Quick & Dirty USB Phone Charger


Hackaday forum member [Dan Fruzzetti] wrote in to share a simple, yet useful hack he built just the other day. He and his wife both have Evo 4G smartphones and they were pretty disappointed in the lack of portable charging solutions available.

Instead of buying something and modifying it to his needs, [Dan] decided to build a quick and dirty charger instead. His ghetto-mintyboost was built into a cheap project box he found at Radio Shack, which is stocked with a set of four D-cell batteries. The batteries were wired in series and connected to a pair of salvaged USB ports mounted on a small piece of protoboard.

Knowing that most portable devices get 5.7v from their chargers already, he was not worried about hooking his phones straight into the 6v battery pack he built. He says that the phones actually charge pretty quickly, and that he estimates he should be able to get about 50 charges out of the box before he needs to swap the batteries.

This is not a complex hack by any means. It is quick & dirty, solves an annoying problem, and it’s dead simple to build. That’s exactly why we like it.

33 thoughts on “Quick & Dirty USB Phone Charger

  1. Cool idea but I would imagine the non-environmental friendly aspect of this makes it less than marketable (hence the lack of quality solutions). Could you do this with a bank of capacitors and a solar panel?

  2. While I have yet to look at the outputs on a scope, the labels of the chargers for my cell phones to me indicate a Pulse Width Modulated output sent to the phone. I assume the PWM circuity is place in the charger to kept the phone to the smallest size possible. This hack provides no means of limiting the charging current, current limiting the reason for PWM being used by battery chargers. This is a well known hack for charging a cell phone in a pinch, but is not a set it and forget means of recharging the battery. The battery could over heat to the point it’s a fire hazard, if left unattended. As most things in life YMMV caveat emptor.

  3. @D_ every single phone charger i’ve seen is just a regulated 5 or so volt output. no pwm, no current limiting (to an extent, short circuit protection excluded). the battery charging circuitry is all in the phone, so no fancy power supplies are needed. just somewhat regulated.

  4. I feel that the figure of 50 charges is highly dubious. Alkaline D cells are typically rated 12000mAh, while the Evo 4G battery is rated 1500mAh. Since all his cells are in series, the capacity is still 12000mAh, and since there’s no boost or buck circuitry anywhere (cell phone charge circuitry are linear regulator devices), this means he will have a theoretical maximum of 8 charges.

    A real heavy duty mintyboost was featured on Hackaday before: http://hackaday.com/2010/05/02/a-more-powerful-boost-charger/ which uses a DC/DC boost circuit with 4 2400mAh lithium cells. But even then that only gets you about 6 charges.

    Perhaps he should consider carrying around a car battery, and a DC/DC buck circuit, that should net him 150 charges no problem.

  5. What I want to know is, given how simple this stuff is (the fact that these cellphones charge over USB with simple USB cables means that there is nothing fancy in the charger), why are the commercial offerings for this so @!#%^&@$%$ expensive?

  6. basically because they can be. I went to radio shack to look for a simple car cigar lighter -> usb port charger and the only thing they had was $34.99. So I found in their parts drawer an empty cigar lighter plug and a 7805 regulator (the guy there didn’t know what that was) and spent $6.50 on parts to do exactly the same thing as their $35 product.

    just preying on people who don’t know what is actually going on inside is all.

  7. I use this for years, as a quad-NiMH cell holder with an attached USB connector. (Was a quick-and-dirty solution from available parts when I needed it.) Got two models over the years, one with flat 4 cells and one with the cells next to each other (round-ish). Saved my posterior for ages as it can power anything that has USB power feed (pretty much everything these days – phones, even some cameras can be recharged that way) and a pocket full of fully charged 2300 mAh NiMH cells can yield a lot of phone airtime.

    Good for traveling all the day and night and day and running GPS on a smartphone for all that time. Except the last few hours, when I found that the last of the battery sets is a lemon.

    Over a decade ago I used to power a walkman from a pair of D-cells. Lasted for MUCH longer than the AA cells it was designed for. Amazing what a few drops of solder, a piece of wire and some electrical tape can do.

  8. “He and his wife both have Evo 4G smartphones and they were pretty disappointed in the lack of portable charging solutions available.”
    they should be disjointed in their researching abilities

  9. @therian: it was more like i was itching for a project :-)
    @D_: you are exactly correct. The input is very different from the phone’s expectations, so the charge must be babysat; I should have mentioned this at the forum.
    @Jonathan: there is no reason you should ever pay the retail price for their crap; a lot of businesses expect accessories to drive a revenue stream. One can get equivalently performing crap from Chinese vendors at about 15% of the cost of US retailers. ‘DealExtreme’ comes to mind as example if one is ok with risk of ‘counterfeit’ branding.

  10. @mike: the phone stopped showing ‘charging’ during the 19th charging cycle in my test, using youtube older daughter to effect quick discharges and recharging while still hot

    However an ‘ideal’ case suggests about 17 charges from dead to full, I mean 50 regular-use charges, say 20%-75% or so. Sadly, i unplug my evo at the 5:45 alarm and by the time i’m at work (7:30) it reads 82%. The evo just degrades its batteries super fast.

  11. I’d run this power through a multimeter, and measure current. Most phones do not like over 850mAh and they may SEEM fine but you can shorten the life or damage the battery. I’d guess this setup is peaking around 3000-4000 mAh which is dangerous. Weaker D cells will limit the danger, less power.

    You’ll get “test charges” till the cows come home, or the voltage drops so low that the phone refuses to top up the battery (probably at 4v when there’s still juice left, even though there is juice left).

    Put a LT1302 or Max 75x in front of this to maintain a constant floor of 5v (you will be over 5v with fresh batteries, as a boost does not limit or regulate).

    Try charging from 0 to 100% – as someone else noted, you will get a half dozen charges, if that.

    I built a traditional mintyboost and carry a few Eneloop AA’s around. Now I am building a series+parallel mintyboost which boosts mAh capacity. You could easily build an 8-cell NiMh pack that powercharges a mintyboost just fine.

    BTW – I can’t say enough good things about Eneloop. They keep 75% of their charge after 6 months… I charge a whole bunch and with C and D sized sleeves and the MB, they’ll power everything.

    This is really NOT economical with D cells, and especially without a boost circuit.

  12. @D_: Perhaps some phones are different, but the majority of phones these days are capable of being charged from USB, and we know THAT doesn’t have any special PWM or current regulation (other than turning off completely when you try to draw too much).

    Perhaps you’re getting confused with NiMH and NiCad batteries, which have a complex charge profile, and charge termination is often based on temperature as well as voltage and currents. Lithium-ion cells have extremely simple charge profiles (constant current to 4.2V, then optionally constant voltage to 0.1C amps). Because of this simplicity, lithium-ion charge ICs are extremely small and cheap, and handle preconditioning, charge and termination automatically. I would be EXTREMELY surprised if a cell phone manufacturer decided to move a 50 cent part to the charger instead of integrating it in the phone.

    (I personally use the MCP73831 lithium-ion charge IC for my projects, which is available in a 2mm x 3mm DFN package, or SOT-23-5, and costs less than 50 cents).

  13. @Cool!: mAh is a measure of energy capacity of a battery. 1000mAh would mean the battery could supply 1mA for 1000, or 1000mA for one hour, although realistically the capacity is lower at higher currents. There is no condition where charging from a battery pack with a higher mAh rating is detrimental to the phone (unless your voltages were wrong).

    The current drawn by the phone is limited by the charge IC. As long as you stay within the voltage ratings of the charge circuit, then the current drawn by the phone will stay at the value preset by the charge IC. There is no situation where you could possible “force” more current into the phone and damage it in that way, other than exceeding the voltage rating and damaging the charge IC.

    The mintyboost does not boost mAh capacity, it trades current draw for voltage. For example a pair of 2000mAh AA cells in series boosted to 5V would be roughly equivalent to a hypothetical 5V 1000mAh battery. Though perhaps what you mean is that mintyboost allows you to access more of the total capacity of a set of cells that you might not normally be able to use because the cell voltages are too low

  14. It may work for this EVO, but I’d be very careful about saying this would work for any other phone. Most chargers for phones are regulated, if not to 5V, to within the USB spec, or < 5.25.
    A lot of 5v power ics have an maximum operating voltage of 5.3v and an absolute maximum of 6V. Four brand new Alkaline batteries could start at over 6.3V. While the charging IC in the EVO might be able to handle this, no device is designed to handle 6.3V.
    Spring for an extra $3 for another series D cell and a 7805 regulator, and it might save you a couple hundred bucks

  15. @macw :) Are you saying every auto appliance electronic tech is preying on the public because the public doesn’t know what’s going on inside? Sorry Hackaday; Radio Shack bashing by those who patronize Radio Shack anyway because no one else in town stocks components, does push a button of mine.

  16. Welcome to 1999, when this device was first available here in the UK from our famous £1 stored.

    Also about the PWM mentioned above / linear regulators…

    Most chargers these days are not linear as when left plugged in, they are bad for the envirinment. You will find modern chargers are switch mode. Which is I guess where the confusion comes in.

    But yes, you can plug pretty much any voltage around 5V into the charger port, the in phones very complex charger IC will sort the rest out.

  17. I don’t know what my £0.49 eBay USB charger uses for voltage regulation, but it certainly doesn’t get warm at all like a 7805.

    And yeah, they shipped it from China for that price.

  18. If I was making this, I’d probably add a silicon rectifier diode in series to drop the output by 0.7v or so. I’m curious as to what happens as the D-cells go flat and the output voltage falls below USB spec though.

  19. If you wish to go the extra mile, you could include a simple 3-pin 5V linear regulator between the batteries and the USB plug, just make sure you use a low-dropout one – the 7805 drops about 2V, ie. it needs at least 7V in to produce 5V out. But then do include a switch as well – linear regulators slowly eat the battery even when idle.

    Or, you could just wire one or two 1A or bigger diodes in series with the batteries – they tend to drop 0.6V each, and eat no idle current.

  20. @ Mike; As I preface my first comment my remarks where based on the labels on the charger for the cell phones I had. The chargers for my portable radios that used nicad batteries had a single solid straight line on the label of the wall charger. the charge circuitry was in the battery pack of some of those radios. without schematics for the phone I have no idea where the charge circuitry for the is located The first time I seen wall charger labels with a solid straight line pair with segmented line was with a Nokia phone using ni-mh battery, a second Nokia using a li-io battery had a charger with a similar label. As does my current LG 260 using a li-po battery.

  21. @D_

    The [solid line with dashed line] as seen here: http://en.wikipedia.org/wiki/Direct_current#Applications
    does not indicate PWM:
    It only means that the voltage will be DC rather than AC, i.e. strictly positive, rather than alternating positive and negative.
    It doesn’t indicate the waveform of the supplied (or required) power.

    90% of the time (if not more), it will be a relatively continuous voltage (say 5.7V), usually produced by rectifying AC, then smoothing with a capacitor, then (maybe) regulating to a fixed voltage.
    The symbol is, indeed, very misleading…

    AFAIK, most phones/etc. have internal charge monitoring, and only require a power source (voltage and tolerance will vary, though)
    Some, hwever, will require ‘magic’ voltages on extra wires to recognise the ‘charger’ as safe (read: genuine, overpriced) and actually start charging.

  22. @dan fruzzetti: My HTC desire also used to eat through ~18% of its battery on the drive to work. The JuiceDefender app has much reduced this problem for me. Give the free version a try – I liked it so much I paid for upgrade :)
    What I think was happening – I’m running several apps in the background which each power up the mobile data radio whenever they want something. The periods rarely line up. The Juicedefender app keeps the radio off for a period, then turns it on (and tells all the apps) – forcing the apps to grab data at the same time and reducing the data radio on time.

  23. @holy_smoke: if that was a reference to my comment, chargers are indeed switched mode, and provide a semi-regulated DC voltage. The lithium-ion charge IC that actually handles the charging of cells on the other hand, is usually a linear device that drops the input DC voltage to whatever voltage is required to charge the cells at the designated current (usually 2.7 to 4.2V range).

    mains -> switch-mode supply (5V DC) -> cell phone -> charge IC (2.7 to 4.2V DC) -> li-ion cells

    Or simply from the article:
    4xD-cell (4V to 6V DC) -> cell phone -> charge IC (2.7 to 4.2V DC) -> li-ion cells
    (and this is fine if the charge IC accepts 6V)

    or minty-boost:
    2xAA-cell (2V to 3V DC) -> DC/DC boost converter (5V DC) -> cell phone -> charge IC (2.7 to 4.2V DC) -> li-ion cells.

    This also implies that you might be ok to fully charge your phone from maybe as low as 4.5V DC source (so actually four D-cells might not be a bad idea, as long as you’re sure the charge IC is ok with 6 to 6.3V.

  24. I’ve found a much lighter solution that’s rechargeable, and higher capacity would be multiple cell phone battery’s. You can typically go to a battery recycle center, and obtain many cell phone batteries. Stack about 5 rows of 5, get about 20 charges, and just build a charging input with regulator, and then you have a more functional system. It’s lighter than those batteries as well. They are flat so you can fit many.

  25. Just an FYI for people complaining that their phones (mostly HTC EVO, and maybe others from HTC) drop a huge amount of charge early in the day: they don’t appear to trickle charge. As soon as they hit 100%, they stop charging and run off battery. There are a lot of questions after this, like how low will the battery get before it starts to charge from the wall again, no solid answers, but repeated testing has shown that leaving the phone connected over night results in a quick drop early on, vs watching it hit 100%, and unplugging right away (or external charging of batteries).

    There are lots of ways around this…
    You could unplug it as soon as you wake up, start to get ready for work, plug it back in 10 minutes later, finish getting ready, then grab the phone.
    Or, top it off using a car charger on your drive in.
    Or, if the phone has enough juice to make it through the night safely, don’t plug it in until you wake up, charge while you get ready.
    Or, use an external charger for batteries, and swap to a freshly charged one in the morning. And again in the evening if you’re a heavy user…

    Using an external charger exclusively also has the added bonus of never having your phone fried by a power surge/lightning strike.

  26. This project has inspired me to build one myself. I had a battery operated fan that takes 4 C cells and I had a usb extension cord. I mated the two and it works great, as a phone charger and still works as a fan

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