MacGyver, Jedi Knights, Ammo Stockpiles, And Candy Crush

Everyone’s favorite machinist, tinkerer, YouTube celebrity, deadpan comedian, and Canadian is back with a tale of popping a few benzos, stumbling around Mexico, and wondering why everyone else on the planet is so stupid.

The hero of our story considered the feasibility of one hundred and eighty-sixth trimester abortions as he stood outside a Mexican airport watching a stockbroker complain about the battery in his cellphone. Meanwhile, cars drove by.

Here’s how you charge a phone with a car battery and an ‘ol Dixon Ticonderoga.

To charge a battery, all you really need to do is connect the terminals to a power source with the right voltage. A cell phone battery needs about three volts, and a car battery has twelve. You need a voltage divider. You can get that with a pencil. Take out a knife, get to the carbon and clay wrapped in wood, and wire the battery up. Make a cut a quarter of the way down this rather long resistor, and there you will find something around three volts.

Does it work? Yeah. It works even better if you have some tape to hold wires onto the cell phone battery when charging. Is it smart? It is if there is no other conceivable way of charging your cell phone. Should you do it? Nah. Video below. Thanks [Morris] for the link.

55 thoughts on “MacGyver, Jedi Knights, Ammo Stockpiles, And Candy Crush

    1. In reality, it is rarely at 12V. 13.8V is likely the float charge voltage i.e. if you keep the car alternator running.

      You are supposed to precharge the Li-ion if it is below 3V or so, constant current charge it until 4.2V, then terminate the charge when current is below a few percent.

    2. Both times you’re kinda right.
      The Battery fails (worst case explodes) if charged too fast (overheating) or too full (4.2V is the absolute maximum). With his Pencil-Charger the current is limited by the resistance of the penciel lead to less than 1 A (13.8V divided by 15 ohms), therefore the “too much current” part is prevented (those phone batterys are charged within ½ h in the device without blowing up, at ~2000 mAh this equates to 4 A current). The overchraging part is prevented by: a) the maximum output voltage of 4,16 V b) him disconnecting the battery long before it was full.

    3. Sure you can. In general, all battery charging is the same. Its that last bit of the charge cycle that is dangerous with Li-Ion (or any battery really), though. If you ever look inside a cheap Chinese direct battery charger for Li-ion batteries, you will find very dangerous circuits.

      I bought a CC (Cheap Chinese) charger for the Li-Ion batteries for my Harmony remote because the cradle for these remotes don’t work very well. 3 batteries started bulging over time so I cracked the charger open. I found that it was nothing more than a transformer to step down the mains, a zener to set the voltage at 4.2v and a current limiting resistor. There was no automatic shut off or even temperature monitoring. The light changed color when the Li-Ion pulled the charging voltage down to 3.7V and you had better take it off the charger immediately.

      I gutted that sucker and installed a Smart Charger IC module in there instead. Batteries charge faster now and actually shut off charging when it is done.

      So you can do it, but it will damage your batteries if you let them ever fully charge. It’s not the right way, but as the article suggests, in a pinch it can get you going again.

      Though, I think I would just create the voltage divider to power the phone from USB and let the phone charger circuitry and regulators handle things.

      1. Lipo constant voltage charge is self terminating, so all you need to do is prevent the charging voltage from going above 4.2V. Unlike the older battery chemistries, where you had to force electrons in with a higher voltage and then figure out when to stop charging, Lipo is pretty easy – just CV charge with a current limiter at 4.2V, and when the cell gets up to that voltage the current flow naturally stops.

        Unfortunately the cheap chargers implement the constant voltage part poorly, or have the voltage set too high, or just waste lots of power through a shunt zener which can fail. Smarter chargers notice when the charge current is nearly zero and switch off.

          1. good that you said that,
            i was about to go “buuuuuuut…”

            its actually _close_ to what pelrun said, buuut
            the current never really will drop all the way to zero!
            it will hover somewhere between 0.5%C and 5%C
            e v e n t u a ll y overcharge.

            even-though the VOLTAGE stays the same, (the 4.20 you mention)
            the CHARGE-LEVEL does NOT stay the same when theres current.
            meaning it WILL overcharge, just v e r y s l o w l y.

    4. What you’re all missing is the cell phone handles the actual charging aspect of the battery, all this is doing is supplying the voltage to do the charging. Remember, the normal usb is putting out 5v dc, so if you really want to complain and split hairs, it’s not the right voltage either…

      In this case a DC supply is a DC supply. Origin is unimportant.

  1. You happens to have a pocket knife, a pencil, pieces of wire, the charge cable and a car (or car battery), but forgot the USB charger!? For someone so prepared and yet so forgetful? Inconceivable!

    3V is the wrong voltage for charging your typical Li-ion battery as you’ll never even get it past the precharge stage. Try 4.2V if you really want to charge it. 5V if you want to charge the phone from USB.

    1. >Make a cut a quarter of the way down this rather long resistor, and there you will find something around three volts.
      Just making a point about the write up.

      Even not considering that the battery draw additional current from the divider hence messing up the math for a plain voltage divider, that 4.1V from a 1/4 divider ratio is very fishy.

      1. The write up said as I have quoted in the post you are replying before: “Make a cut a quarter of the way down this rather long resistor, and there you will find something around three volts.”

        I didn’t watch the video for a crappy hack, but the editor *should* do that to make a write up to make sure he has the right numbers.

  2. I wonder how many cars don’t have a 5v source these days.

    Also: why not simply power the phone directly from the voltage instead of charging the battery? Then you don’t have to wait 30 minutes.

      1. And you never got a USB convertor plug? I mean I don’t know any shop that doesn’t have those constantly on sale, from the supermarket to the clothing stores.
        You are practically forced to get those things. And they are cheap as hell.

        But I have no reason to doubt your words, so perhaps there are many that held out and don’t have such. But it does surprise me..

    1. My sister has a 2015 kia that does not have a 5v port, i dont even have a converter in my car … my samsung S5 has more than a full day of charge on it … never been left out in the dark even on a 12 hour car trip using GPS and bluetooth audio
      as battery life increases its becoming more and more obsolete
      Not to mention the cheap ones you get at like 7-11 i would not trust them as far as i could throw them … they are linear converters and can easily overheat and fail high

    2. You couldn’t realistically run a phone directly in this manner. He said in the video that the pencil was about 50 Ohms end-to-end and he notched it about 1/3 up from the bottom. This would mean that 2/3 * 50 Ohms = 33 Ohms on the top end of the voltage divider. Any current that the phone needs will have to flow through the 33 Ohms of resistance. The current during boot is going to be high and current * resistance = voltage drop. The phone will likely experience “brown out” resets continuously as 33 Ohms is a fairly high source impedance.

      I once had a non-E.E. friend trying to power a nice Canon DSLR from a car battery. He used a dummy battery in the camera with wire-leads and cigarette lighter adapter to drop the voltage down to whatever the camera needed. The camera wouldn’t turn on… On a hunch, I told him to go to RadioShack and get some big electrolytic caps so that the supply voltage wouldn’t drop as much when the thing tried to boot. Sure enough, that fixed it. His situation was better than the one you mention; using a regulator to power a device vs. using a voltage divider.

    1. They would protect against over current, under and overvoltage, but not overcharge. That’s the common misconception.

      You can slowly damage a Li-ion battery by not terminating the charge. The protection circuit doesn’t protect against that as it doesn’t keep track of how long the charging cycle is. The constant voltage part stresses the battery.

      1. my comment was related to this stuff we’ve just seen.. not on regulary base charging.. i know it’s just a short term second layer protection.
        btw you’ll find out that there are some chips on those tiny bms that will disconnect the battery from the charging pads if heavy overvoltaged. Many people, multiple and getting better bms to avoid them catching fire for their misconceptions about tech :)

    2. Cell phone batteries have a thermal overload fuse. Try to drive too much current through the battery (charge or discharge), and the fuse opens. That’s a safety device to protect against short circuits, not a BMS. and hardly fail-proof. It’s very easy to overcharge the battery and start a fire without tripping the fuse.

      Laptop computer batteries are a somewhat different animal. They also gave the thermal fuse, but also have a controller than maintains a record of the battery’s age, temperature, state of charge, etc. This controller talks with the computer which actually performs the charge as directed by the battery pack controller.

  3. A cellphone draws several hundred milliamps of current to charge a battery. This has several implications for this “hack”.

    A voltage divider can supply only as much current as what is passing through the resistance, so recharging a cellphone at any significant rate (>250 mA) requires that the resistance of the pencil is below 48 Ohms, but that also means there’s enough power loss in the graphite that the pen will likely catch fire.

    So I took a pencil and measured it, and found it at about 75 Ohms. Connecting it to a 12 Volt source quickly heated it up, and the current hovered around 140 mA, which would charge a smartphone in mere 7 hours! Leaving it for 30 minutes would give me about 5% more charge, but I wouldn’t leave this thing unattended for 30 minutes for fear of causing a fire.

      1. In a desperate emergency situation, dialing 999/000/112 etc. bypasses the phone’s battery management system and lets you run the battery to the ground, which gives you more talk time than 30 minutes on a pencil charger.

          1. I find that when my phone runs out of battery, it shuts down, but allows you to re-start it in case you need to make an emergency call.

            If you don’t start the call right away, it of course shuts down again.

          2. I used phones in the past that did not have that feature, but maybe it’s standard now.

            Either way, the point here is that it’s odd how in movies and TV shows nobody can make a simple voltage divider.

            If you don’t want the phone story, how about you are out of cell tower range and your icom needs power then? Same concept applies.

    1. If you are in a desperate emergency, then you wouldn’t be waiting the 30 minutes for charging before trying to get help. I supposed you could get stuck and snow in in a huge storm…

      To have 12V handy means that you are likely in a car (or some kind soul’s car), so it isn’t much to shop for a $2 charger + USB cable in a dollar store than trying to hold piece of wire trying to make contact to the battery by hand.

    2. yeah very few have mentioned the true feasibility of using a pencil as a VR. I encourage everyone to give making a pencil VR a try. Experiment. But keep it well away from your face.
      Not only is a hot pencil a probability, but fire is actually possible.
      While it is an interesting hack to demonstrate that conductors and resistances are all around us, it is not practical, and occasionally unsafe.
      The problem is in the led chemistry. It varies somewhat from pencil to pencil. And your resistance is wrapped in kindling.

      I HAVE caught a pencil on fire once this way, and smoked more than a few. In most cases, my loads were meager.
      Its a fun demo for kids in a controlled environment, but not a good hack to perform “on the road and in distress”

  4. All well and good until the pencil begins to smoke and the phone battery explodes…
    Like “recharging” AAs or better yet coin cells with 12V 2A for a quarter of a second at a time a few times in a row until it gets as warm as you dare, just to get that much needed extra half an hour.

    1. Although this might work in practice for a “regular” battery, carbon/alkaline. Lithium tends to get a little too hot a little too fast. As for phone batteries I doubt the protection/charging circuit would like it much. I guess one could always break the phone battery open and bypass it for charging then reconnect it to put it back in the phone. I’d add a disclaimer but I like Tesla coils, and those moments when physics and chemistry make you feel just a little nervous far too much to be a great fan of health and safety.

      1. Depends on the car. Number of cylinders. How heavy the pistons, crankshaft, camshafts and valves are. The compression ratio. The tension of the springs in the cylinder head. The weight of the flyeheel etc. All these factors, and more, change the torque needed to crank the engine. The car also needs electricity for the fuel pump and ignition system.

        1. It is true, those are all factors. However even a small car engine’s starter will draw over a kilowatt of power, and most draw 2 to 3 times that. Even a stack of cell phone batteries won’t have enough power to turn over even the smallest car engine.

      2. The best it would do is to slow charge the car battery up enough that it would start. Don’t expect the phone battery to delivery tens of times its rating of current directly. Asking the lithium battery to delivery more current than its rating would only cause the protect circuit to shut the pack down or start a fire.

    1. So, I had a look for a cheap unnamed car battery from a reliable source, apparently they can supply 360A while cranking, I don’t think there’s any way of getting that from a single lipo but you could conceivably get it from some supercaps. Just gotta charge the caps from the cell phone battery then hook em up in whichever orientation you need to get 7 or 8V for cranking. Admittedly these aren’t things you can buy from a gas station but you could plan ahead. Supercap starters seem to be a semi-common thing for reducing car weight and allowing cold diesels to start more easily (doesn’t lose charge in the cold so easily, can if needed push hundreds of amps more than required in warm weather)

  5. For me this is a usefull thing to know and hope to never use. Many of the commenters label this as an unsafe way way to charge a Phone. The notion of macgyver makes me to beleive this is only for extreme emergencies such as being locked up by a villan. If I remember correctly this is one of the safer hacks that would be used. In reallity macgyver would pump the full 12v through the Phone battery creating an explosion beating the bad guy without calling for backup…

  6. Stop bashing. Is it the right way to do it if you have all modern tech available in a hackerspace or techshop: NO. Is it something you could do during an apocalypse or on a deserted island: YES definitely. And yeah you could also attach the 12v->4v ish divider directly to your phone too if you needed to call instantly. Anyway loved this and it was funny too ;)

  7. Anyone able to do this successfully already knows enough to do it without being told how. Who hasn’t exploded the odd pencil by the time you get to the age of being able to solder? Making a video of it so that people that don’t know or understand are able to do it is a bit dodgy though.

    Having said that I did like it, particularly the commentary.

    1. Actually, I would just the pencil as a series resistor. 9V (12V-Vbattery = 8 to 9V) applied across a 50 Ohm pencil passes about 180 mA (requires a 1Watt pencil), Well under the 500mA USB output of most chargers, and 2000mA of high current chargers. Charge for an hour, or until the pencil gets really warm.

      Fast charge? Use a really short 10Ohm pencil for 900-1200 mA. After 15 minutes, a flaming pencil (10 Watts!). or burnt fingertips from holding the wires on the battery contacts (which ever occurs first) will give a decent charge to a dead battery. More than enough to operate a phone for >15 minutes.

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