Technology in terms you understand. Sign up for the Confident Computing newsletter for weekly solutions to make your life easier. Click here and get The Ask Leo! Guide to Staying Safe on the Internet — FREE Edition as my thank you for subscribing!

Why Don’t All USB Chargers Work on All USB Devices?

Question: I guess I don’t really get USB charging especially with our Apple devices. My little mind says that five volts is five volts; apparently, some five volts are different than other five volts. We have more Apple devices in our house than an Apple store. OK, maybe not quite, but for convenience, I’ve bought a number of third-party USB charging devices, including those little cube shaped USB chargers. What blows my mind is that some devices work on some chargers, but others won’t work on the same chargers. “Charging is not supported with this accessory” is a frequent message. There seems to be no logical pattern to this. Initially, I thought it was related to the change in charging that Apple did a few years ago when they dropped support for 12-volt stuff. I think that was around 2007, but all of our devices are newer than that except my trusted iPod video 5G; that charges on almost all of the USB chargers that I’ve bought. Why is five volts not five volts?

To begin, five volts is five volts. That’s an absolute measurement.

But there is more to it than that, as you might imagine.

Let’s look at it in more detail.

Become a Patron of Ask Leo! and go ad-free!

More than voltage

The issue here is that there’s more to charging than the voltage. In fact, it’s the USB specification that actually might get in the way.

The other factor at play here is current. Typically, this is labeled on the charger as some form of amps or milliamps. Current is the amount of electricity, not the voltage, but the amount of electricity that the charger or device is able to provide.

Now, the USB specification actually calls out that a USB 2.0 port on your computer must be able to provide 500 milliamps, or ½ of an amp, of current at five volts. In fact, it apparently specifies that it can only provide 500 milliamps. It’s not allowed to go above that.

For some devices that require more power, you may have seen those funny USB cables that have two USB plugs at the computer end. Each of those plugs can then take 500 milliamps of power at five volts for a total of one amp.

Usb electric charger plugChargers play by their own rules

USB chargers may or may not follow that same restriction. Many do not, and often offer the ability to provider 1 amp, 1.5 amps or even more.

I believe that hubs technically do need to follow the limit. They’re supposed to only provide 500 milliamps per port on the hub. If a device requires more than 500 milliamps to charge and the charger or the port is providing only 500 milliamps, then it won’t work.

And yes, it can be detected by the device. That’s probably where you’re seeing the error message.

Sometimes only speed is affected

Often, devices charge slowly (if at all) on a 500-milliamp source and more rapidly on a more powerful source. The chargers that come with my phone (these are “wall warts” – the kind that plug directly into a wall outlet) are usually capable of putting out 1.5 amps at five volts. That’s three times this USB minimum of 500 milliamps.

My phone charges relatively quickly when plugged into one of those. When I have it plugged into my computer’s 500 milliamp USB port, my phone still charges, but more slowly.

Output amperage is all part of the (often very) small print on the USB charging devices. That’s something you want to pay close attention to. That’s probably the difference that you’re finding between all of these different chargers and USB ports that you’re using to plug in your different USB charged appliances.


Do this

Subscribe to Confident Computing! Less frustration and more confidence, solutions, answers, and tips in your inbox every week.

I'll see you there!

15 comments on “Why Don’t All USB Chargers Work on All USB Devices?”

  1. I have a 12V car charger that wasn’t working – it wouldn’t charge my phone. It turned out to be the USB cable; swapping the cable for a better quality one got the phone and charger happily working together. Always worth a go.

  2. My Sony e-reader (a few tears old) will only work from a Sony charger or from a real computer; it apparently uses the data lines to verify. A “wall wart” (lovely name) is just ignored. I don’t know whether other manufacturers are playing the same trick.

  3. Many a time I have a failing rechargable battery – and many a time I put the sucker in the freezer for a few days, then have another go at charging. Sucess rate about 80%. I know why but you might be better at explaining it than me, I get long winded.

  4. I have an ipod touch 4G, that required the two data cables to be tied together, before by cell phone charger would charge it(the cell phone charger was rigged with a USB socket from a spare device). I got the wiring instructions from the web and knew the risks. I have been charging it for about a year and everything is fine. (Not for everyone, you will need a soldering iron and know how and risk of destroying your device, USE AT YOUR OWN RISK!).

  5. I’ve got an interesting case. My brand new BMW X1 2014 has a USB port for multimedia integration and works fine with the original iPhone 5S cable and with those cheap crappy ones from China (already broke 3 cables in 1 month). I’ve Decided to buy a Belkin cable to use in the car and leave the original cable at home. But the cable does not work in the car. It works great on laptop’s USB 2.0 and 3.0 ports and on Apple’s charger though.

  6. I just bought a 4Sight 4SK606 Dash Camera. Bottom line- can you recommend a WALL USB Charger compatible with this camera? I’m supplied NO specs anywhere either on the camera, or the pathetic ‘instruction manual. The manufacturer’s instructions are extremely skimpy (that’s another subject) but here’s the real problem. They supply you with two (2) cords with which to recharge the camera’s battery. One is hooked up to the car’s cigarette lighter/power outlet; the other is a USB type cord. My question is do you, or anyone out there now the specs on this thing? They are listed NOWHERE. I have tried plugging in to my computer via USB port, but it apparently DID NOT charge even after 6+ hours. It did turn on once I connected it to my car’s cigarette lighter. All I can tell you is that it apparently can be recharged through my cigarette lighter port in my car- trouble is, especially in winter, I’m not too keen on leaving the ignition on for 6 hours for fear of draining my battery. Any assistance gratefully appreciated. I’m 64+ and not techno-savvy! Thanks in advance.

    • Most devices which require USB chargers don’t come with any instructions on chargers is because all USB wall and car chargers are supposed to work with all USB chargeable devices. The only exception I’ve found were that some of the cheaper chargers weren’t strong enough, and even some cheap ones worked well.

      • Quite right in theory, but we have several phones tablets and cameras, some charge on all the chargers, others won’t charge on some, there seems to be no logic about it, out Samsung phones are fussy about the cables when using Samsung Kies to connect to the computer.

  7. If I understand you correctly, I can use a 5 volt 2.1 amp supply as an external power source to run my video cam (Zoom Q2n-4K) which asks for 5 volt 1 amp. It’s via a USB cable.
    Or am I missing something,?

  8. I am trying to charge a speaker that requires 5V and 3A charger with 5V 500mA USB outlet. How long is it going to take , will it charge at all?


Leave a reply:

Before commenting please:

  • Read the article.
  • Comment on the article.
  • No personal information.
  • No spam.

Comments violating those rules will be removed. Comments that don't add value will be removed, including off-topic or content-free comments, or comments that look even a little bit like spam. All comments containing links and certain keywords will be moderated before publication.

I want comments to be valuable for everyone, including those who come later and take the time to read.