My time traveller has a maybe 20-minutes-into-the-future smartphone and laptop. They can store large amounts of data, on the Petabytes level or higher. The time traveller can effectively store the entirety of the current day's internet and library of congress or other information on his laptop.

The problem is, they have a limited battery life. On very conservative use, it would be around 2-3 weeks before it runs out. Any DC current can charge the time traveller's equipment, though it supply a minimum of 5W of power on average to charge.

The so long as the time traveller's equipment is working, he can consult his database to help him construct devices necessary to charge his equipment. eg. Give him a motor and he would be able to rig it so that he can charge his equipment.

However, once the battery dies, he would no longer be able to do anything except the anything say, a college graduate would be able to do.

How far can this time traveller go back in time before his electrical equipment becomes unchargeable?

EDIT: I did not expect a solar powered solution, though it seems like a pretty good idea. However, some of the answers gave rise to some clarification I should make about the time traveller's equipment.

His laptop and smartphone require a DC current to charge, simply because a AC current would discharge when the polarity gets reversed on the other side of the waveform. (average current is 0) Essentially, this would mean that a rectifier would be neccessary if a AC current is used.

The idea is that the laptop and smartphone are off-the-shelf from the time traveller's time, so such external power methods would not be incorporated by default. The batteries are high-capacity such that although the laptop and smartphone consume significant amount of power, they can last for a relatively long period of time. Their actual specs are that on general continuous use, they last about 24 hours. But if you turn them off and on just for minor uses (such as just referencing some data), they can be expected to last 2-3 weeks.

In a sense, this question could be converted to: when is the earliest time that a DC power source with at least 5 watts of power (or more likely, enough a rectifier) can be obtained within 2-3 weeks?

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    $\begingroup$ When traveling into past, take also solar charger, they are cheap. $\endgroup$ Commented Dec 20, 2014 at 17:36
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    $\begingroup$ Does your time traveller carry a standard "wall wart" AC charger for their phone / laptop? I'm asking because modern switching power supplies like that tend to be pretty robust, and will happily convert anything from about 100 to 250 VAC, at around 50-60 Hz or so, to a nice clean regulated 5 VDC / 12 VDC, or whatever your laptop needs. If they need to generate the correct DC voltage themselves (and didn't bring a regulator for that), it gets trickier (and the risk of frying some sensitive electronics by mistake goes up). $\endgroup$ Commented Dec 20, 2014 at 22:39
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    $\begingroup$ Another portable option is hand cranks like on some emergency flashlights and radios. This way, even if you are terrorizing the locals by blocking out the sun with an eclipse, you can still charge. $\endgroup$ Commented Dec 20, 2014 at 23:18
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    $\begingroup$ A solar charger or hand crank could solve this problem quite easily, but aren't things you'd expect someone to carry around all the time. If you want the power thing to actually be an issue in your story, he'd either have to travel in time while poorly-prepared (e.g. it was an unexpected journey), or lose his planned power source upon arrival. On the other hand, if you're asking, "do I need to be concerned about this to have a plausible story?" the answer is something like "no, it's easy to prepare for: plan A, B, or C." $\endgroup$
    – Tim S.
    Commented Dec 21, 2014 at 3:45
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7 Answers 7


If you want to charge your laptop off of mains power, Edison Illuminating Company will be happy to provide you with 110-volt DC power in 1882, provided you're willing to do your work in Manhattan.

If a private power source is acceptable, you can pick up a dynamo in the late 1860s, or look for a factory that will let you tap into theirs.

If any commercial power source will do, you should be able to pick up a truckload of Daniel cell telegraph batteries by 1840.

And if you're willling to build your own, a copper-zinc voltaic pile can be made as early as 1400 BC in Judea, although you might have a bit of trouble convincing a smith to smelt the zinc ore straight, rather than using it to produce brass. If you prefer to buy your zinc in metallic form, you're probably stuck with 1100s India.

Building a generator can be done at any time: you just need to find a location with native copper, then spend months or years turning it into wire of suitable quality.

  • $\begingroup$ I think with your statement to "build a geneator at any time" is not quite right. I assume the time traveller only stays on earth while time travelling. If so, you can't build a geneator before earth itself was created. Therefore I you check history of earth: en.wikipedia.org/wiki/History_of_the_Earth#mediaviewer/… you can't start before 3.8 to 3.5 billion years back when atmosphere was build. $\endgroup$ Commented Dec 22, 2014 at 12:00
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    $\begingroup$ Building a generator is considerably more difficult than one might assume. Lodestone, naturally occurring magnetic rock, is not only rare, but fairly weak. Generating 5W out of a motor built out of lodestone and crude wire is going to be an exercise in futility. The fabrication of stronger magnets requires electricity, thus a chicken and egg style problem. Batteries, as weak as they are at any given time, will probably be far easier to make and use than a generator. $\endgroup$
    – Adam Davis
    Commented Dec 22, 2014 at 16:51
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    $\begingroup$ @AdamDavis, a lodestone isn't the only option for your magnet. You could use native gold, native copper, and the citrus fruit of your choice to make the world's most expensive electromagnet. $\endgroup$
    – Mark
    Commented Jan 1, 2015 at 21:44
  • $\begingroup$ How easy is it to find native copper? Can he dig with his hands? I'm not sure. $\endgroup$
    – Mikey
    Commented Mar 20, 2015 at 20:38
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    $\begingroup$ Can you make enough electricity with a lodestone to energize an electromagnet? If you've got that, can you use that inside a generator? Or do you have to make a magnet first? $\endgroup$
    – user3082
    Commented Sep 25, 2015 at 21:36

This depends on what else your time traveler is bringing. Solar energy should be pretty reliable in this case.

  • It is highly portable, especially for only 5 volts
  • The Sun is always around (even if hidden by clouds, it is unlikely it will be hidden for three weeks)
  • It is getting cheaper

Of course, if you are traveling to the Arctic or somewhere around there in the winter, you get no hours of sunlight. (Thanks @MikeScott for bringing that up in a comment). If you don't have a really large battery, your time traveler might not be able to travel there for more than 3 weeks very well.

Basically, if you bring the right equipment, you can go back as far as you want in most places.

Thanks to @PeterMasiar in the comments for suggesting solar power.

  • $\begingroup$ "It is unlikely it will be hidden for three weeks." Not if your time traveller is visiting Ireland in winter, it isn't. Average daily sunshine hours in Belfast in December: 0 $\endgroup$
    – Mike Scott
    Commented Dec 21, 2014 at 10:51
  • $\begingroup$ @MikeScott That's why I said "unlikely". Those areas would probably be off-limits for long stays. $\endgroup$ Commented Dec 21, 2014 at 13:16
  • $\begingroup$ I'd figure it's more accurate to say solar source portability is proportional to the current capacity, not the voltage. $\endgroup$
    – MikeB
    Commented Dec 21, 2014 at 18:11
  • $\begingroup$ Belfast is a long way from the arctic (54.6 degrees north), and there are over seven hours between sunrise and sunset even today, the winter solstice. It's not the latitude that's the issue; it's the weather. $\endgroup$
    – Mike Scott
    Commented Dec 21, 2014 at 19:23
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    $\begingroup$ Modern solar cells work fairly well in overcast conditions. They are widespread in Germany, which is also somewhat dark and cloudy in winter. $\endgroup$ Commented Dec 22, 2014 at 11:23

Can your petabyte laptop's power supply accept dirty, unregulated AC? Around the first century AD, the Greco-Roman world was aware of magnetism, and Hero of Alexandria described an aeolipile, a basic steam engine. Your time traveler might be able to work with the technology at that time to fashion a crude generator.

Otherwise, if your petabyte laptop accepts only DC, I can think of three options.

Photovoltaic Cells

Use photovoltaic solar cells, as others have mentioned. If your time traveler doesn't want to bring them along, there are plenty of DIY articles online. (You'll need to check on material availability.)

AC Generator

Build an AC generator, then rectify the output to DC. All of the rectifier designs that I know use diodes. Diodes are a semiconductor technology, so I don't think you'll find them much prior to the 20th century.

Andrejako notes in a comment that:

There was a number of non-semiconductor rectifier designs available before the semiconductor revolution, but that doesn't make them much easier to manufacture. There were mercury-arc rectifiers, there were the original diodes from 1900s, like the Fleming-valve, then there were many improved versions like tungar bulbs. Then there were also copper-oxide and selenium rectifiers as well. In any case, most of that stuff is a forgotten technology and it isn't very simple to create. On the other hand, they all do look much cooler than modern diode-bridge rectifier.

These mostly look like vacuum tube technologies, so they won't take you much farther back than the latter quarter of the 19th century.


The Baghdad Battery dates to between 250 BC and 250 AD. Each cell provides 0.5V: they could be ganged in series to provide whatever voltage you need, but I see nothing on current rating, so I don't know if you could pull 5W.

There's also the traditional science fair lemon battery. At the 0.9V and 1mA specs mentioned in the article, you'd need about six-thousand of them to power your laptop, but you should be able to travel back as far as copper and zinc are both available.

  • $\begingroup$ There was a number of non-semiconductor rectifier designs available before the semiconductor revolution, but that doesn't make them much easier to manufacture. There were mercury-arc rectifiers, there were the original diodes from 1900s, like the Fleming-valve, then there were many improved versions like tungar bulbs. Then there were also copper-oxide and selenium rectifiers as well. In any case, most of that stuff is a forgotten technology and it isn't very simple to create. On the other hand, they all do look much cooler than modern diode-bridge rectifier. $\endgroup$
    – AndrejaKo
    Commented Dec 21, 2014 at 10:35
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    $\begingroup$ Thanks for the historical information -- I learned something, and those technologies would certainly look mad-scientist next to a modern laptop. I hope you don't mind, but I've included your comment in my answer. $\endgroup$
    – pmcoltrane
    Commented Dec 21, 2014 at 16:53
  • $\begingroup$ Through history, Lemons weren't always as widely available as they are today. $\endgroup$
    – MikeB
    Commented Dec 21, 2014 at 18:13
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    $\begingroup$ Vinegar should work about as well as lemons for a battery, and should be available, or easily made, anywhere there's wine, cider, or similar. But of course your sensible time-traveller would prepare, as those of us who go backpacking/camping do today, by checking his local REI or other outdoor gear store :-) $\endgroup$
    – jamesqf
    Commented Dec 21, 2014 at 19:25

One Laptop Per Child (OLPC) computers were designed to be run in exactly such situations. They were originally designed with a built in hand-crank to provide power in any environment - even indoors or perpetually overcast!


I'm going to take a shot at near-term technology forecasting, and say that your time traveller's near future smartphone and laptop are probably charged over some version of USB (unless they're Apple devices, in which case they'll probably use some more-or-less equivalent proprietary system).

The common everyday USB port hides quite a bit of complexity, with the same wires carrying both power and data, and with a complex hardware/software protocol for negotiating communication and power delivery parameters. This makes "hotwiring" a primitive power source into a USB connector pretty difficult, and likely to risk damaging the electronics if you do it wrong.

Fortunately, however, our intrepid time traveller probably won't have to do that, not unless they also forgot to bring a charger for their devices. There are several types of such chargers, each of them good for different situations:

  • Standard AC "wall wart" chargers: Good anywhere you can get AC.

    Modern switched-mode AC chargers tend to be designed for international use, and are thus rated to accept anything from 100 to 250 volts AC at anywhere from 50 to 60 Hz (and may work with sources somewhat past those official limits, too). That means they'll be compatible with most AC systems used since the late 1800s.

    In a pinch, if you happen to know the general theory (or carry a copy of Wikipedia on your laptop), you could even try to build your own AC generator (or maybe team up with a local down-time inventor to do so). As long as you can find enough copper wire for the coils somewhere, and have a general idea of what you're doing, it should not be too difficult to build something that produces a voltage and frequency roughly in the needed range.

  • Automobile chargers: Good anywhere you can get DC.

    The DC voltage provided by a typical car "cigarette lighter" socket is nominally 12 volts DC, but in practice can fluctuate anywhere from 5 to 15 volts. Thus, USB (and other) chargers designed to be plugged into such a socket feature a switched-mode DC-to-DC converter that takes the "dirty" power from the car socket and converts it into a nice, clean, regulated supply. As with wall chargers, car USB chargers also contain circuitry to regulate and negotiate the current, and to take care of all the other fiddly little details of the USB protocol.

    All this makes such a charger the perfect tool for powering modern electronics from a primitive DC supply. Just hook up some down-time batteries in series to get a voltage somewhere near 12 VCD, plug the charger in and go. Mind you, it helps if you know how to build an efficient battery — some of the earliest batteries, while technically capable of generating electricity, had really lousy power densities.

  • Solar chargers: Good anywhere there's sunlight.

    For powering your electronics "off the grid", it's hard to beat a solar charger. These are sold for travel use, for places where no other source of electricity is easily available. Internally, they generally feature a modern solar panel and a DC-to-DC converter, just like in the car chargers, for regulating the output voltage. You're not going to be able to build one using primitive technology (building an efficient solar panel is still a cutting-edge research topic today), but if you happened to pack one with you, you're pretty much set.

  • Hand-crank chargers: The last resort.

    If you need to charge your phone off the grid, and don't fancy carrying a solar panel with out, you could always go for one of these. They basically contain a small generator and a power regulator, and are mostly meant for emergency use (and/or as gimmicks).

    If you don't fancy developing your muscles (and/or a repetitive stress injury) by winding a tiny crank for hours and hours, you could always hook the crank up to something like a water wheel, and let it grind away while you do something more useful. The main risk with these things is that they may not have been designed for long-term continuous use, and the moving parts might wear and break down after a while. If you were good with your hands, though (or had someone who was to help you), you might be able to rebuild a working generator from the parts.

  • $\begingroup$ Bog-standard USB separates data and power on different wires. Some chargers use resistor networks between the power and data lines to signal the device to switch to a higher-current mode, but the data lines still aren't carrying any significant current. As far as I can see, this is still true for USB BC and USB PD. $\endgroup$
    – hobbs
    Commented Dec 21, 2014 at 6:52
  • $\begingroup$ Yeah. You can simply solder a couple of wires directly to the power USB pins, if you are desperate. The issue is that a primitive power source might have trouble generating and regulating voltages as low as 5 volts. You don't want to hook a transformer directly to a dynamo and feed it into a USB port. $\endgroup$ Commented Feb 25, 2015 at 19:22
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    $\begingroup$ Those "modern switch-mode AC" chargers will also function off of DC.. like say Edison's 110V DC, since internally the first thing they do is rectify the input, and then send it into a large filter capacitor. $\endgroup$ Commented Mar 18, 2015 at 14:37

He should have a photovoltaic surface on every suitable surface such as the outside lid.

I have a shake-to-power flashlight for emergency so I don't have to worry about batteries. A kenetic device can also be integrated.

The emergency rock-bottom devices would take a long time to be useful. OTOH the long battery life you mentioned, and the possible capacity of a chemical cell, means it doesn't use much power and sunlight should power it completely.


If you go back too far and can't find any metal for your generator / battery, you could build a Piezoelectric Generator, and spend you days banging rocks together!

  • $\begingroup$ Piezoelectric generators produce utterly trivial amounts of power -- the sort the average time traveler will be able to build will have a power output measured in picowatts, or nanowatts at best. That's enough power to (maybe) run a wristwatch, but recharging a laptop or a cell phone is right out. $\endgroup$
    – Mark
    Commented Aug 6, 2018 at 21:47
  • $\begingroup$ Not any more. New Piezoelectric designs are on the way. $\endgroup$
    – Hoytman
    Commented Aug 10, 2018 at 6:25

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