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Why does my phone charge more quickly on some chargers?

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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 it’s being charged.

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

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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 chargers take your line voltage – typically 120 or 220 volts – and convert it to 5 volts. It’s that 5-volt side that is then connected to your device to charge it. 5 volts is not only the same on all chargers, it’s actually part of the USB standard. Power provided over USB cables is 5 volts, period.

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 5 volts.

As it turns out, amperage makes all the difference.

Volts and amps

Voltage and amperage are confusing concepts. An admittedly over-simplified1 metaphor looks like this:

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

  • Voltage is akin to the size of the hole that the water comes out of.
  • Amperage is analogous to how hard you pull the trigger.

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

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

When it comes to USB connectors, the “size of the hole” is fixed at 5 volts. In a way, that means we can line up or plug in any two holes of the same size, and they’ll fit. (Perhaps you’re using your squirt gun to fill a bottle. If the holes aren’t the same size, things could get messy.)

So the only thing we can control is amperage.

The USB specification

It turns out that USB, while a very nice, albeit occasionally frustrating, standard for connectors used for USB, actually gets in the way.

It specifies that the voltage should be 5 volts, and that the amperage provided by a computer’s USB data connection should not exceed half an amp, or 500 milliamps (500mA), where a milliamp is one one-thousandth of an Amp2.

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 device3. 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

Every charger has its output specifications printed on it somewhere, often in incredibly tiny print. For example: Output 5.1V (5.1 volts4) and 750mA. An adapter with that rating can provide 1.5 times the power of a standard USB data connection from your laptop. Thus, it would charge your device faster (though not necessarily 1.5 times faster, as things are rarely that simple).

Power rating on a USB charger

Another 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. It’ll almost certainly charge your device faster.

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 nothing more than small computers. They use more power when they’re working hard on then they do when they’re idle.

What does it mean to “work hard”? Well, that varies. Watching video, playing a game, running a WiFi hotspot, or just running a poorly written app on your device will use more power than when the device being charged is doing relatively little. In most cases, a sufficiently powerful charger will provide power faster than it’s being used, and your device will charge, albeit 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. I’ve experienced this running my phone as a WiFi hotspot (very power intensive), using only my laptop’s USB connection for power, which is the minimal 500mA. The phone lasted longer – several hours, instead of just one – but at the end of the day, the battery was still nearly empty.

Best practices

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

  1. 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.
  2. Try to make sure that 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, simply to get the general concepts across. And for the record, my degree is actually 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: Apparently up to a maximum of five amps.
4: The standard is indeed five volts, but some variance is allowed, so 5.1 is OK.

39 comments on “Why does my phone charge more quickly on some chargers?”

  1. The limit on current from computer USB outlets is a nuisance. But my Dell computers have at least one USB outlet with a ‘lightning’ symbol (kind of jagged thing). These provide more than 0.5 A and are therefore technically not USB compliant but very useful!

    Leo – your articles are brilliant but I’m not sure you got the pressure and hole size the right way round on your water pistol (as we call it over here) analogy. I think of voltage as pressure and amperage as flow rate.

    • That’s why the footnote – I agree it can be viewed both ways, but this allowed me to then also use the size-of-the-hole analogy as well. Analogies are, by their nature, imprecise.

    • Technically your Dell is very much standards compliant, the lightning symbol means it’s a charging downstream port in accordance with the USB Battery Charging Specification and must provide between 1.5A and 5A. It’s always the responsibility of the portable device (PD) to negotiate how much current it can draw from the source. The source basically has 3 jobs, 1. maintain 5V (-ish), 2. tell the PD how much current it can draw, 3. shutdown and recover if it tries to draw more than that.

  2. In addition, there is a prescribed hardware handshake that can occur between the charger and the device. When this handshake is present, the charger and phone can “agree” upon a higher charge rate than the default USB “powered” connection.

    Naturally, Android and Apple devices have different handshakes. When looking at chargers, having a charger than understands the handshake procedure for your device can strongly impact the charge time.

  3. Once again, you’ve explained something very confusing to me in a very clear and concise manner. Thanks for making understanding computers and electronics just a bit easier for the pc/electronic-illiterates like me!

  4. Power from a USB socket can get very confusing. As Leo says, the average PC struggles to produce more than 0.5 Amps from a USB, laptops sometimes less.
    The latest ‘USB’ wall-socket style charger that came with my tablet outputs 2 Amps, and I often get an ‘insufficient power to charge device’ message if I actually use my tablet while it’s plugged into my PC.
    This ‘power usage’ is especially relevant if you try to use the various power banks on the market, as they often don’t give out more than 1 Amp – effectively they will only slow down the battery use, not charge it up. Be warned.

  5. Just to top it up.

    At times, it happens that the quality of the usb cable becomes a limiting factor too when the cable is of sub standard quality.

    The phone will charge but a lot slower evrn though connected to a powerful charger.

    Though not foolproof; but if your android phone says USB charging even when plugged in to electrical mains then you have a faulty sub par cable. Check your status under –

    System / Battery and check the staus under Battery usage and it should say “charging on AC” and not “USB”

    Ravi.

  6. Actually Bob, in your water pistol analogy, you got volts and amps the wrong way round. The pressure is analogous to volts (potential difference or electromotive force) and the size of the hole is more analogous to resistance. The larger the hole, the lower the resistance and, therefore, the greater the flow. If you think of a car light bulb with a very thin filament, the amperage is low, say half an amp for 5 watt running light but if you put a screwdriver across the battery terminals, although it’s only 12v, the battery will deliver it’s maximum amperage and melt the screwdriver.

  7. Thank you, Leo, for this explanation. May I point out that this topic highlights one of the most poorly understood concepts in the field of electricity/electronics, and it stems primarily from what I would call non-standard terminology. In virtually any other context, the rateof an action is expressed in conjunction with the time value: gallons/minute, kilometers/hour, feet/second, etc. In the case of the flow of electrons, however, the correct expression of rate of flow is not “coulombs/second”, but the equivalent “Amperes” or “Amps”.
    Because a mobile device’s battery represents an accumulator that accepts an incoming supply of electrical energy and doles it out on request during operation, I think a better analogy than a squirt gun might be a toilet tank. It discharges upon request, and refills at a rate (Amps) determined by the supply pressure (voltage), the opening of any control valve in the path (the charger’s own limitation), and the size of the supply pipe (the cable to which Ravi referred). The pressure is preset by the USB standard, so the charger’s capacity, coupled with the cable’s resistance, determine the rate of flow (Amps or milliamps) available to fill the “tank”.
    It is also true that the target devices are able to determine on their own whether they are connected to a data device such as a computer, or to a charging device, and they generally adjust their charging rate accordingly so as not to overload a computer’s USB port. A simple search on the Web will return more information on this than any mortal could possibly need.
    These observations are not intended to “educate Leo” — it’s clear he understands it already — they’re just offered as another way to look at the situation for those trying to understand. I hope they help someone.

  8. Thanks Leo, I had wondered why one charger worked quicker than the other. I’ve also found switching your phone off while charging makes a huge difference. It’s not as if you’re not going to miss anything while it’s charging, and it only takes a few seconds to switch it back on when charging is complete.

  9. Good article, but the water analogy got switched. Voltage is like water pressure and amperage is like water flow. So the comparison should read:

    Voltage is akin to how hard you pull the trigger.
    Amperage is analogous to the size of the hole that the water comes out of.

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

    Pull the trigger harder. (Increase the voltage.)
    Make the hole bigger. (Increase the amperage.)

  10. You have your voltage and current A over H Leo. Voltage is pressure, Amperage is volume. The 1000s of volts from a car ignition is not fatal because there is only micro amps, no current to speak of, so no power and that is the killer, when the wattage gets too high. Power is Watts = amps X volts. Charging is controlled by the voltage, try putting 5 volts to a flat phone battery and you would have way over 500 ma. The same applies to a 3/4 charged phone, 5 volts is much lower. Without the charger voltage being above the battery voltage no charge can occur.

  11. With the USB fast charger does it matter if you use the cable cord that came with the fast charger and does cable cord matter or can any cable cord be used to fast charge a phone and the same result?

  12. I have a charge status app on my phone the results are always different when I unplug then plug it back in no change in what the phone is doing, same cable same charger one second its 800ma unplug.. plug it in refresh and it says 500ma if I keep unplugging I can get the whole range from 100ma to 1000ma.
    is it the app not reading correct amperage or is my phone really charging at what ever amperage I leave it on

    • My experience with phone boosters and battery saver programs etc. is that they don’t perform any useful function. I sometimes wonder if they even consume more resources than they actually save. Some might work, but so far, I’m not convinced.

  13. I thought Leo meant pundit; an expert in a particular subject or field who is frequently called on to give opinions about it to the public.

    • A pedant is one who makes an ostentatious and arrogant show of learning. I guess some pundits can be pedants. πŸ™‚

  14. What about heat? Fast charging can create more heat which could reduce the life of a battery. Some cheap chargers rely on the temperature rise to stop the charging.

  15. I think it’s the other way around … The speed of the water in that squirt gun is relative to the voltage of a conductor…. And the amount of water that flows through that hole is relative to the current in a conductor….. πŸ™‚ … That’s why they used thicker wire for high current applications …

  16. Thank you for the article, I suspected that but was too lazy to check the Amp.
    However, I now checked all of my cheap chargers, and all of them are 1 Amp.
    I was wondering why some were charging nearly double as fast as others.
    Turns out it’s the cable.
    I bought a couple of long (3 m) USB cables, and using these, makes all the difference.
    I tried all 4 chargers, with short cables and these long cables. It’s the cable.
    Isn’t it strange? Is it possible that the cable is parasiting so much energy from the charger output???

  17. what about new adaptive chargers that deliver various volts and amps depending on how far along the battery has been charged? but the real kicker is how does the charger know how much power and what voltage and what amps to supply the device? there are no extra leads for this or does it use the data lead cable to tell the charger what voltage and current to supply at any given time and when to switch to trickle charging?

    • @ Bonkers ~ What you are essentially referring to as ” Chargers ” are actually [ and technically ] Adaptors which ‘only’ supply power to the Phone. The Charger is actually inside the phone.. Some of these adaptors come with the option of 9v 1.67 mA as well as 5v 2 mA. Such Adaptors which are shipped along with the phones have the capability of either ” fast charge ” or ” normal charge “, hence the two specifications as above. Such phones have the option of turning on fast charge which are built in under settings [ Battery ] When you opt for Fast charging, the charger in the phone will draw current as per the higher rating i.e 9v 1.67 mA. If you want to know which is higher….5X2=10 and 9X1.67= 15.03. Based on this principle, your phone gets charged faster. I have this adaptor for my phone too, but never use it. Reason ~ it generates heat which is harmful for the Lithium batteries. A normal charge serves the purpose and I am never in a hurry.

  18. Hai Charge spot , u have seen it wrong …your Mobile charging also depends on your battery life and usage..The battery after the mobile’s warranty period gets a slight weak.and goes on .You may have battery fault also

  19. Here are some observations that may sum up the discussion:
    1) The wall (120V) and car (12V) adapters have evolved as the power needs and device batteries have grown so they are not all the same.
    2) Using the adapter and cable that came with the device is usually the safe way to go.
    3) The current accepted by a smartphone is controlled by the smartphone so adapters with greater output may make little difference.
    4) The USB cable has four wires: two for power and two for data. If the adapter puts certain voltages on the data lines the phone is told ‘I can give you can 2.1 amps (or whatever) if you want it’ The phone accepts it if it can handle it. Most car adapters have 1.0 and 2.1 amp slots, the only difference is the control voltage on the data wires in the 2.1 slot. This means the phone that only wants 1.0 amp will accept 1.0 amp from either slot.
    5) Your charge time will be about twice as long if you use a 500 milliamp charger when a 1000 milliamp (1.0 amp or greater) adapter came with the phone.
    6) Your charge time will be about twice as long if you use a USB port on a computer as the power source.
    7) Your charge time will be about twice as long if you use a 6 foot USB cable instead a 3 foot one.
    8) Your charge time will be about twice as long if you use a generic unshielded cable (usually flat) instead of a shielded (generally round and thicker) one.
    9) Your charge time will be about twice as long if you have the screen on. If you charge while doing power hungry things (streaming video or navigating) the device may overheat and shut down. (My S4 does)
    10) All this may soon be irrelevant as USB-C replaces what you have, but the dollar-store car adapter still works fine.

      • 7 is definitely wrong.
        8 is partly correct.
        Cheap adapter cables, whether from the dollar store or the impulse item section of the grocery store checkout line or online tend to use lesser gauge wire than the one that came with your device or those found at a more reasonable price in a phone or electronics store.
        It’s been awhile since I had Electronics in school, so I don’t know how much effect the gauge of the wire has on the flow rate through that wire, but I would think it would be measurable.
        Regardless, the lower gauge wire, coupled with less and lower quality casing and shielding means less wires conducting electricity to start and many more wires with lost or intermittent conductivity due to breakage with use.
        So even if your cheap cable works sort of ok to begin with (likely 250-650mA* vs 1000-1850mA*), it will probably be reduced in effectiveness by 50-100% within a week of average use, or maybe a couple months if you’re lucky or very careful.

        * As measured by AccuBattery charging app for Android

  20. I have a Samsung Galaxy. It can be connected in 2 ways. One is by a micro USB cable, and the other is a full USB 3 cable. If I use a USB 3 cable and plug it into a USB 3 port, will it charge faster?

  21. Using the manufacturers recommended charger usually works better for whichever phone you’re using. This could mean that the phone has a current limiting circuit to ensure you are using the recommended charger. Just saying.

  22. hello sir… what should i do when i charged the android phone in 7 volts output? because it is already short circuit instead of 5 volts i used 7 volts but my android phone got damaged what should ill replace?sir what do you think will be the damaged of my android phone?

  23. what does the line above the three dashed lines mean, it has to do with AC /DC????? right?? my Gf’s is charging super slow and it has two long lines, one atop the other instead of one and 3. could you please try to explain to me what that would be?

  24. HI. I have a samsung galaxy s7edge and i have tried many chargers and cables. I never got more than 4v and 1amp for charging. I just dont understand why. Most of the chargers were quick charge 3.0 even the original charger is rated to 2 amp and another thing that I can’t understand is why on charger says 5v 2a or 9v 1.67a if the usb is limited to 5v so what is the point of 9v if u can’t use it. And why the phone is not able to receive more than 1-1.1 amps. Another question is why not even the voltage isn’t reaching 5v and is going up to 4-4.2v

    • Maybe? I couldn’t tell you which uses more power overall: a higher current charging for a shorter time versus a lower current charging for a longer time. The different would likely be insignificant.

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