BU-411: Charging from a USB Port

Become familiar with the limitations when charging a battery with a USB charger.

The Universal Serial Bus (USB) was introduced in 1996 and has since become one of the most widespread and convenient interfaces for electronic devices. Compaq, DEC, IBM, Intel, NEC and Nortel contributed to the developments with the goal of simplifying the interconnection of peripheral devices to a PC, as well as to allow a greater data transfer rate than was feasible with earlier interfaces. The USB port can also be used to charge personal devices, but with a current limit of 500mA on the original design, this might have been an afterthought.

A typical USB network consists of a host that is often a PC and peripherals such as a printer, smartphone or camera. Data streams in both directions but the power is unidirectional and always flows from the host to the device. The host cannot take power from an outside source.

With 5V and 500mA available on version USB 1.0 and 2.0, and 900mA on USB 3.0, the USB can charge a small single-cell Li-ion pack. There is, however, a danger of overloading a USB hub when attaching too many gadgets. Charging a device that draws 500mA connected together with other loads will exceed the port’s current limit, leading to a voltage drop and a possible system failure. To prevent overload, some hosts include current-limiting circuits that shut down the supply when overdrawn.

The original USB port can only charge a small single-cell Li-ion battery. Charging a 3.6V pack begins by applying a constant current to a voltage peak of 4.20V/cell, at which point the voltage peaks and the current begins to taper off. (See BU-409: Charging Lithium-ion.) Due to the voltage drop in the cable and connectors, which is about 350mV, as well as losses in the charging circuit, the 5V supply may not be high enough to fully charge the battery.  This is a minor problem; the battery will only charge to about 70 percent state-of-charge and deliver a slightly shorter runtime than with a fully saturated charge. The advantage: Li-ion will last longer if not fully charged.

Standard A and B USB plugs, as illustrated in Figure 1, feature four pins and a shield. Pin 1 delivers +5VDC and pin 4 forms the ground that also connects to the shield. The two shorter pins, 2 and 3, are marked D- and D+ and carry data. When charging a battery, these pins have no other function than to negotiate current.

USB Plug

Figure 1: Pin configuration of standard A and standard B USB connectors, viewed from the mating end of the plugs.

Pin 1 carries +5VDC (red wire) and 4 is ground (black wire). The housing connects to the ground and provides shielding. Pin 2 (D-, white wire) and pin 3 (D+, green wire) carry data.

Besides the standard type-A and type-B configurations with 4 pins, there are also the USB Mini-A, Mini-B, Micro-A and Micro-B that include an ID pin to permit detection of which cable end is plugged in. The outer pin-1 is positive and pin-4 is negative. USB cables are generally standard type-A on one end and either type-B, Mini-B or Micro-B on the other. The new type-C connector described later features 24 pins and runs on the USB 3.1 standard.

Power Delivery

USB 2.0 with a current of 500mA has limitations when charging a larger smartphone or tablet battery. Keeping the smartphone running on a bright screen during charge could result in a net discharge of the battery as the USB cannot satisfy both. Connecting a high-speed disk drive requires more than 500mA and this can create a power issue with the original USB port.

In 2008, USB 3.0 relieved the power shortage by upping the current to 900mA. This current ceiling was chosen to prevent the thin ground wire from interfering with high-speed data transfer when drawing a full load.

With the need for more power, the USB Implementers Forum released the Battery Charging Specification in 2007 that enables a faster way to charge off a USB host. This led to the dedicated charger port (DCP) serving as a USB charger, delivering currents of 1,500mA and higher by connecting the DCP to an AC outlet or a vehicle. To activate the DCP, the D- and D+ pins are internally connected by a resistor of 200 ohms or less. This distinguishes the DCP from the original USB ports that carry data. Some Apple products limit the charge current by connecting different resistor values to the D+ and D- pins. 

To support charging and data communication when using the DCP, a Y-shaped cable is offered that connects to the original USB port for data streaming and to the DCP port to satisfy charging needs. This appears like a logical solution but the USB compliance specification states that the “use of a Y-cable is prohibited on any USB peripheral,” meaning that “if a USB peripheral requires more power than allowed by the USB specification to which it is designed, then it must be self-powered.” The Y-cables and the so-called accessory charging adapters (ACA) are being used without apparent difficulties.

The question is asked: “Can I cause damage by plugging my device into a USB charger that delivers more current than 500mA and 900mA?” The answer is no. The device only draws what it requires and no more. An analogy is plugging in a lamp or a toaster into an AC wall plug. The lamp requires little current while the toaster goes to the maximum. More power from the USB charger will shorten the charge time.

Sleep-and-charge Mode

In most cases, turning the computer off also shuts down the USB. Some PCs feature the sleep-and-charge USB port that remains powered on and can be used to charge electronic devices when the computer is off. Sleep-and-charge USB ports might be colored in red or yellow, but no standard exists. Dell adds a lightning bolt icon and calls it the “PowerShare” while Toshiba uses the term “USB Sleep-and-Charge.” The sleep-and-charge USB ports may also be marked with the acronym USB over the drawing of a battery.

USB 3.1 – Type-C Connector

As with most other successful technologies, USB has spawned several versions of connectors and cables over the years. USB chargers do not always work as advertised and charge times are slow. Incompatibilities between competitive systems exist, willingly or by oversight.

Companies overseeing USB standards are aware of the shortcomings and brought out the type-C connector and cable based on the USB 3.1 standard. Rather than using four-pins as in the classic type-A and type-B, the type-C connector has 24 pins and is reversible, meaning it can be plugged in either way. It supports 900mA and, on command, delivers 1.5A and 3.0A over a 5V power bus while streaming data. This results in 7.5 and 15 watt power consumption respectively, as opposed to 2.5W using the original USB (current times voltage = wattage). The type-C can go up to 5A at 12V or 20V, providing 60W and 100W respectively. Figure 2 shows the pinout of the USB Type-C connector.

Figure 2: Pin configuration of USB Type-C connector.
Side A and B are mirror images. Some pins are connected in parallel to gain higher power and more reliable connections.

New devices come with the USB-C connector and USB 3.1, but consumers beg for two or three regular USB 3.0 ports on their gadgets to support what worked so well in the past. USB 3.1 is backward compatible with USB 2.0 and USB 3.0 and the classic type-A and type-B connectors. While in transition to the type-C, adaptors are available to convert, but expect lower data transfer speeds with adapters than what USB 3.1 offers.

With the availability of higher currents and voltages on the Type-C system as compared to the Standard A and B connectors, damage to a device can be afflicted when giving a wrong digital command. The commands may come from a device or an adapter requesting modified power demands. It is advised to only use compatible or trustworthy brands when experimenting with higher voltages and currents in USB connectors.

Last Updated 2016-11-25

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On August 9, 2011 at 3:22pm
John B. wrote:

Thanks I allways wanted to know which side caried the positive charge !


On August 10, 2011 at 6:54pm

Charging from a USB

On December 28, 2011 at 10:35am
Bob Stuart wrote:

Have you done any research on AC powered USB chargers?  My family is accumulating a number of USB charged devices (i.e. mobile phones, game controllers, e-readers, etc.) and I’m wondering if any AC powered USB charger can be used with any of these devices or if there are limitations.  Thanks!!

On January 4, 2012 at 12:33pm
barak wrote:

Charging Li-Ion From USB trough Series 0.7V Diode will give 4.3V which is the charging voltage (~4.2V),
enother option is to use PNP transitor with the Base connected to the gnd or to 10K thermistor in order to protect from high temperature (50deg),

what do you think about this?

On February 10, 2012 at 12:27pm
David Olive wrote:

I need to find out charge time of lithium battereis versus depletion…ie…if it takes
2 hours to get a 100% depleted battery back to full (or close to) charge, how fast will it take a battery that has been depleted by only 80% back to full charge….I am trying to design a high powered dual bank battery based system and need to know how long it will take to charge one bank of batteries based on depletion %?

Also, do I need to “rest” the battery between charge & use cycles, or can I drop the battery into the circuit as soon as it is charged…ie…will I diminsh teh life charge cycle times by not giving the battery any “rest” between cycles?


On June 1, 2012 at 7:17am
Angela wrote:

This is not quite true, the data pins in the USB specification play an important part in charging circuits. Depending on voltages applied to those data pins, current limiters on the host device can operate anywhere from 500mA to 1000mA. By default, with no data pins used, 500mA will be provided, but the computer may conserve power once draw becomes significant. By telling the computer that there’s a device pulling power (by applying standard voltages to these lines), the computer will be able to allocate further units of power out, in 100mA increments. There’s some changes with the USB 3 spec, so I’d recommend reading them if you’re planning on using USB charging.

On September 6, 2012 at 11:48am
Cole wrote:

Thats one thing I’ve been noticing a lot lately…for example i got a new droid razr and if i use an older usb it will connect and be accessible by computer or if u use a usb car charger it will connect and say charging…but will never really increase at all… Now if i use the motorola usb it works great… So some things have to be using the data connections also…I’ve also hard wired an Micro usb into my truck directly but it was basic 2 wire cable + and - and it also would say charging but never really did…Makes it frustrating when you save old cables and u end up having to get new ones anyways.

On September 21, 2012 at 1:08am
nadeem wrote:

i want to charger chip but farst i want chip rate becouse again quantty you ak

On October 23, 2012 at 5:46pm
Chaiyan Pongcharoen wrote:

Very Good and education info.

On December 5, 2012 at 11:52am
dave_shah wrote:

Can Anyone say me How can i plug the red and white(i.e Here pin 1 and 4) wires to my cellphone batries or direct to cellphone to get charge my cell phone

On February 1, 2013 at 10:21pm
Steve wrote:

I always charging my devices using usb ports only. its never created any issues. I refer to my friend and relatives also. steve @ http://www.indcel.com/

On February 2, 2013 at 8:18am
jimmy owen wrote:

very good website learned a lot info

On February 8, 2013 at 10:29am
Jay Sandler wrote:

Cole: (if you haven’t gotten an answer already) I believe what you are experiencing is that some devices require higher amperage to charge their batteries - especially if the device is powered on while charging…. While nearly all USB based chargers output 5v, if you read the output current, it varies quite a bit… A simple flip phone may come with a 250ma charger, your Droid (as my Droid 2 does) may come with a 1000ma charger. I also have a Motorola GPS watch which came with a charger that looks identical to the one that came with my Droid, but it only outputs 800ma.  I think my iPad charger outputs 2 or 3 amps.

Which brings me to my next question… Does anyone know if the charge circuitry is built into modern smart devices?  A USB port is not a charger, just a power supply in a sense, and those cheapo car chargers for $5 at the gas station can’t be too high tech so I wonder if today’s smart devices just expect 5v and take care of the rest… Anyone know?

And my last question is… I want to try making a portable charge pack for 5v charged devices.  ( I know I can buy these, but its a project I want to make)... I wondered if just using a 5v battery as a source would be workable or would I be better off with a 12v battery and charge circuitry (like that used in a car charger).

Thanks for any help!


On February 8, 2013 at 11:22am
Cole wrote:

J: That makes sense….but I’ve also noticed a difference in just the cables themselves .....For example..I had a micro usb from my old blackberry storm and with everything else the same it couldn’t keep up like the stock motorola cable….now what you said makes sense but is there differences in basic usb cables? If so there is no need to go buy off brand cables to save a couple bucks anymore if its not made the same…


On February 8, 2013 at 10:18pm
Jay Sndler wrote:

From what I know, unless the USB cable is defective, they should all be the same…. EXCEPT, there are actually at least 2 versions of micro-USB connectors.  I’ve seen diagrams online that make them seem obviously different, however in real life, to me anyway, I can hardly tell them apart.  I can’t remember which is which, but I think a b-type socket is what is on a Droid (and most devices of seen) but the other type is an “a” or an “ab”.  I think the ab will accept either cable, but the b requires a b cable…

I have only one device I know of, a Vizio Tablet, that has what I think is an AB socket… My Droid charger will fit in the Vizio, but the connection is kind of loose… The cable that came with the Vizio, however, though it fits in the Droid (albeit not as easily as the correct cable if you pay attention) will not charge the Droid…

The chargers that came with both devices feature standard USB ports and will charge either device with the correct cables.

Otherwise, it has been my experience that every other USB powered/charged device I own seem to work well with any of my chargers (save for the low current chargers that I have… Including the car charger I bought from Verizon with my Droid 2 which only outputs 600ma and does not seem to charge the phone while its powered on- frustrating)

Maybe you have some devices that are using “A” or “B” or “AB” connections and they are not all the same.

On February 15, 2013 at 10:02am
Seth Fasig wrote:

To answer Jay’s question, yes. In almost every modern phone or other device that can charge off a USB there are internal components that control charging. The phone monitors how much charge is in the battery so it knows how much current to pull or when to stop pulling (though they will still pull a small amount) based off the batteries charge. This is no to say that using an incredibly cheap charger is a good idea though, some may not have the ability to supply the necessary current and some really bad charges may risk shorting and attempt to pull far to much current through your device, of course that is a worst case scenario. As for Cole’s question, in addition to the different types of cable you can get that jay talked about, the size of the cable could theoretically be the problem. It really depends on the device. Take, for example, the iPad. It requires massive amounts of current, therefore it probably has very strict charging circuitry built into it (I really don’t know if it does but my guess would be it does). Now with some device that must keep such an accurate account of the current flowing into it, that small 350mV drop through the wire could greatly impact the supply. Consider, V=IR. You need 2 amps and the USB supplies (ideally) 5V so the resistance must be 5/2 =2.5 ohms. Now take that means the if the supply voltage drops to 4.6V you are only drawing 4.6/2.5= 1.84A that is a considerable difference, not to mention that 350mV could increase/decrease depending on the length and gauge of the wire. just something to consider.

On February 16, 2013 at 2:32am
Frank wrote:

Nice article and website, very useful information! I got some questions, I am building a USB loading station for 4 smartphones that is indirectly powered by a solar panel through a 12V lead battery.

For the lead battery, I have a battery charge controller, that keeps the voltage for charging at the right current with build in protections. But from the lead battery trough usb ports to the smartphones i have some trouble designing the right configuration. Because how can i control the amperage/voltage through the usb to be the right, and what is the right amperage (1000mA and then it will be tuned down further by the smartphone? or 0.3C sow 600mA?) . And does every usb then needs its own regulator? or can you build them in series (what if one is only in operation?) I cant find any standard components for this like the 12V charge controller. Thanks in advance.

On June 13, 2013 at 1:14am
Fer wrote:

I have an LG Optimus G which has a non-removable battery, so I want to extend the battery life to make my phone last longer. So, a part from turning off features when not needed I ask myself how to propertly charge the phone. So I have three possibilities, can someone help me?
1) The phone comes with a 1.2A charger
2) The phone can be charger from USB-PC 500mA
3) I can use other chargers I have in my home. I own a 0.7A charger from Samsung

1 & 2 are supported by LG instruction manual. 3 is not supported.

so, what method should I prefer to make my battery life last longer??? is it safe 3?

On July 10, 2013 at 11:58pm
Brett wrote:

question,Can you charge at a 10C for a 200mAh Lithium battery via a USB 5.0

On July 24, 2013 at 3:42pm
Shai wrote:


A li-ion phone charger shuts itself when the battery is full . This is done because the overcharging the li-ion battery is very dangerous ... it can explode .

What about charging a li-ion phone ( or device ) from the USB port of a pc ...

Does the pc knows to stop charging when the battery is full ?

On August 7, 2013 at 12:37am
Vase wrote:

Hello. I usualy charge my phone using a USB cable (I have a smartphone witha a micro usb connector and conect it via usb to the computer). I wonder if I can wire a 5V power supply to the usb instead of the computer. Do I need a current limitater circuit?
Thank you!

On November 4, 2013 at 8:53pm
justo wrote:

used 4 diodes,2 going to + battery line, 2 from the - line to a usb port.used the data usb modem cable. Without the diodes the battery overheats with them takes time to charge but safe to run both as modem and charger without the usb hub warning power failure.

On November 24, 2013 at 5:35am
pete kaye wrote:

Yesterday I had a smoketastic e-cig battery explode in front of me after about 1 hour on charge via USB port.can a nyone explain .The computer was damaged and Iwas hiot by the xploding cartridege .The remains burst into flame and needed an extinguisher to put out. I have photos! I could have been seriously hurt as my face was only 12” away.
Any comments???

On February 27, 2014 at 7:24pm
Sanjeev wrote:

I have a Samsung ATIV 10.1” Tab 3 that has a pin charging port for which I have a wall AC charger. I want to know if tgere is a USB charger available. My information is that the Tab has a 12V power requirement and the USB system can charge only upto 5 V. Need clarificatory help that will lead me to hunt for a USB to Samsung proprietory pin charger or abandon my search. Thanks in advance.

On April 13, 2014 at 11:41am
stephen wrote:

There’s a typo at, “With the USB supply current limited to 500mAh…”  mAh should just be mA

On May 26, 2014 at 9:10pm
Jerry wrote:

I have an MP3 player which charges from USB.
I was wondering if there’s any way of monitoring the level of charge in the battery, so as to avoid over charging & possibly harming the battery.

On June 21, 2014 at 1:02am
Evan Giles wrote:

There are two ways of listing power for electrical devices input or output, if the input is
500 mA then so long as the device you attached to it supplies this amount or more you can use it, if the output is 500 mA then that is all you will get out of it and if you attach some thing that requires more it will simply take longer to charge so long as you don’t use the device at the same time then device short the charger because you will pull to much current so the rule more current in is better then less out

And just so you know I have had two friends blow the charger for their Laptops not because they were under powered but because they are not designed to run them continuously and they simply shorted out

On July 27, 2014 at 10:50pm
rejoice clark wrote:

I have an android tablet with a3.7V Li-Polymer battery. I can’t get the battery to take a charge, Therefore I can’t get my device to power on . Plugged in or not. What can I do?

On August 12, 2014 at 4:03am
PeteTy wrote:

standard spec on usb port is 5v 500milliAmp

some of the motherboard plugs are merely connected to the 5v buss (100A or so)
the wire in the cord is very small  

newer motherboards sometimes monitor current draw thru the bios and you may receive an error if you draw more than 500 ma

usb hubs sometimes give that error with 3-4 devices connected, but most hubs have a plug for external power

the usb chargers in the wall or on the car cigar lighters are sometimes rated at 1 or 2 Amps and these usually use high frequency switching regulators

if you need 10A or so at 5v from your computer you are better to use one of the 4 pin HD molex plugs and you have a choice of 5V or 12v (big wire)
these connecters are free since most drives are sata now

On August 12, 2014 at 4:30am
PeteTy wrote:

Most Li-ION battery packs have rather sophisticated circuits that shut down the pack if any cell is below 3.5 Volts

if you can get access to the cell themselves you can sometimes brig them back if they havent dropped below 3.1V
(be VERY careful monitor voltage and current with kelvin connections and temperaterature)

if the cell voltage is below 3V DO NOT try to charge it

if you can charge the offending cell to 3.5V the normal charger can take over

if you are a bit safety conscious do your charging outside away from anything combustible in a metal bucket filled with dry silica sand
it the battery case is cracked and water hits the lithium you WILL have a fire

a company known as Boeing ( 787 Dreamliner ) decided they should write the FAA standards for charging Li-Ion batteries and ignored all other standards ( ntsb doe erda epri )

On August 12, 2014 at 5:04am
PeteTy wrote:


drug dealers supply their poisons to their victims with absolute disregard to the victims health or well being

The tobacco institute is an organization to maximize profit
they have managed to get surgeon generals fired at least 9 times and have many congressman paid off

Dont expect nicotine delivered by patches pills gum or ecigs to be any safer to you as an addict (these are also higher profit items)

burns and shrapnel damage are quickly treatable and curable (perhaps a wake up call)

The addiction to nicotine is also curable but may take years. it is well worth the investment and will be much better and less expensive in the long term


On August 12, 2014 at 5:33am
Jerry wrote:

And this has what to do with the charging of USB batteries?

On August 12, 2014 at 6:18am
PeteTy wrote:


usb is a source of 5V to charge/power many devices

USB is actually a serial communication protocol with a small power source

since USB is on all sorts of things from cars to tv’s to phones to picture frames it is universal, more universal than actual mains power

“USB batteries” is an unfortunate oxymoron used to start the thread

this “thread” does not have a tree from the original article

most unorganized “threads” such as this one that dont reply to specific questions
you may put @JERRY on top to help the not thread organized reply system become somewhat organized

the shelf at the library has Dewey decimal
PDR is next to aromatherapy
Innumeracy is in 510[math] it belongs in psychology or religion not math

On August 12, 2014 at 7:05am
Jerry wrote:

Sorry if I misunderstood how the system worked here.  I was just under the impression that the general subject matter was meant to be batteries (USB charged or otherwise), and I was confused as to where the subject of tobacco came from.

On August 12, 2014 at 7:08am
Jerry wrote:

After a few re-reads I think I may now understand the relevance of the comparison with safety.

On August 12, 2014 at 11:23am
PeteTy wrote:


yes using a usb port to charge a LiIon battery to run a heater that delivers nicotine

kind of like any auto safety ignores the fact that more than 1/2 the fatalities involve booze drugs or cell phones ( DWI impaired includes cell phones)

ONLY someone like FORD has the resources to pull this off
if you remember the most rollovers of any vehicles were ford broncos
Firestone took the hit for that

we need Gun control since as many guns are involved in injuries an deaths in a year as cars in a day

a lot of new cars have blue tooth they would be safer with cell phone jammers and legal in 44 states

that is to say cell phones while driving are illegal in 44 states


On August 12, 2014 at 11:39am
Jerry wrote:

Couldn’t agree more about the mobile phone jammers.  Quite right that they should be illegal.  I used to work as a car delivery driver, when mobile phones were still pretty much in their infancy, but even then then influence it was having on the safety, or lack thereof of driver yacking away - more often gesticulating wildly whilst doing so.  I understand why it wouldn’t be practical to jam the entire car, as there could be passengers that it would affect as well, but to work on devising a jamming ‘bubble’ in the driver’s section & making that a compulsory attachment would be ideal.

On August 25, 2014 at 4:42pm
PeteTy wrote:

@jerry perhaps the fone jammers should a 10 ’ bubble beyond the car

ive seen people reach across the seat or into the back seat to roll up windows and slap kids around ...
a minor thing like rolling down a window and holding a fone 3’ away is not a deterrent
even if a cop should notice
Italy fines anyone with a cell fone in a public building
concert or movie with cell fones ringing is kind of annoying

but still it didnt help a lot no most concert halls and librarys and court rooms have jammers

cell fones are UHF a few 10s of milliwatts with a line of sight range of under a mile

walkie talkies in a GIjoe play set are 1/4 or 1/2 watt with a range of 3-5 miles

cb car radios are limited by law to 5W an lower frequency HF so can skip around the globe

most truck stops sell CB linear amplifiers from 100 to 2000 watts

laws do nothing

jammers work

On September 13, 2014 at 12:46am
iBattery wrote:

My iPhone charger is 5V 1A, which is a 5W. iPad charger is rated as 5.1V 2.1A 12W.
If I use iPad charger to charge my iPhone, would it cause problem?

With 2.1A, would it pumping in too much current to the battery.

Note: iPhone battery is around 1400 mAH.

On September 13, 2014 at 8:21am
PeteTy wrote:

the ipad regulates the charging so absolutely no problem

the usb supplies 5.0V
Li-ION can go to 4.5v or so without damage
some usb chargers and hubs are rated 3-4A
some motherboards dont monitor current so can be 70A-120A

On October 10, 2014 at 9:49am
vishnu wrote:

can anyone pls tell me if the small usb charging port found in mobile phones are unidirectional or bidirectional in terms of power flow by design??

On October 29, 2014 at 11:55am
Ron wrote:

I use an iPhone as a GPS distance finder while playing golf.  The battery on the iPhone dies before the end of the round as GPS uses a lot of power.  I have tried wiring the red and black wires of a USB cable directly to the golf battery (26AH) but it provides no charge current to the iPhone.  Am I missing something?

On November 24, 2014 at 12:37pm
Irelia wrote:

I wonder if the cable itself makes a difference. For some reason my powerbank charges with my tablet charger + the cable that came with the charger significantly slower than with the cable (which feels rather cheap) that came with the powerbank. the tabletcharger has 1.4amps so the cable should be designed for it right? I even checked it with a multimeter, the current is about 33% higher with the cable of the powerbank. with other devices like my tablet and phone which also charge at around 1 amp the cables make no difference. Tried it several times, always the same problem.

also all cables I tested have got all pins connected (plus and minus, data + and -, and of course ground) and are all USB 2.0 cables.

On November 25, 2014 at 4:53am
PeteTy wrote:

#20 wire is 10.1 ohms per 1000’
this is about the largest you will see in usb cords
if the cord is 5’ long thats 5’out and back for 10’ total
.1 ohms for the wire
i=1amp you start with 5 volts the most you get at the end of the cord is 4.9v
with 2 amps its 4.8v
about 4% voltage loss

power is I^2*R with 1 amp youve lost 0.1 watts
with 2 amps you lose .4 Watts

multimeters in the digital age imply killohms 3 1/2 digits th ohm i a 1 or zero

the old analog ones you had a zero and perhaps 5 ticks until you see a 1 and a knowb to set the zero on zero ( negate the loss in the test leads)

if you can measure the resistance of the cord you can find the voltage loss for any particular current

or you can measure the voltage at the charger and the voltage at the “powerbank” and divide to find the resistance

ethernet Thicknet #10 wire(rg8) is good for about a mile
ethernet thinnet #16 is good for 700’
ethernet with twisted pair telephone wire is good for 300’
USB is good for 15’

do the arithmetic

On December 8, 2014 at 2:31pm
Orlin wrote:

I have a query - I tried to charge a smartphone (with original 1000 mA charger) through USB port on my laptop.  The charge went up with good speed to about 50%, then it stalled at about 53%.  Half an hour later, when I checked, it showed 46%.  I switched the phone off, and then it started to go up again.  Three more points - (1) I have been using an older USB cable (not sure if that matters, not the original one); (2) during the charge I also used the laptop for browsing and running live stream; and (3) when the phone was switched on, the WinXP would find the ‘new hardware’ every 10-20 sec. (I presume the phone drew too much power and WinXP shut the port off periodically)

My query - if the USB port is unidirectional,  how could the battery dis-charge during charging.  Could someone explain, please?

On December 8, 2014 at 3:17pm
Orlin wrote:

Follow-up on my above comment - with the phone switched off, the battery charged up to 57%, then went down to 56% charge, and went on an on like that.  I disconnected it and switched off the phone - the battery index was still at 56%.

After reading some more (quite a helpful site - thank you for the effort), I have one possible explanation - due to the constant (every 10-20 sec.) port switch off (see item (3) in above comment), the battery has performed great number of mini-cycles (more than 1000 in my calculation), which in turn has distorted its gauge.  Any other explanation will be greatly appreciated.  Also any advise how to get the gauge back to initial state. Thank you.

On December 11, 2014 at 1:25pm
tucker wrote:

actually the usb port is not uni-directional, I’ve used it for both directions in multiple electronic projects

On January 12, 2015 at 11:40am
Bhanu pat. wrote:

Can a u.s.b charger be used in place of 5.5V panasonic cordless phone charger?

On January 26, 2015 at 8:43am
Joe wrote:

I recently bought a Sylvania portable dvd player for a trip for my wife and I.  I’m just curious if anyone knows if the USB port will charge the battery without damaging it?  We have two 7 hour plane rides and the battery wont last that long.  I was thinking of buying one of those speaker/usb charger things but I want to make sure it will work first.  any info would be GREATLY appreciated.  Thanks!


On April 16, 2015 at 1:36pm
mihvoi wrote:

@Irelia : it could be the cable resistance or the negotiation: http://meaningofstuff.blogspot.ro/2015/04/why-my-smartphone-is-changing-very-slow.html

On April 20, 2015 at 11:07am
jayvl wrote:

The wording at the end makes it seem like USB 3.1 allows for 3A charging on the 5V line, concurrent with data. This is not accurate, it is the c-type connector and the PD2.0 specification that allow this—according to wikipedia. So a normal USB 3.1 A-type connector should still follow the PD1.0 spec and not be able to charge faster than 2A.

Also the A-type and B-type connectors have 9 pins in USB 3.0, not 4.

On May 31, 2015 at 3:09pm
Larry Becque wrote:

This article mentions “Can I cause damage by plugging my device into a USB charger that delivers more current than 500mA and 900mA? The answer is no.”  While theoretically this statement may be correct, I have a practical example that disproves this. 
I have been using a 2.1A rated USB car charger to charge my phone for months. http://www.harborfreight.com/universal-usb-car-adapter-61546.html It charges much faster than another that I had with a lower 500mA rating.  No problems charging the phone at all.  Then I bought two USB power banks which are rated 2A for charging input and 2.1A output.  http://www.ebay.com/itm/5V-2-1A-USB-Power-Bank-Case-Kit-4X-18650-Battery-Charger-DIY-Box-For-Phone-MP3-4-/400896416020?var=670439291743&_trksid=p2056016.l4276 The first time I used the car charger to charge it started smoking heavily within a few minutes.  It was so severe I thought the batteries may have caught fire so I took it apart and discovered that an IC chip on the PCB had burned completely.  I noticed afterwards on ebay comments from other sellers of similar devices that if you plug them into a car charger that doesn’t limit power they will blow and they will not warrant replacing the unit.  I’ve used the second power bank now and charged it with a 2.1A rated USB AC wall adapter charger with no problems.  So I conclude that there is definitely a problem with current not being limited, but not sure which device is at fault, the car charger or the power bank.  Both devices work fine with other devices, but not with each other.

On June 1, 2015 at 8:05am
PeteTy wrote:

the answer is yes IF both the usb and the device working are functional

“For-Phone-MP3-4-/400896416020?var=670439291743&_trksid=p2056016.l4276 The first time I used the car charger to charge it started smoking heavily within a few minutes.”

the older12v to 5v regulators are usually shunt based (ie 7805) these can fail to a short and get the full 14.2 volts to the device you are trying to charge.
if the car battery cable is loose and the car is running, you may get far higher than that 15V at the cigar lighter

the new regulators are commonly switch / chopper based ie on/ off 5/12 which only dissipate heat from the ir drop in the switch
but both ways can fail to a short. giving the full 12v nominal voltage

the cigarette lighter is fused at 40a nominal 14V or so and can set most anything on fire
especially a 5V device

some of the european/american power adapters for 220V to 120v
use a diode to change 220vac to 120v dc that will run ac devices that are NOT transformer based

a transformer look like a dead short to a dc source…
an induction motor is also a direct short on dc
most small hand tools are dc motors with brushes (universal motors)
true dc motors the poles are solid iron rather than laminated

any source of electric power that is properly wired and fused is made to keep the house wiring from setting the 2x4 studs on fire if they run over current


On June 1, 2015 at 8:44am
PeteTy wrote:

some devices dont have charger/maintainers/conditioners

ie plug it in to charger if charges to full then turns OFF

my kindle charges from the 2a charger in about 2 hrs or 4 hrs from usb port

the led goes from yellow to green
now if you go back in a week or month the led is still green…
but if you unplug it at turn it on its obvious it NOT a full charge
plug it back in and the light is yellow it turns green again when it reaches “full” charge

it will remain green until its unplugged a week,  a month or year later later
and the battery may run down enough so it cant be recharged (Li-Ion)


On June 10, 2015 at 12:43am
Rana G wrote:

i have phone 3gs mere paas iska charger nahi hai siraf iski aik lead hai jo kay laptop se bht slowww charge ho rahi hai tofrndzz kiya koi mujhay bataae gak me laptop k usb port ka vault barhane kliye kiya karu
plz reply…..

On September 10, 2015 at 12:28am
Drake wrote:

Great article! I was just browsing and looking through some usb port related reviews coz a friend was convincing me to buy one of these USB Sockets. Has anyone here tried one of these? My friend bought it from here http://usbsockets.com.au/ . What do you guys think?

On October 6, 2015 at 6:45am
Jon wrote:

1) Well but the li-ion battery has a “Charge limited volts” specification at 4,2V +- 0,05V. If USB ports have output 5,0V and cca 350mV are losses, it is still cca 4,65V. This is not a problem for the smartphone battery although it has a 4,2V limitation?
2) I have constructed a small solar charger with output 5V, max 1000mA. While charging a power bank (li-ion, 3,7V), it delivers 4,65V and max 750mA. Am I damaging its battery? The charging current is VARIABLE - because of moving clouds etc. from 40mA to 750mA. Is it OK?

On October 7, 2015 at 3:01pm
Larry Becque wrote:

There needs to be a CCCV (constant current, constant voltage) battery charging circuit between the USB port and the battery being charged.  This will limit the voltage to no more them 4.2V during the end of charging.  Same thing with the solar charger.  Take a look at:  http://www.ebay.com/itm/5A-Lithium-Charger-Step-down-5A-5V-32V-0-8V-30V-Power-Supply-Module-LED-Drive-/141249782084?pt=LH_DefaultDomain_0&hash=item20e324a144

On October 8, 2015 at 12:16am
Jon wrote:

thanks for your response. Do I understand it right that for charging a li-ion battery I need this CC CV circuit and the LM2596 circuit I´m using now is suitable only for DIRECT powering other devices?
And still - am I harming the battery by charging it without CC CV module at 4,65V - 5,0V? And how? What´s happening inside the battery while it is getting harmed when incorrectly charging? Well - I haven´t read all the articles on batteryuniversity so fat, if it´s there, please just tell me and I´ll look it up…

On October 8, 2015 at 12:28am
Jon wrote:

I need to clarify - now I´m using LM2596 circuit without the CC CV ability - this type: http://www.ebay.com/itm/LM2596-DC-Adjustable-Step-Down-Power-Module-LED-Voltmeter-USB-2-54mm-Needle-/161025990411?hash=item257de59f0b
So the whole solar charger is a solar array 10V/1000mA and this LM2596 step down regulator with voltage regulated to 5,0V.

On October 9, 2015 at 6:08am
Jon wrote:

I´m sorry, I search through BU chapters and articles and found many pieces of useful information. No need to answer my former questions… I´ve never seen so professional and accessible web like BatteryUniversity so far, thank you for it!

On October 10, 2015 at 3:18pm
Larry Becque wrote:

Take a look at the article at http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries
The voltage limit is very critical which is why charging circuits use both CC CV techniques.  Over-voltage will either cause the battery to explode, catch fire or shorten its life.

On November 18, 2015 at 11:38am
paul wrote:

Since ATX power supply can provide 5V line with higher current than 0.5A, would it be safe to splice a ATX molex cable with a micro-USB and use that to charge a high-demand phone/tablet?

On February 21, 2016 at 8:44am
Laura wrote:

Great article! It helps me a lot especially the clear context about USB charging and standards. What makes me surprised is that you keep information updated and even including USB type C and Power Delivery in this “Battery-central” blog. It’s amazing and thanks a lot.

On May 13, 2016 at 3:30am
prem thapa wrote:

I appreciate a lot for this great article. But i am confused about USB charging. How can we make the current flow from 5V USB to a battery bank which can supply 5V to peripheral(s) ? is there any logic/technique which can make current flow from 5V to 5V ? dont we need a small voltage difference to make current flow ? i guess current flows from higher potential to lower potential. please, anyone give me some idea.

On May 26, 2016 at 2:57am
Abdul Rauf wrote:

I am using Iphone 5, i got a situation with its charging. Whenever i charge the phone with usb 2.0 it charges smoothly even when screen is off. But whenever i charge the cell with either 1A charging adapter or 2A, it stops charging when the screen goes off. It only charges as long as screen is kept on. I even replaced the battery but the problem still exists. While charging with adapter and screen is off it starts discharging after sometime. Any ideas whats the problem and how to overcome this?

On June 2, 2016 at 6:41am
Lyn wrote:

I’m looking at buying an mp3 player but I won’t often have access to a computer to charge the mp3 via a computer USB. Is it OK to use an USB that is plugged into the mains power of a house? (I’m obviously a novice, your information is good for those who know a lot more than me).

On August 22, 2016 at 10:21pm
JHANE wrote:

I wonder because my Globe wifi was damaged. Its not charging anymore through Honda City 2016 model. I used dual USB cord socket. I went to Globe center to check my battery but my battery was okay. We used another Globe wifi but the problem still happened. Upon charging to dual USB port my wifi didn’t work anymore. Even I try to charge to another power supply, I can’t use my wifi anymore. Please help how can I solve the problem.

On September 6, 2016 at 4:20pm
Nicholas MacNaughtan wrote:

HI I have a PCB board that requires a Mini USB for power, however i was wondering if i could use some wires and connect a Micro USB instead also the Mini USB has 2 rows of pins 3 on the first row and 2 on the second I can’t find a diagram showing what these connections are please help quite new to this. regards Nick

On October 8, 2016 at 2:26am
ProDigit wrote:

Make some article about using a 5V USB plug to extend laptop battery life.
I have the EEEPC901, and my battery life is about 6 hours.
I know by connecting a hub to it, that powers an external harddrive, or printer, I can supply power via an external port.

However the laptop (or netbook) by itself without apparatus attached, does currently not benefit from connecting to a 5V USB charger port.

I want to see if my 6 hours could possibly be extended to maybe 7hrs, by providing a constant 5V 500mAh to the laptop, relieving it from some power demands (eg: wifi, SD Card, bluetooth, audio jack, or screen.
The CPU might decide to drop providing voltage to the USB port, since the voltage is higher than usual (maybe 5.1V from the charger), essentially perhaps telling the voltage controllers to cut USB power completely as the external USB charging voltage might provide enough to relieve 1 USB bus/hub.

The math behind this?
The battery connected is a 19V, 6000mAh battery.
This equates to about 96Wh, spread over 6 hours that’s 16W usage (each hour).
The charging voltage and current are: 5V, 500mAh gives 2.5W.
2.5W added to 16W = 18.5W, should extend the battery to:
6 * 18.5 / 16 = 6.9hrs.

I know that the laptop probably won’t get the full 2.5W benefit, but perhaps it gets a percentage of the supplied power, and extends a bit more.
In worst case scenario, the provided USB voltage is lower than the laptop, and the laptop would be providing the charger with power. In this case it would drain.
But most of the times, a laptop in power saving mode, should have a lower voltage on the USB port, than an AC charger port.

Any comment on this?