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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 '17 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 '17 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 '17 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 '17 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 '17 at 12:13

12 Answers 12

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Maybe use -

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

https://en.wikipedia.org/wiki/Galvanic_cell

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 -

https://en.wikipedia.org/wiki/Standard_electrode_potential_(data_page)

For 5Volt we can use -

Ba(OH)2 + 2 e− ⇌ Ba + 2 OH− (−2.9v)

and

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 '17 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. $\endgroup$ – a CVn Dec 14 '17 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. $\endgroup$ – a CVn Dec 14 '17 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 '17 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 '17 at 21:19
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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.

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    $\begingroup$ "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 '17 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 '17 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 '17 at 16:20
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    $\begingroup$ Carbon (ashes, charcoal) can be used to make resistors - though not calibrated ones :) $\endgroup$ – rackandboneman Dec 16 '17 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 '17 at 16:55
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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.

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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.

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  • $\begingroup$ 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 '17 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 '17 at 1:18
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    $\begingroup$ When life gives you lemons... make a battery. $\endgroup$ – uhoh Dec 15 '17 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 '17 at 16:40
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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.

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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 $\endgroup$ – workoverflow Dec 18 '17 at 14:05
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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).

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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.

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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).

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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.

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  • $\begingroup$ This doesn't really answer the question. $\endgroup$ – kingledion Dec 18 '17 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 $\endgroup$ – George Schneider Dec 18 '17 at 3:54
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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.

Needed materials:

  • Lemons
  • Copper
  • Zink
  • Wiring of some sort
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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...

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