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Why Does My Phone Charge Slowly on Some Chargers?

All chargers are not created equal.

Why do some chargers work faster or slower than others? We'll take a look at some basic electrical concepts to understand what to look for in a charger.
Photo of two modern smartphones side by side on a wooden table. The smartphone on the left displays a vivid lightning bolt icon on its screen, symbolizing fast charging. The smartphone on the right shows a detailed snail icon, representing slow charging. Both phones are connected to their respective chargers, with cables leading off the table.
(Image: DALL-E 3)
Question: I have several USB chargers for my mobile phone. I’ve noticed that one will charge my phone quickly — like in an hour if it’s really dead — while another will take several hours. And connecting a USB cable between my phone and laptop will also charge it, but that seems slowest of all! What gives?

Two things are at play here: how much power your charger can supply and how much power your phone is using while being charged.

I’ll warn you: for the first, at least, you’re going to need a magnifying glass or extremely good eyesight.

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USB charging speed

The charging speed for your phone depends on the power output of the charger and the phone’s power consumption. All USB chargers convert line voltage to five volts, but differ in amperage (current). Higher amperage usually means faster charging. Computer USB ports offer slower charging because they are often limited to 500mA. USB-C chargers can offer higher power options. Keeping your phone idle also speeds up charging.

Getting amped

Chargers vary a great deal in size, shape, and quality. But one of the most important and often unnoticed differences is in how much charging power they provide.

All USB chargers take your line voltage — typically 120 or 220 volts — and convert it to five volts (some include additional voltages as well; more on that shortly). It’s that five-volt side that is then connected to your device to charge it. Five volts is not only the same on all chargers; it’s actually part of the USB standard. All USB power sources like chargers must provide five volts.

Where things differ is in the amperage that the charger provides. That’s a measure of how much electricity, or “current,” can actually be provided through the wire at five volts.

As it turns out, amperage makes all the difference.

Volts and amps

Voltage and amperage are confusing concepts. Here’s an admittedly over-simplified1 metaphor.

Consider a squirt gun — a plastic toy you fill with water. To use it, you squeeze a trigger to shoot the water out of the gun.

  • Voltage is how hard you pull the trigger.
  • Amperage is how quickly or how much water comes out.
  • Resistance is the size of the hole you’re shooting water out of.

You can get more water out faster either of two ways:

  • Pull the trigger harder. (Increase the voltage.)
  • Make the hole bigger. (Reduce the resistance.)

When it comes to the USB power sources we’re talking about, the “pull of the trigger” is fixed at five volts (5V). It can’t “push” any harder.

Resistance is a function of the circuit design — both that of the charger and that of the device being charged. A charger capable of providing one amp of current has a bigger “hole” than one able to provide only half an amp, or 500 milliamps (500mA), where a milliamp is one one-thousandth of an Amp2. The charger’s circuitry controls what it can provide.

How much amperage is used depends on the device being charged. A device may use less than 500mA, all of one amp, or anything in between. If it “wants” one amp and the charger is capable of providing only 500mA, then 500mA is what it gets. I’ll talk in a moment about whether that’s a good thing or a bad thing.

The USB specification

The USB standard gets in the way.

It specifies that the voltage on a standard USB socket should be five volts, and that the amperage provided by a computer’s USB connection should not exceed 500mA. You may recall we used to see USB cables that had two connections to the computer. These were designed for devices that needed more than 500mA and up to 1 amp. The two 500mA connections were combined to provide the required current.

Once upon a time, that was enough. But today, while that’s enough to run many devices, it’s not really all that exciting for charging devices. In fact, it’s definitely the low end and results in a very slow charge for many devices.

USB chargers, however, have no such limitation.

USB chargers

It’s up to the power adapter’s manufacturer to determine the capacity of their device. Providing more power means using more expensive components, so many USB power adapters err on the side of “not much”.

Here’s how you can tell what you have.

Power rating on a USB charger
A charger providing 5.1 volts with a capability of 750mA. Click for larger image. (Image:

Every charger has its output specifications printed on it somewhere — often in incredibly tiny print. An adapter with the 750mA rating shown above can provide 1.5 times the power of your laptop’s standard USB connection. Thus, it might be able to charge your device faster (though not necessarily 1.5 times faster, as things are rarely that simple).

Power rating on a USB charger
USB charger providing 5.3 volts with the capability of providing up to 2 amps of current. Click for larger image. (Image:

Another, as shown above, might offer 2.0A (two Amps, or 2000mA). This charger is capable of providing four times the amount of power as a standard USB data connection. If your device is capable of using that, it’ll almost certainly charge your device faster.

5V, 5.1V, 5.3V?

If you look carefully at the example chargers I’ve included above, you’ll note that they have voltage ratings ranging from 5 to 5.3 volts. In reality, the USB specification allows for a certain amount of tolerance. Interestingly, 5.3 volts is slightly above that tolerance. This is one reason it’s a good idea to use the charger that comes with your device. In this case, the charger came with a Samsung phone that I assume can tolerate the slight excess.

The reality is most devices will tolerate some amount of variation around five volts beyond the specification. The problem, of course, is that we don’t know how tolerant which devices are and what the impact of being off-target would be.

5V, 9V, 15V, 20V, 28V, 36V, 48V?

A new standard for USB power delivery specifies not only several different voltages but a maximum current of 5A. This standard applies only to USB-C style connectors.

By default, the power supply provides 5V. However, devices are allowed to “negotiate” or specify that they want something stronger. It’s this specification that allows your laptop to be charged via a USB-C connection, probably using 48V, when that same power supply might be used to charge your phone at a lower voltage.

The implication is that your phone, if USB-C enabled — and if so designed — can request a higher voltage. As a result, it may be able to charge more quickly when this type of power supply is used.

The impact of a mismatch

I talked about voltage issues above. Fortunately, in the USB world there’s little chance for things to go truly wrong.

However, the most common question I get when it comes to power supplies and charging is about amperage. Specifically, what happens if the “amp” rating for the power supply or charger doesn’t match that specified by the device being charged?

It depends.

  • If the amperage rating of the power supply or charger matches that required by the device, no problem.
  • If the amperage rating of the power supply or charger is greater than that required by the device, no problem. The device will simply use what it needs.
  • If the amperage rating of the power supply or charger is less than that required by the device, there are three possibilities.
    • The device will charge more slowly or continue to discharge as you use it. This is most common for USB power and charging.
    • The device will act as if no charger is attached at all.
    • The device will be damaged. This is rare when it comes to USB power and charging.

It’s truly difficult to harm anything with USB charging, which is one of the nice aspects of the specification. At worst, you just won’t get the charge you need.

What the device is doing at the time matters

While the charger you use can definitely make a difference as to how quickly your device charges, so will what that device is doing at the time.

Most smartphones, for example, are small computers. They use more power when they’re working hard than when they’re idle. Watching video, playing a game, running a Wi-Fi hotspot, or just running a poorly written app on your device will use more power than when it’s doing relatively little. In most cases, a sufficiently powerful charger will provide power faster than it’s being used, and your device will charge — although more slowly if it’s running one or more of those power-draining applications.

In the worst case, a charger may not be able to keep up, and the device will continue to use its battery, albeit at a slower rate than if not connected to a charger at all.  For example, my car’s USB power output is not enough to keep up with my phone if I’m actively navigating with the screen on constantly and listening to a podcast at the same time. Left this way, the phone will have less of a charge when I’m done than when I started. If the phone is doing nothing, however, I’m brought up to 100% relatively quickly.

Do this

Knowing what we now know, there are things we can do to charge our devices more quickly,

  • Choose a charger with a higher amperage rating for its output. At this writing, if I were purchasing a charger, I wouldn’t get anything that puts out less than 1.5 Amps. It’s quite safe to have “too much”; the device being charged will only take as much as it can handle.
  • If your device is USB-C capable, consider getting a USB-C charger that has greater capability. Chargers listed as “100W” (100 watts3) or higher typically have the multiple voltage option that your phone might also be able to use.
  • Try to make sure the device is as idle as possible. Putting a mobile phone into airplane mode, for example, turns off all of its radios and can reduce power consumption significantly.

The net result in either case: a faster recharge.

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Footnotes & References

1: Before the pedants pull my analogy to shreds, realize that it’s an over-simplification on purpose to get the general concepts across. And for the record, my degree is in Electrical Engineering, so I do have some background in the concept.

2: I’ve seen it written as high as 900mA, but 500mA seems to be a very common implementation.

3: Watts is a measure of voltage and amperage combined, and termed “power”. Watts equals volts times amps, so that 48V charger capable of providing 5A is capable of providing 240 watts.

23 comments on “Why Does My Phone Charge Slowly on Some Chargers?”

  1. Very, very interesting, and dumbed down so I understand. Thanks!

    You commented that you use your car’s USB to charge your phone. We have concerns that doing so would enable the car to root around in any information in our phone .. Are we right? wrong in that opinion?

    We don’t let our phone attach to our car for any reason, we personally don’t see the need as we use it as a phone and text machine only, don’t even purchase data.

    Would be interested in your opinion of the security issues (or not) of charging via the car’s USB.

    We always use the OEM charger at home, but we do carry a cable that will not transfer data just in case we need to charge away from home.

    • With your “cable that will not transfer data,” it is perfectly safe to connect your phone to your car.

      The protection provided by all such no-dataline connectors works in both directions: The car can’t send any data to your phone, and your phone can’t send any data to your car.

      Hope this helps…

      • Most phones and tablets default to charging-only mode, and you have to tap to authorize data exchange. If you are unsure if your device does that, plug it into your computer and see if the computer recognizes your phone’s drive without authorization. It’s extremely rare this will happen.

  2. I remember when I had my first HTC smartphone and plugged it into my car to use the GPS, I’d sometimes get a message that the phone wasn’t charging enough and would soon run out of power. That was probably because I was using a cheap insufficient charger. You need a good charger for your car. GPS + music + whatever other apps you are running require a lot of AMPS.

  3. Excited by this post for several reasons:

    1) Knowing Leo answers questions on phones, not just computers

    2) Knowing Leo, like me, uses a Pixel

    I’m glad to get the explanation as I’ve noticed the USB-C port on my work computer charges much, much more quickly, than the USB port on the table at my work.

    The only thing I wasn’t sure of is what if you use your USB-C laptop charger to charge your USB-C of a phone? Will it burn it, or can it control? I don’t dare take a risk.

  4. Fast charging will shorten the life of the battery.
    I don’t mean the charge won’t last as long, I mean when your battery will actually die and refuse to work any more.
    Go to The Battery University website to see how and why.
    In a nutshell “The lithium-ion battery works on ion movement between the positive and negative electrodes”, the faster this happens, the sooner your battery will die.

  5. Good article as usual. I’m still confused about multiple outlet chargers. They seem to be very unclear about how much power they can provide per outlet. Some might provide fast charging, but only for one outlet whereas the others are standard (slow) charge rates.

  6. Leo, regarding your power description metaphor; the one I usually hear of uses a water pipe to describe electrical terms.
    For amperage, a half-inch pipe (500milliamps) will deliver about half the water per second that a one-inch pipe (1000milliamps/1amp) will at the same pressure (voltage).
    For voltage, Water pressure (Voltage) is the amount of force being applied to the water stream (amperage), or how hard it is being pushed through the pipe
    Power (Wattage) is the product of pressure (Voltage) times pipe size (amperage).

    For me, this is the most accurate (and complete) corollary I’ve found.

    Regarding phone chargers, I always use the one that came with my phone exclusively. I’ve never had a phone charger fail, but if that should ever happen I’ll make sure to get an exact match from the phone’s manufacturer if I can. If not, I’ll look for a replacement that can provide at least the same output amperage to my phone at the same voltage (I know the voltage is defined in the USB specification, but I’ll verify compliance by checking it anyway) :).

    Over the years I’ve learned that it’s always best to use the right tool for any job, and the right tool to charge my phone is the charger that came with it, or an exact replacement. I record the charger’s specification in a text file and store it in the cloud (OneDrive) along with all the other information I keep for my devices (PCs, Tablets, my new Video doorbell, etc.) and their peripherals so it’s available from any device/OS I’m using when needed, and so it’s less likely to get lost when transferring ownership of/replacing any device or in the event of any device’s failure. This requires a bit of effort when I get a new device, but it has proved to be well worth it when things go wrong. Additionally, when I transfer ownership of any device I own to someone else, I move the information I’ve collected/stored for it to a USB stick so I can pass it along to them too. If a device fails, I know where it’s information’s stored so it’s easy to delete when the device is properly disposed of. These measures help me to manage the device information I have on hand and insure that only information for functional devices I currently own is being stored (my form of procedural house keeping).

    I know others may not think to go as far as I do about record keeping, but I strongly recommend doing so, at least to some degree. It can make replacing a device or peripheral with the correct replacement much faster and easier. If you don’t want to use the cloud, find a safe way to record and keep your device’s replacement information, but no matter how you store it, when something goes wrong, you’ll be glad you did.

    Ernie (Oldster)

  7. If the phone is plugged into a PC (standard USB, not USB-C), doesn’t the PC lower the amount of charging power because it’s also prepared to handle data too? I know if I use a data-blocker device, I can get a PC to charge my iPhone just as fast as if I had it plugged into the little white brick. Without the blocker, it can take HOURS to charge the phone. Certainly that’s not all amperage related.

  8. Another great article, Leo!
    There is another factor to consider in selecting a stand alone charger – the amount of power the charger itself consumes, particularly while plugged in but not charging. It can vary substantially between chargers rated for the same output. Wall chargers are frequently plugged in 24/7. When family visits, there will 8-10 plugged at my house.
    The input information on the base of the charger isn’t much help, as it shows the maximum power the charger might draw while in use. Instead a device like a “Kill-o-watt” can measure actual power use over an entire day or week. You might be surprised.
    That said, my wife’s Apple chargers have quite low power draw while idle. My Pixel didn’t come with a charger, so . . . .

  9. Leo –

    Great article. Where you talk about “a new standard for USB power delivery…” I’m assuming you are referring to fast/quick charging technology. My understanding is that to take advantage of this technology, the charger, cable, and the device to be charged all need to be compatible with the technology.

    What if only the device to be charged is not compatible, can the charger still be used to safely charge the device (albeit more slowly)? Or does doing so risk damaging the device?

    Thanks and happy Thanksgiving.

  10. This is a good article. I think it is important to clarify the advantages/disadvantages of slow or fast charging. Fast charging will generate a bit more heat. Heat degrades lithium batteries (as in all our smartphones).

    So, if you have the time, shut your device fully off and go with a slow charger. Reserve the use of fast chargers for the times you really can’t afford the time of waiting.

    Lithium batteries “treated right” (i.e. proper charging and discharging) can last substantially longer, perhaps 4x as long as would otherwise be the case.

  11. I don’t know, but I try to use the charger that came with the phone. Of course, those get lost. I DO know not to use an “Anchor” charger, that will cook the factory battery on the first charge!

    • I know the best advice is to use the charger that came with the device, but lately, since most USB devices no longer come with chargers, the best you can do is use a charger that came with an older device. The quality is high and will provide correct voltage and amperage. The bottom line is stay away from dollar store chargers and cables.

  12. In my Experience
    The Charging Cable is just as important an many times the issue.
    I have seen people with 2.1A Chargers saying their phone takes forever to charge because they are using a cheap cable only capable of 0.5A or a damaged cable giving even less amps.
    I think this should be highlighted / included when explaining to people as they do not realise.

  13. Bought a 65W power bank to charge my laptop as it has a 65w output charging block that came with it. To my surprise and frustration, it wouldn’t charge, just continue to run down. Suspect that the equal outputs of both devices caused them to not recognize each other. Finally had to go with a 100W power bank to get my laptop to cooperate and take the charge.

    Ironically, when I tried the 65W power bank later, it appeared to work, so go figure.


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