable voltage. Definition of ampere won't help me unless I get two wires of infinite length.

So question is: How can I create a stable current source to charge my electronic devices without exploding the batteries?

This is related to question asked in I was thrown into the middle ages, how do I power my time machine? , but answers there don't seem to say much about voltages used. Phone batteries might need pretty specific voltage, and I'm mainly interested how I can either check the voltage of my source or build one having stable 5V (or some other that won't destroy my batteries).

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    $\begingroup$ "To create current I can use magnets and copper wires. [...] This will however give me alternating current": Why will it give you alternating current? You know how to make an AC generator but you don't know how to make a dynamo? What kind of electrotechnical engineer are you? $\endgroup$
    – AlexP
    Dec 14, 2017 at 9:45
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    $\begingroup$ "What kind of electrotechnical engineer are you?" The kind that gets sent to medieval world and forgets what powers the bikes d; Dynamo seems good idea, but getting right voltage and keeping it stable might be a problem, unless i get some capacitors (paper and tin foil, but do they have it in my local medieval shop?) $\endgroup$
    – crueltear
    Dec 14, 2017 at 9:54
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    $\begingroup$ Forget about your phone, medieval times are known for their very bad cellular networks and GPS satellites are still waiting for their launchers. $\endgroup$
    – mouviciel
    Dec 14, 2017 at 9:58
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    $\begingroup$ @mouviciel: Luckily for me, I'm more interested in portable light source, casual games for playing when locked in dungeon, and ability to emit disembodied voices, which strangely seem to scare medieval people. $\endgroup$
    – crueltear
    Dec 14, 2017 at 10:07
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    $\begingroup$ Related question: worldbuilding.stackexchange.com/questions/74991/… This can be used for guidance. $\endgroup$
    – a4android
    Dec 14, 2017 at 12:13

15 Answers 15


Maybe use -

A Galavanic cell , which uses chemicals which are available during medieval times .


This electrolytes in this case is zinc sulfate and copper sulfate , but it can be also any other thing which has ions in it like maybe urine .

Correct Voltage maybe obtained by trial and error method by varying the concentration of the electrolyte .

We can calculate the potential using half cell reactions based on the material used -


For 5Volt we can use -

Ba(OH)2 + 2 e− ⇌ Ba + 2 OH− (−2.9v)


F2(g) + 2 H+ + 2 e− ⇌ 2 HF(aq) (+3.05)

resulting would be around 5V.

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    $\begingroup$ The trouble with LiPo batteries and trial and error is that error tends to explode. $\endgroup$
    – Separatrix
    Dec 14, 2017 at 9:49
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    $\begingroup$ @Separatrix I see no mention of LiPo batteries in this answer. Yes, you'd be using the galvanic cell as a voltage source to charge a LiPo or LiIon battery, but that still has the battery's own protection systems in line with the charging current. Shouldn't be a problem if you get the voltage close enough to meet the USB 5V spec. $\endgroup$
    – user
    Dec 14, 2017 at 10:28
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    $\begingroup$ @crueltear Lithium-based chemistry batteries have far more elaborate charging protections than a simple fuse. Electrical Engineering has a question Why is there so much fear surrounding LiPo batteries? the answers to which would be relevant here. $\endgroup$
    – user
    Dec 14, 2017 at 11:50
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    $\begingroup$ @crueltear As AlexP says, you can rely on the chemistry to some extent. The easiest would be a lead-acid battery - lead and oil of vitriol (sulphuric acid) were both readily available. With no load, the voltage on a lead-acid cell will always be close to 2.1V. $\endgroup$
    – Graham
    Dec 14, 2017 at 12:56
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    $\begingroup$ @KareemElashmawy For complicated stuff, yes. But lead-acid is astonishingly easy, and medieval alchemists were perfectly familiar with producing both to a fairly decent standard. $\endgroup$
    – Graham
    Dec 14, 2017 at 21:19

It looks like your primary problem is measuring voltage accurately which is tricky with only medieval resources at your disposal.

The actual construction of an analog voltmeter is actually not too hard, if you have measured components. If you look at the Wikipedia page for Voltmeter you can see that you can make an analog voltmeter by just adding a resistor to a galvanometer, which is essentially a compass.

You can then calibrate the voltmeter with you phone's battery, hopefully you can remember what the voltage is and that it can be taken out.

Your biggest problem here is finding a resistor of a known resistance. You'll probably need to cannibalize some of your own electronics to find one, because you'll need to know the resistance pretty well if you want to get an accurate volt reading.

  • 7
    $\begingroup$ "Your biggest problem here is finding a resistor of a known resistance." How about headphones with known impedance? If someone gets transported to medieval times with phone, it's likely he'll have them - remembering impedance however is most likely audiophile thing. Overall, this answer seems really nice. $\endgroup$
    – crueltear
    Dec 14, 2017 at 11:50
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    $\begingroup$ I don't think you need a known resistor. If the known good 5v source causes the improvised voltmeter's needle to deflect 30 degrees or 10.5 degrees it doesn't matter, you mark where it deflected to and combine cells until you get past it then make a voltage divider from any centertapable resistor (like an iron bar or even a wet rope) to get to the same deflection. However the 5v source isn't straight from the battery, that almost certainly has some oddball voltage that gets regulated to 5v. $\endgroup$
    – user25818
    Dec 14, 2017 at 18:08
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    $\begingroup$ Voltage is fixed for given chemicals (at typical concentrations). If you knew which chemicals made which voltages, there would be no guesswork (at least the error would be insignificant) $\endgroup$
    – Bohemian
    Dec 15, 2017 at 16:20
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    $\begingroup$ Carbon (ashes, charcoal) can be used to make resistors - though not calibrated ones :) $\endgroup$ Dec 16, 2017 at 15:59
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    $\begingroup$ @crueltear impedance is effective resistance to a alternating current. It is possible for a circuit to have high impedance but little resistance (a coil of wire), or low Impedance but infinite resistance (a capacitor). Headphones can fit either category, depending on the specific piece of equipment. $\endgroup$
    – pojo-guy
    Dec 16, 2017 at 16:55

The thing you need is a Voltaic pile. It's very simple and you can make it out of any two different metals that you have available. Copper and zinc would be best but copper & iron will work. The advantage is that you don't need to be able to measure the voltage - just add more layers until the charging light comes on your phone.


Depends where you are. If you're in Southern Italy you can probably get hold of lemons. Add copper and iron nails and you have a simple cell. Chain them in parallel or series till you get the required voltage & current.

  • $\begingroup$ Welcome to WorldBuilding! If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$
    – Secespitus
    Dec 14, 2017 at 23:34
  • $\begingroup$ The question is not just about getting a power source, that's already assumed to exist. How do you know the voltage and current of each cell or of a given battery? $\endgroup$
    – Nij
    Dec 15, 2017 at 1:18
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    $\begingroup$ When life gives you lemons... make a battery. $\endgroup$
    – uhoh
    Dec 15, 2017 at 12:41
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    $\begingroup$ That's going to be a lot of lemons. But the problem then is you need a lot of current. $\endgroup$
    – Rob Rose
    Dec 15, 2017 at 16:40

Unless you are an electrical engineer, you don't.

There are too many smart en creative engineers between you and the medieval tech you find yourself in for one man (or woman) without the right knowledge to create the right electricity (voltage & amperage, etc).

If you take the path of chemicals, you will have to work with acids. Good luck and have fun! Corrosive stuff, protect your skin and eyes. But you don't have the modern gear and probably don't know how to make them. How to game without eyes?

Water wheel / windmill is safer(ish). Big moving parts that can crush you with ease. Remember, no workplace safety guidelines! Gaming with missing fingers can be interesting, I'll grant you that. And to get the right voltages is no mean feat. And you have very little or no way of checking the current.

And that leads me to power spikes. Your modern gear trusts its power source quite a bit. And the medieval power source you created is anything but stable. Prepare to damage, short circuit or otherwise kill your beloved gear.

Then what to do? Turn off your gear, and try to get back to a time where you have a stable power supply. Maybe turn up at Alexander Graham Bell's door, he might be able to help you. Tesla will work for sure.


If you're in a medieval fantasy setting with magic, you might be able to use necromancy magic to turn your dead battery into an undead battery, and power your phone that way.

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    $\begingroup$ "Everybody knows magic and electricity do not work together." -- loosely quoted from a Harry Potter book, Hermione probably $\endgroup$ Dec 18, 2017 at 14:05

A LiPo/LiIon battery is never ACTUALLY discharged to 0V (this would leave it beyond repair) - your discharged battery (if you can access the battery terminals directly) can be used as a voltage reference of close to 3V per cell, so you can compare other voltage sources (eg by a bridge circuit - all you need is SOME voltage indicator (headphones!) and some changeable resistor (eg carbon)).

Also, so called "charger" plugpacks are almost never the actual charger - which is in the phone/laptop, and which will take care of handling the battery correctly unless you feed it GROSSLY out of range voltages.


Following on from AmruthA's answer, you are better building a voltaic pile, rather than two half-cells in jars. These are stacks of zinc-felt-copper discs, with the felt is soaked in brine. A diagram of them looks like this https://upload.wikimedia.org/wikipedia/commons/0/06/Voltaic_pile.svg (stackexchange isn't letting me upload the image).

A practical pile is placed between two large wooden plates, a bit like an old-fashioned egg-timer. The pile has to be pushed together to make sure that the discs are in close contact, but without enough force to expel the brine from the felt.They look like this https://upload.wikimedia.org/wikipedia/commons/5/54/VoltaBattery.JPG

Zinc was purified by 1300 in India, but pure zinc wasn't exported to Europe until 1600. A zinc-copper pile produces about 0.7V per cell. In the absence of zinc, you could use copper-tin, but that produces a considerably lower voltage, about 0.13V per cell. However, tin was readily mined in Cornwall (SW England).


You can construct a voltaic pile, an early battery. All you need is copper, zinc, and salt water, all of which can be found in medieval times. Each copper/zinc/salt water soaked rag produces .75 volts. Put seven together in series, and you have 5.25 volts, enough to charge the phone... (they charge using 5 volts DC through the USB port).

Of course, if you're in medieval times, you'd also have to invent a cellular network and maybe GPS satellites (plus a way to get them into orbit), or the phone won't be of much use. That might prove to be a bit more difficult...


Charging your phone is easy, meeting your specific requirements is close to impossible

How to generate percise and stable voltage

There are a lot of ways to create some current but the easiest method to create a really homogenous voltage in a way that we can tell exactly how strong it will be, should be to use the thermoelectrical effect (in form of the Seebeck effect). It occures when two different connectors are exposed to differenct temperatures at either end. The resulting voltage is comparably low bu it is only affected by the material of the conductors and the total difference of temperature - and using phase transition, we can make the temperatures highly stable.

To use this phenomenon we only need conductors of different metal, two containers of water, ice and fire wood. Make the water in one container boil at east a little bit and make sure there is at least some ice in the other one (this way they will be exactly 100°C and 0°C respectively). Than connect both of them using e.g. iron whire and use e.g. copper whire to connect both poles of your charger with one container each. Those two metals are easily accessable in medival Europe and result in percicely 12.5µV per Kelvin whothc is a slight problem but will turn out in our adventage later.

12.5 µV per Kelvin won't be enought so we need a thermopile, connecting several conductor pairs. Since Kelvin is based on Celsius and Celsus is based on the temperature diffference between freezing and boiling water, we get the most exact 100 Kelvin difference there will ever be and thus get a resulting 1.25 mV per pair of connectors. To reach 5V, we would need 400 pairs of conductors that must not touch each other. Again, this is somewhat inconvenient but it does not require any unusual materials and gives us a voltage of precisely 5V that will not change over time (unlike any method that is based on Galvanic redox). Another drawback is that ice is not necessarily readily available in medival Europe. In winter, far north or near mountains it is available and using an ice house or ice cellar you could secure a supply throughout the year. Ice houses and similar constructions have been invented centuries before the medival ages even bagan but they didn't become popular in Europe before the 16th century so you won't be able to rely on profound methods being in place. But then again that is not wat you need. You only need enought thermal insolation to keep the ice from melting completely and even that might not be necessery depending on when and where you end up.

How to assure the percice current

Given the voltage, the current is dictated by the resustance. Unfortunately there is no way for me to know the resistance of your phone, the average resistance per lenght of wire for the contuctors you will use and how much of it will be used. Tht is even assuming that Galvanic elements have no inner resistance neither have connections between conductors (whitch will add up for any method but especially with the n=400 thermopile I suggested).

As long as noone can provide a model that allows us to predict the resistance with a reasonable percision, meeting your requrements (even as they are a rather big interval) is practicly impossible. I do belive that you would be able to charge your phone using my method but that goes for any other method as well, as long as it allows you to calculate the resutling voltage.

  • $\begingroup$ kudos for suggesting using temperature as voltage generator :) Temperature differences are readily available (nearly) everywhere you want. Take a glacier mountain stream and set up a campfire next to it, you can charge your phone. $\endgroup$
    – Anderas
    Apr 17, 2020 at 8:26

Refer to section on Gravity Cell in this page: http://members.kos.net/sdgagnon/te4.html

Telegraphy was the first electrical technology to spread (widely) beyond the laboratory. It was entirely powered by batteries at each station along the (iron) wire.

Copper and Sulfur (both well known to the classical world) should be relatively easy to obtain in the middle ages. (Remember that your paper money, debit-cards, and bitcoin will be met with laughter at best.)

Zinc on the other hand was poorly understood as a distinct metal. Despite having been used in various chemical compounds for thousands of year, you have to wait until the Enlightenment for the actual metal: in Europe "German chemist Andreas Marggraf normally gets credit for discovering pure metallic zinc..." [1746]

If you plan on smelting your own Zinc, be advised that the fumes present a health hazard. For this reason, Welders are told early on to avoid welding Galvanized Steel.

As you have remembered to bring your electronic amusements, your best choice is to remember to bring solar cells, and hand- or wind-cranked generator(s).


Lets just take a second to think about the implications of this during that time period. electricity in a time where you would be killed for any number of reasons including looking at a king or queen wrong. I would ditch the phone and use my engineering experience to carve out a kingdom of my own. Build infrastructure, machines, unfortunately weapons will be needed. As an engineer i am sure you know the basics behind a rifle and can work one together in some fashion. keep in mind you know how to make steel. so a better question is why do you want a useless phone when you can rule the damn world.

  • $\begingroup$ This doesn't really answer the question. $\endgroup$
    – kingledion
    Dec 18, 2017 at 3:49
  • $\begingroup$ Here is an answer. Logically speaking you would likely get killed well before you get the chance to even buy supplies. You have no money and nothing to trade for it other than what you have...how will you survive? still a better answer than the one below it that suggests using magic... you will need a kingdom before you can start playing with electric and not get killed for it out of fear @kingledion $\endgroup$ Dec 18, 2017 at 3:54

Build an electrostatic generator of roughly correct power level. Build a Leyden Jar. Connect the generator to the jar. Connect the jar to the phone.

Use the generator to charge the jar until the voltage gets high enough and the phone starts charging. Experimentally discover the rate you need to run the generator for the phone to keep charging.

Your phone can detect the voltage and regulate the current. Just provide a power source and something that prevents voltage spikes above what the phone can safely handle. Adding a resistor to prevent over current might be good also.


Batteries and Mobile Phones

Potato batteries, lemon batteries, apple batteries, it really is less complicate than it seems because modern mobile phones know to shut down the power source as soon as the internal battery is full, so you can't overcharge except if you really send the sparks flying. You shouldn't pile dozens of potatos, but you will be able to find out by try and error how many potatoes you need to stack until your phone shows something on the display or the charging LED. You will need some metals, though...

Versatile Charger

If you still have your charger with you, it's even better. They have as input 110-240 volts with 50Hz to 60Hz; so, that's total flexibility. It's in express made so that they fit both the American and Europe systems without problem.

Their flexibility makes that you can input any oscillating voltage and the charger will reliably fill your device without destroying it.

Making a alternating current generator

So if you are able to find some lengths of copper, silver or gold wire and a magnet, and you find a way to rotate the magnet inside a selfmade spool, you can just attach your charger with very little precaution and it is highly probable that it just works. Don't exaggerate, though - if you accidentally produce a thousand volt it will still burn out. Your character probably wants to use a very small iron heater as resistor to discharge exceed voltage. Hint: If the sparks bridge more than 2-3 mm (0.1 inch) then it's too much. You can use this as sort of a security precaution, too: Put your two wires in one spot as close as 2mm, not isolated, and any current that's too high will spark away.


Now, getting a useful length of wire in the middle age is a real constraint. You can't use steel or iron; they would heat up too much. Could be a nice first experiment for your character, though.

Silver is just perfect (best conductor) but expensive, copper is still quite ideal but getting it into wire form is as difficult as for silver, gold is less ideal, more expensive still but easy to make into a wire. Your character will spend a lot of money for blacksmiths just for the material and to make wire.


Worst problem will be to find a magnet. If you can rotate a magnet inside a spool, you've won. You can even magnetize a bigger block of iron once you have the first one down. But where do you find the first usable magnet? Well, your character probably has to go back back to the potato battery to magnetize the first piece of iron.

What do you want with a smartphone in medieval times?


I suggest forgoing the idea of creating generators/dynamos/alternators and simply going for a simple battery:

Create a Lemon battery

enter image description here

According to sources a single lemon battery cell creates around 0.9V in voltage. If you add 5 of them in series you should get some 4.5V which might be enough. Then simply scale the entire thing up in parallel to increase the current until you reach the needed amperage for your phone.

Edit: I even found this proven in a YouTube video.

Needed materials:

  • Lemons
  • Copper
  • Zink
  • Wiring of some sort

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