Imagine a theoretical scenario where I am transported backwards in time 500 years with nothing but a mobile phone containing all of human knowledge. Clearly I want access to this valuable resource but darn I forgot to bring my charger.

Using only things readily available in the 1500s (and let's say in the area of Europe, although I'm happy to have location moved if that would make a big difference to your answer), how would I construct a device for charging the battery on my phone?

I'm not overly fussed about efficiency, i.e. time to charge is not really an issue.

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    $\begingroup$ The circuitry inside the phone is necessary for accurate battery charging and so should not be avoided because reproducing them without semiconductors would likely be impossible. Perhaps crafting a 5V DC generator (requires some wire and magnets) and connecting to it via USB has a chance to work. $\endgroup$
    – sharptooth
    Commented Oct 21, 2019 at 9:38
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    $\begingroup$ In principle only the 5V and GND wires have to be connected to the USB plug ... probably by just clamp the wires together (there were no soldering systems at that time). The biggest problem I think is how to get (more or less exactly) 5V DC. Maybe with use of some fruits or potatoe (I remember you can get electricity with them) $\endgroup$
    – Michel Keijzers
    Commented Oct 21, 2019 at 9:41
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    $\begingroup$ Did you take a plug to plug into the phone with you? What about the multimeter to make sure you get the polarity correct? $\endgroup$
    – Solar Mike
    Commented Oct 21, 2019 at 10:09
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    $\begingroup$ How exactly do you intend to store all of human knowledge on your phone? Sure, you can access all (well, a lot) of human knowledge with it, but that's only because the internet and satellites, etc. exist. Without that infrastructure in place, the amount of info that you can hold on a phone is severely limited. Even if you solve the charging problem, that phone isn't going to be nearly as useful as you might think. $\endgroup$ Commented Oct 21, 2019 at 19:43
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    $\begingroup$ @Finbarr : please stop with this nonsense. It's like 500 years in the future every question about the early 21th century would be commented as "just be careful to not go close to a school because then you will surely die in a school shooting" - yes, witchcraft trials occasionally happened, but they were rare abuses of power, not a regular day-to-day occurrence. There were plenty of alchemists and self-claimed miracle makers who were never accused or tried for witchcraft. $\endgroup$
    – vsz
    Commented Oct 22, 2019 at 4:34

12 Answers 12


You could emulate Volta's pile if you could find some zinc and copper metal, and some acid, however the purity of such items might be pretty questionable. Which means you might get more voltage than you expected and damage the phone, with no possibility of repair. The cells Volta made produced about 1.1V so 5 of them would be about right (perhaps a bit high).

Of course if you had a 7805 or similar voltage regulator you could just add a whack of cells and regulate it down, or if you had a voltmeter you could check it, but if you're assuming that kind of preparation you may as well bring a solar charger along with you and use the sun.

If you could get access to some thin wire (perhaps a master jeweler would have drawing dies) and some lacquer or enamel for insulation, plus some iron and other bits you might be able to make a crude galvanometer which would allow comparison of a charged battery to whatever you are trying to come up with.

Given the likely high cost of reasonably good supplies in that era, probably you'd best start by finding a patron to fund the operation. And making yourself understood might be difficult.

Edit: A better approach might be to build a Volta pile or similar to add up to a nominal voltage in the 250 volt DC range. That could then be connected to a standard charger (despite being DC the chargers will still work). The enormous advantage of that approach is that the chargers have a very wide tolerance of input voltage, from 85 to 250V RMS or so, so anything from about 110VDC to 350VDC should work (square root of 2 relationship, and some ripple allowance). A 250 cell Volta pile should provide sufficient voltage - nominally around 275VDC (and lots of energy if the cells are of reasonable size) without taking risks on the phone itself, and would be very safe for the charger too. Not so safe for humans, easily lethal, which might make for an interesting twist to a story line. While, technically, this is not "building a charger" as the question asks, I think it well answers the spirit of the question.

Then you could go ahead and find some monks (or other literate people with time on their hands) to begin transcribing from PDF to manuscripts, assuming you don't want to keep all this knowledge to yourself.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Monty Wild
    Commented Oct 23, 2019 at 10:03
  • $\begingroup$ Given OP forgot to bring their charger, I'd think it a bit of a stretch for them to happen to have a 7805 handy... >:-D $\endgroup$
    – KlaymenDK
    Commented Oct 24, 2019 at 7:28
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    $\begingroup$ They are still wondering what these devices labelled MMMMMMMDCCCV, found in Pompei, are..... $\endgroup$ Commented Oct 24, 2019 at 15:04
  • $\begingroup$ I'm having trouble finding any corroborating detail for the claim that you can feed a wall wart DC and still have it work. Got a link? $\endgroup$
    – Phil
    Commented Oct 24, 2019 at 19:58
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    $\begingroup$ Modern(!!!) cellphone wallwarts (not early 90s stuff) are PSMPS circuits. First thing they do with the AC mains input is rectify it! A simple rectifier is OK with being fed DC from the start. $\endgroup$ Commented Oct 24, 2019 at 20:49

Don't try to charge the battery inside the phone.

Build a 10 cell lemon battery with copper and lead electrodes, which produces ~4.35V with a maximum current of about 1~5mA. Remove the battery from the phone and connect it to your lemon battery with the correct polarity (copper electrode is positive).

At this low current there is no risk of damaging the battery until the voltage gets over 4.2V, so charge it until it has enough capacity to be usable but not risking over-voltage (eg. 50%) using the phone to tell you the state of charge. At 1mA (24mAh per day) this will take several weeks.

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    $\begingroup$ I doubt a phone will even acknowledge the power source at 1 mA. $\endgroup$
    – Mast
    Commented Oct 22, 2019 at 6:06
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    $\begingroup$ @Mast that's one reason for taking the battery out of the phone. Lithium-ion batteries have almost 100% 'coulombic efficiency' and extremely low leakage, so even 1~5mA will charge a phone sized battery.This is also why 'trickle charging' a Li-ion battery is dangerous, because it will keep accepting charge until it blows up even at very low current. Of course you could always wire several lemons in parallel, or extract the juice and use large plates in jars to increase the charge current if you are impatient. $\endgroup$ Commented Oct 22, 2019 at 6:21
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    $\begingroup$ The charger circuit is in the phone, not in the battery. Removing the battery from the phone and charging it externally eliminates any effect the phone's charging circuit might have had. $\endgroup$ Commented Oct 22, 2019 at 10:29
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    $\begingroup$ Charging modern LiPo batteries without charging circuit is A Terrible Idea. $\endgroup$
    – Mast
    Commented Oct 22, 2019 at 11:15
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    $\begingroup$ With a modern phone, have fun trying to get the battery out of the phone without special tools and without destroying it. $\endgroup$ Commented Oct 23, 2019 at 10:56

You could build a dynamo.

One of the simplest designs for a dynamo is the homopolar generator, also known as a Faraday Disc.

Faraday Disk

You'll basically need a:

  • magnet
  • copper disk
  • commutator brush

Line enough of them up and you should be able to produce the necessary voltage and current. With a skilled jeweler, you could then create gold wires to hook up to the +V and GND pins of the USB connector on your mobile phone.

The harder part of the problem is generating the correct amount of voltage/power to charge the device. Fortunately, the battery-charging circuitry on mobile devices should protect it from too much over-voltage, and if you use a galvanometer in series with one of the connectors, then you should be able to see the point of "deflection" where the voltage has reached the threshold to allow current flow.

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    $\begingroup$ " if you use a galvanometer" Granted, construction of a galvanometer is not earth-shatteringly difficult, but how do you calibrate it? If you don't, how do you actually use the galvo, since you don't know what the deflection means? $\endgroup$ Commented Oct 21, 2019 at 16:47
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    $\begingroup$ @WhatRoughBeast you can cheat a bit with the phone: spin very slowly at the start, and speed up until the phone starts to display that its charging. Mark the galvanometer "do no exceed". $\endgroup$
    – mbrig
    Commented Oct 21, 2019 at 17:41
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    $\begingroup$ Homopolar generators are good at producing very low voltages at very high currents. For this reason they are rarely used. I think a standard motor using a wound armature would work better, though it would be more difficult to construct. The more separate coils you have, the smoother the output. With 5 or more you might be able to charge a phone. Still I think a battery will be easier. $\endgroup$ Commented Oct 21, 2019 at 23:27
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    $\begingroup$ A dynamo is going to kill your phone if you don't use something resembling an electrolytic capacitor as well, to level-off the voltage. $\endgroup$
    – Mast
    Commented Oct 22, 2019 at 5:55
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    $\begingroup$ This is the solution I would go for. While other solutions might work better, they rely more on materials which aren't always available close to each other. This solution needs copper for the electrical parts, wood for the structure and a magnet. Copper should be fairly wide spread in civilized areas, wood is abundant and a magnet can be created from hammering iron if none is available. You can even connect the device to a waterwheel for constant power and control the voltage through the spin rate. $\endgroup$
    – user48721
    Commented Oct 23, 2019 at 12:29

It is possible to make a battery using electrolytes from vinegar, apples, etc.

Depending on the type used you can get different voltages. If they do not match 5 V, put them in a series or parallel combination to get close enough to 5 V. A side issue is to create a Volt meter to measure voltage output (you don't want to damage your mobile phone having a too large voltage). However, calibrating it might be another problem (see remarks of C1sc0 and JRE below).

This way it should be possible to get a 5 V electricity source (which is DC).

See e.g. https://www.cool-solar-stuff.com/solar-diy/diy-vinegar-battery-lights-led/

What you need is:

  • zinc or aluminium; I'm sure aluminium was not present in 1500 (confirmed by JRE, production from 1824), but zinc might be (not the element, discovered in 1746, but the alloy was known for much longer)
  • natural products should not be a problem
  • copper wire, also available at that time
  • plastic bottle caps, not present, but I'm sure anything made of non metal can be used (maybe wooden cups?).

Check comments of Bytes, Dirk Bruere and analogsystemsrf below for details/alternatives.

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    $\begingroup$ Hmm, I guess the factors controlling voltage output would be your electrolyte (probably controllable by making up your own salt solution), metal purity (a bit more difficult at that time) and general construction technique (probably fairly well controllable). $\endgroup$
    – Bytes
    Commented Oct 21, 2019 at 10:10
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    $\begingroup$ Zinc and Copper would provide 1.1V per cell $\endgroup$ Commented Oct 21, 2019 at 10:14
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    $\begingroup$ You'd want a voltmeter rather than an ammeter. We're back to the same old "do I match the voltage or current of the powersupply to the device?" But, you haven't got much chance if calibrating your voltmeter so it will be of limited use. $\endgroup$
    – JRE
    Commented Oct 21, 2019 at 10:32
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    $\begingroup$ No aluminum in 1500. Aluminum metal was first produced in 1824. $\endgroup$
    – JRE
    Commented Oct 21, 2019 at 10:44
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    $\begingroup$ Assuming your phone is easy to open and access, you could build an analog voltmeter with a coil of wire and a ferromagnetic dial indicator. Then, calibrate the voltmeter by measuring the voltage on the battery cells in your phone (around 3 volts) and use that to extrapolate to 5 volts. $\endgroup$
    – Dragongeek
    Commented Oct 21, 2019 at 21:57

You've picked a very difficult thing to do.

Besides the electrical problems, you also have the mechanical problems of making the connector. USB micro connectors are very precisely made, and you are going to have to connect wires to some very fine pins.

I think what you want is beyond what you could reasonably accomplish. I mean you've got like two days of battery time before your phone dies and you lose access to all the information you need to build a charger. You won't be solving all the inherent problems and getting something made quickly enough to do you any good.

I'd suggest you make it a bit easier.

Take a charger with you. The typical wall wart to USB charger you get with your phone.

Now all you have to do is to get enough voltage to it so that it can operate.

"But my charger needs AC," you protest.

Well, it is designed for AC. Most of them will run, though, if you give them enough DC.

Build voltaic cells as described in the other answers here. Stack them to get about half the lowest rating of your power supply, and add stacks in parallel to get more current.

Connect your crude battery to the AC input of the charger. It will then reduce and regulate the voltage to the 5V your phone needs.

Building just that part will also take longer than your phone battery will last without recharging, but the task is simple enough that you can accomplish it without access to the data stored in the phone.

It will take you a good while to accumulate the metal pieces, and build your batteries. You don't have to worry too much about what kind of metal. Pretty much any two metals can be used in a battery - some combinations are better, some will mean you need really tall stacks to get enough voltage together. You just need to be sure you really only have two types of metal in your battery stacks - impure metals can cause you lots of problems.

There's no insulated wire available, so it'll be much easier to attach to the AC prongs on the charger than to the USB pins.

The whole contraption will be dangerous as F. A DC voltage high enough for this trick to work is high enough to be fatal to anyone who manages to touch the (exposed and uninsulated) plus and minus poles of the battery.

Once you have a usable way to charge your phone, you can look into building something better - with the ability to keep your phone operating until that something better is finished - and expect that to take years.

The charger can operate from a lower voltage, but it might not like it. You'll have to weigh getting information from your phone against the chance that your charger will croak the next time you hook it up to the battery.

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    $\begingroup$ I think you may be underestimating the engineering skills of the time. The first watch was made in 1505, after all. en.wikipedia.org/wiki/Watch_1505 $\endgroup$ Commented Oct 21, 2019 at 11:30
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    $\begingroup$ @JohnGo-Soco: If anything, I (and everyone else) is drastically over estimating how easy it will be to simply survive. You show up in funny clothes, speaking a funny language, and you need the assistance of the best craftsmen and finest materials - but you have no money, and no letter of introduction to get you into the society of people who have the things you need. $\endgroup$
    – JRE
    Commented Oct 21, 2019 at 12:12
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    $\begingroup$ I agree that it would be nearly impossible to measure the voltage of the battery. $\endgroup$ Commented Oct 21, 2019 at 17:39
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    $\begingroup$ Any competent jeweler in the 1500s should be able to make a USB breakout connector. The smallest features on a micro-USB connector are roughly the same size as the fine wires used to create gemstone fittings; if you've got a charging cable to go with your phone, a type A connector is probably within the capabilities of a copper smith. $\endgroup$
    – Mark
    Commented Oct 21, 2019 at 22:16
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    $\begingroup$ It's literally 5 pins of which only 2 are needed to charge. I think you can manage: 4.bp.blogspot.com/-yi-dbdKLfNc/Vfzp4g13yVI/AAAAAAAAACU/… $\endgroup$ Commented Oct 21, 2019 at 23:14

A lot of these answers are forgetting something very important.

The phone contains all of human knowledge.

Assuming you are transported back with a full battery, look up and write down everything you can on electrical engineering in the time the phone is still running. Once you have written down all of the necessary information, get to work in building whatever you need.

  • Gather the needed materials, without showing the phone around and without bringing too much attention to yourself
  • Find somewhere isolated and safe so you can build your machine without being discovered
  • Build it and use it to charge whenever you need it
  • Once charged, use the phone's human knowledge to continue improvements, if necessary
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    $\begingroup$ This is recursive, i.e. what are the instructions that would be quickly written down from said repo of human knowledge? $\endgroup$
    – Steve
    Commented Oct 22, 2019 at 3:23
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    $\begingroup$ "Assuming you are transported back with a full battery, look up and write down everything you can on electrical engineering in the time the phone is still running." I will guarantee you the battery will run out before you're finished. Can't be done in a couple of hours, no way. $\endgroup$
    – Mast
    Commented Oct 22, 2019 at 5:58
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    $\begingroup$ Besides, you'd need more than just electrical knowledge. You'd need geological and mechanical knowledge to find/identify the resources and make the required tools. $\endgroup$
    – Mast
    Commented Oct 22, 2019 at 5:59
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    $\begingroup$ How do you know this very post and its subsequent answers aren't the very piece of human knowledge the OP's character looks up using this phone? Now that's meta... $\endgroup$
    – Logan
    Commented Oct 22, 2019 at 10:03
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    $\begingroup$ On the bright side, the phone doesn't need internet access since it already contains all of human knowledge. So, it should last a little longer than a typical charge. $\endgroup$
    – Zwuwdz
    Commented Oct 22, 2019 at 18:32

Depends on the phone.

If you give an LG G7 One 5 volts, it will happily charge away at whatever current it can reasonably pull. As other answers state, coming up with 5 volts (ish) is certainly doable, and the internal resistance of a battery from that timeline will take care of any over-voltage you accidentally create from impurities.

If you give a Samsung Galaxy S7 or a Blackberry Dtek 50 5 volts, it will start trying to ask the charger if it's a samsung charger/blackberry charger etc and will leverage the USB protocol to do so. Opinions about this aside, this makes it nearly impossible to charge these phones from a diy standpoint (there's no way you could forge silicon to do USB communications in 1500).

If you can get inside the phone (go find a jeweller/watchmaker and wow them into making you a torx bit with files and high carbon steel), you can relatively easily build a charging system that will charge the battery, and manually modulate the current/voltage by building a potentiometer with a strip of carbon/lead from a pencil (is charcoal conductive enough for this? A quick google says so). Take another pencil's lead to solder some leads to the battery (a mild steel screwdriver in the fire for a while as an iron, or any chunk of steel with wood nailed to it for insulation) and make some rosin by boiling pine sap (thanks again, google).

I imagine from this answer you can tell the different phones I've had over the years.

Edit:I was wrong, my phone charges(albeit slowly) from a dumb charger (data lines floating). It will not recover from dead (I tested this with the dtek 50 which died overnight) without the original charger. This is an important point and could factor in. The phone mustn't die completely at any point.

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    $\begingroup$ I have a hard time believing this answer. If what you say is true, you could not use a dumb power bank to charge an S7, or a cheap old USB charger. Are you sure you don't mean that you can't charge fast without negotiating? $\endgroup$
    – pipe
    Commented Oct 21, 2019 at 19:53
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    $\begingroup$ I have a "dumb" charger I've installed in my car. Even fully powered off, both of those devices will not charge, and will actually slowly discharge, when plugged in. That's also with the apple 3 style resistor network or the data lines shorted together $\endgroup$ Commented Oct 21, 2019 at 20:08
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    $\begingroup$ Yeah I don't buy this. I saw countless S7 phones charge with generic chargers. Sure, they'll try to negotiate a fast charge over the data lines, but if nothing is responding there, they'll happily charge at 5V. $\endgroup$ Commented Oct 22, 2019 at 3:22
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    $\begingroup$ My Samsung Galaxy S7 will charge on any dumb powerbank or plug charger with just the power lines connected, limiting itself to 500mA of charge current. Your observation sounds more like a problem of the charger itself. The only thing I've found that several manufacturers will not allow is bringing a fully-discharged (read auto-poweroff by lack of battery) phone back on without the original charger. This bit me with a Nokia Lumia and an older Sony more than once. $\endgroup$
    – WooShell
    Commented Oct 22, 2019 at 14:16
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    $\begingroup$ You short the 2 data lines together and it will charge in dumb mode. $\endgroup$
    – cybernard
    Commented Oct 22, 2019 at 20:24

Any battery in a phone removable or not is a module with it's own protection circuits which will cut it off when the voltage/current/temperature is out of range. So it is safe to charge/short circuit the battery directly.

The battery's voltage range is like 3.0-4.3V which is wider than the phone input port range which is like 4.75-5.25V so it is easier to charge the battery directly with a poor power source.

The most import advantage of battery is that it is a voltage source at normal state (not cut down by protection circuit), i.e. it is a voltage potential plus a low resistance which is <0.1Ohm or even <0.02Ohm. So you can just drive the battery with any power source through a large enough resistor and it will be charged since low resistance battery plus large exteral resistor means that voltage of the battery contact and the current will fall in the range reqired by the protection circuit for a very high chance.

On the other hand, the phone charging port is a high resistance port by default, it is just some capacitor and voltage sensing circuits. It only begin to charge the battery when the input voltge is in range like 4.75-5.5V. This means that with a poor power source (e.g. large resistance/low loading capacity/unsure voltage), the chance that the phone will beginn to charge is low.

Even the phone begin to charge the battery, since most phone use switching power to charge the battery, it is the charging circuit that decides how many current to draw from the power source, this means that the chance of overload of the poor power source is high,and once the input voltage falls bellow like 4.75V, the phone will stop to charge.

On the other hand, if you charge the battery directly, the current draw only depends on the battery's resistance and the external resistor you use, you can limit the current with external resistor. This also means that you can't use resistor when charging the phone port. External resistor mean that you can use a much wider range of voltage and unstable power source.

Some answers mentions the connector problem, it is just impossible for you to wire out a micro-USB or type-C or Lightning connector.

Like @Zero's anwer, remove the battery and charge it.

Most phone have non-removable battery so the first difficult is to remove the back cover of the phone, you may need to somehow destroy the back cover for many cases but without rendering the phone fully unusable.

With some sort of 3.5-6V(or even higher) poor power as mentioned by other anwers, the next tool needed is a resistor which can be made from long thin conducting wires which I'm not sure is available. You can begain with high resistance and check out wether it works after charging for a while and cut the wire shorter to try low resistance. No need to worry about over voltage or over current, unless the voltage is too high and the resistance is too low and you break the maximum rating of the battery protection IC.

  • $\begingroup$ Warning: Over V MAY be OK but may not. | SHort circuit may cause an in cell overvoltage fuse or control circuit to operate and effectively kill the battery. $\endgroup$ Commented Nov 2, 2019 at 16:19
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    $\begingroup$ @Russell McMahon Can you provide some reference? Short circuit won't kill the cell, that's the most important function of the protection IC. The fuse (if any, I suppose there is no fuse in smartphone battery) will only function in case the protection IC fails, i.e. a second level protection. $\endgroup$
    – jw_
    Commented Nov 3, 2019 at 3:56
  • $\begingroup$ I was referring to the condition described in your first paragraph - a single battery on its own with whatever protection it has being internal to the item. In that case some examples have a series fuses designed t one time limit gross discharge for safety reasons. I agree that with many batteries and situations inbuilt protection circuitry will prevent this happening. (They usually use back to back N Channel MOSFETs to form a "switch" and a current sense resistor to. ) $\endgroup$ Commented Nov 3, 2019 at 23:04

Why can't you take out the battery and connect it to Volta's pile (4 of them), that is 4.4V when fully charged, this is more than enough for Li-Ion battery operating at 4.2V, you can ignore 0.2V overvolt I'm sure battery can handle that. As soon as you connect pile to battery, pile's voltage will drop.

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    $\begingroup$ Because it would explode. LiPo batteries are very prone to exploding. $\endgroup$
    – Aron
    Commented Oct 22, 2019 at 2:50
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    $\begingroup$ @Aron citation needed $\endgroup$
    – TylerH
    Commented Oct 23, 2019 at 13:27
  • $\begingroup$ @tylerH just YouTube lipo battery explosion. There is a wealth of examples of explosions. Also, guess why they don't let you bring big lipo batteries on planes. $\endgroup$
    – Aron
    Commented Oct 27, 2019 at 8:09
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    $\begingroup$ @Aron When someone asks you to source a claim, "go search for it" is not an acceptable response. If you're the one making the claim (on, like, every answer suggesting it here), you need to provide the evidence for the claim. $\endgroup$
    – TylerH
    Commented Oct 28, 2019 at 13:47
  • $\begingroup$ @TylerH the fact is, the above, was not an answer and thus should not be held to the same standard. Furthermore, exploding batteries, would, I argue, fall under common knowledge. Especially after a spate exploding phone deaths and self immolating Samsung Phablets. However, here is your citation batteryuniversity.com/learn/archive/lithium_ion_safety_concerns $\endgroup$
    – Aron
    Commented Oct 28, 2019 at 16:36

If we have a phone with all of human knowledge, then I’m surprised that nobody has suggested searching the Internet archives on the phone for the Reddit topic where this question was asked and the answers given.

  • $\begingroup$ This is reflexive and somewhat sarcastic, whilst many might appreciate the humour, what the forum requires is well defined answers that explain themselves and justify why they are correct, actual references are appreciated. Your post does not answer the question. When you have a few spare moments, take the tour and read-up in the help center about how we work. Welcome to the site, you are capable of more than you think. (From review). $\endgroup$ Commented Oct 22, 2019 at 23:35
  • $\begingroup$ Ah my apologies. The “How to answer” part does say to have fun, but in retrospect I was answering how to find the answer not how to solve the problem; similar to parts of some answers above In story telling I’ve found the how you find the answer is sometimes just as important as what the answer is. I’ll be more careful in being more clear in the future, thank you $\endgroup$
    – Cralis
    Commented Oct 23, 2019 at 0:22
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    $\begingroup$ All human knowledge doesn't contain how to make Reddit search good. $\endgroup$ Commented Oct 23, 2019 at 1:08

Since the device has all human info, I'm assuming that it is advanced enough to have inductive charging. Inductive/wireless charging would be safer than using a USB connection and is simple to build from wires. Combine that with the battery ideas and you could charge a phone until enough wire and other basic materials could be acquired to build a small hydro-electric generator. One of the wind turbines for pumping water from wells in northern Europe could also be adapted to generate power.

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    $\begingroup$ Maybe I'm wrong but isn't there a negotiation before it will charge inductively? It's not just a simple varying magnetic field, but the phone communicates with the charger before agreeing to charge $\endgroup$ Commented Oct 22, 2019 at 20:27

...with nothing but a mobile phone containing all of human knowledge

I know this is all just a thought experiment, but it's worth pointing out that although we have ACCESS to "all of human knowledge" via things like Google, that information is contained on some very HUGE server farms, not in your cell phone memory itself.

enter image description here

Even applying Moore's law to the increase of memory storage capability, your mythical "cell phone" device capable of storing all of human knowledge is not going to exist for a few hundred more years, especially considering that the amount of "all" is ever increasing as you try to get there. And even if that happens, I seriously doubt it will be possible to power it with 5VDC. Right now Google has about 1 million servers, and assuming that each one consumes about 850W of power, all of that knowledge needs around 850 MEGAWATTS of power to make it accessible.

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    $\begingroup$ The more I dig, the less I find this to be true. I'm finding the deeper I dig into a topic the more valuable books become because the hurdle for publishing a book is so much higher (not to mention money to be made from selling it) so there's a lot more the time, care, and experience invested into writing a book than something online. $\endgroup$
    – DKNguyen
    Commented Oct 21, 2019 at 20:27
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    $\begingroup$ While there's indeed a great deal of information in world data centers, it's debatable whether billions of cat videos actually constitute knowledge. $\endgroup$
    – Dan Bryant
    Commented Oct 21, 2019 at 21:17
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    $\begingroup$ Is this an answer to the question? The data on the phone is not relevant to how we would charge it in 1500. $\endgroup$
    – Harabeck
    Commented Oct 21, 2019 at 21:54
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    $\begingroup$ You would be surprised just how many books I can hold on a 512GB microSD card. The average kindle book is 1.87 MB so that's over 270,000 books on your phone. More than enough to hold all the important knowledge (especially if you skip the fiction section) $\endgroup$ Commented Oct 21, 2019 at 23:09
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    $\begingroup$ The compressed size of the text of English Wikipedia is only about 16GB (en.wikipedia.org/wiki/…). This gives a ballpark figure for the amount of storage required for all (useful) human knowledge. $\endgroup$
    – Dancrumb
    Commented Oct 22, 2019 at 0:12

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