I'm considering to buy a 20 watt charger for my iOS-devices. Obviously it charges a device two to three times faster then the 5 watt chargers, which I currently use.

I just wonder: Might it reduce the life-time of the battery? Or is it even dangerous (strong heating up, explosions)?

Should I definitely get an original Apple charger? Or are others vendors safe too?

  • You should probably spin off your second question. Comparing Apple to non Apple is entirely different than choosing Apple adapters at multiple power levels.
    – bmike
    Dec 7, 2020 at 5:39
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    The "charger" is not a charger at all, it is a power supply The charger is in your iOS device, and it receives its power from the power supply. The iOS device might "talk" to the power supply to figure out how much current it can deliver, and tell the internal charger to go into slow charging mode it the power supply is too weak. But it's still always the internal charger in the iOS device which decides what to do.
    – Klaws
    Dec 7, 2020 at 11:25
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    Just to provide technical background, when charging it is the role of the charger to provide a constant voltage (5V, 12V, 20V, depending on the specific use case). As long as it provides that, it has done its job, and the phone will charge safely. However, pull too much current (above the 20W), and the hardware will not be able to maintain the voltage. The decrease in this voltage could harm the device. That being said, there's a lot of safeguards in place to avoid harm to expensive devices.
    – Cort Ammon
    Dec 7, 2020 at 17:56

7 Answers 7


I'm considering to buy a 20 watt charger for my iOS-devices. Obviously it charges a device two to three times faster then the 5 watt chargers, which I currently use.

I'm not sure about that, the BMS on the iOS device should alter its power draw to match what it's programmed to accept, given the current state of the battery.

If 5 W is the most it can charge at, then plugging it into a 20 W charger isn't going to force more than 5 W in.

If your device is capable of drawing more than your current charger provides then you'll see some improvements in charge time with a higher capacity charger but not necessarily by the ratio of how many watts the new charger provides vs the old one, ie. 20/5 = 4x faster. The BMS has to accept 20 W and even then things are a bit more nuanced as 20 W would be the maximum, not continual power draw.

Might it reduce the life-time of the battery? Or is it even dangerous (strong heating up, explosions)?

It really shouldn't, unless there's an existing flaw in the battery or charger.

Should I definitely get an original Apple charger? Or are others vendors safe too?

I think that's going to be tough to get a definite answer for unless you have a teardown on the specific device, from someone qualified to comment on its quality. I've seen some teardowns of USB chargers with terrible isolation and there have been cases of people being shocked while using their phone as it's charging.

I'd put money on there being plenty of properly made, safe chargers out there. The question I ask myself is: do I feel safe plugging a $1,000+ phone into this to save a few dollars, or is my peace of mind worth the ~$30 for an OEM part that I've seen a teardown confirm that at least the design is sound?

  • A higher rated charger won't cause harm, but also won't provide benefit to justify the cost.
    – Pedro
    Dec 6, 2020 at 9:07
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    Isn’t slow charging (slightly) better for the battery? Of course the BMS will keep currents below actually harmful levels, but it will still heat up and I’m not sure what’s going on inside the battery chemistry wise …
    – Michael
    Dec 6, 2020 at 15:43

A higher wattage charger will not harm the device, but it will also not charge a device any faster than a matched rating charger.

The power or current rating of a charger (or any power source) is its delivery capacity. The power or current rating of a device (or any load) is what it draws. No matter how much power or current a source can deliver, a load will only draw up to its own rating. Conversely an under-rated source will be insufficient for the load. In general, using an over-rated charger is a waste of money, but it won't cause harm.

USB is USB is USB, the brand doesn't matter, & any exception is equipment that differs is breaking USB spec. USB-C delivers 5 to 20 Volts (negotiated between source & load) at up to 5A. USB-A tops out at 3A & is 5V only. That's why USB-C can charge devices that support it faster.

If you have multiple iOS devices I suggest a better choice would be a multi port USB-C charger with ports of at least 5W capacity.

P = V x I, where P is power in Watts, V is Voltage, & I is current in Amps (all 3 units are capitalised because they're people's names). So a 5W USB-C charger could be delivering 5V at 1A or 20V at 0.25A, as negotiated between the devices. A 20W USB-C charger could deliver 5V at 4A or 20V at 1A, as negotiated. A top rated USB-A charger will deliver 5V only at 3A = 15W, but many supply lower current for lower resultant power.

  • Indeed, as long as the voltage is the same, 5V. We have 20W = 5V x 4A, and the newer iPhones consume about 2.4A where they charge from 0. So 1.6A won't be used, no problem. Note that charging newer devices is very slow with only 1A. So the new 20W charger will make a difference.
    – Déjà vu
    Dec 7, 2020 at 16:37
  • @e2-e4 USB-C negotiates voltage between devices, from 5V to 20V. A truly compliant USB-C charger, even the most basic wall charger, supports the full voltage range & that negotiation. The fact that voltage is no longer fixed (as it was with USB-A) is why there has been a shift to giving charger ratings as Watts (= Volts x Amps). Amps is only truly meaningful as a rating when Voltage is fixed.
    – Pedro
    Dec 12, 2020 at 3:47

20W USB chargers are not dangerous to use with an iOS device. A non-defective USB charger coupled with a non-defective iOS-device will not overheat, explode, or reduce the life-time of the device.

It is perfectly valid and normal to use a 20W USB-charger with an iOS device.

Any standards-compliant charger is fine. Apple chargers follow the same standards as chargers from other vendors.

You might want to stay away from the cheapest, no-brand chargers, as the quality is usually not that good.

Note: A device won't necessarily charge faster with a higher capacity charger. Each type of device has a maximum wattage that they can draw from a charger - having a charger with a higher rating doesn't speed up anything.

For example an iPhone 12 can take advantage of the full 20W rating, where as an iPhone 7 only draws 10W.

  • But a higher rated charger will not charge faster than a matched rating charger.
    – Pedro
    Dec 6, 2020 at 9:25
  • I didn't said that they would.
    – jksoegaard
    Dec 6, 2020 at 9:33
  • The original questioner is believing, incorrectly, & probably hoping it will.
    – Pedro
    Dec 6, 2020 at 9:47
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    I have added to my answer that you shouldn't in general believe that. The original questioner might not be incorrect, as he could very well have iOS devices that support faster charging than 5W - but we cannot know with the information available to us.
    – jksoegaard
    Dec 6, 2020 at 10:16
  • @Pedro that's not true. Apple delivered its iPhones, for instance, with a 5W (1A) charger, as Op specified, which clearly under-performed! They can take more than that (like 2.4A for a new iPhone).
    – Déjà vu
    Dec 7, 2020 at 16:41

Buying a non apple charger is fine, as long as you take a reputable brand. Anker is known to be good for exemple.

Avoid cheap Amazon or Aliexpress no name chargers at all cost!

As far as battery life is concerned, yes, it will have a small negative effect on battery life: Faster charge means more heat, which is known to have a small cumulative effect on battery life.

I don't think it's particularly worth worrying about since the phones are designed to be charged this way.You could also mitigate this buy using 5W charging for night time, and 20W for quick day time top ups.

Don't forget that your phone battery WILL degrade anyway, and that the battery can be exchanged (for a fee, or free under warranty)


The iPhone 12 has a 2815mAh battery. At a nominal voltage of 3.7V, this is 10.4Wh. Charging at 20W is equivalent to a C rate of 1.92 (20 / 10.4). Typically for maximum lifespan, Li-ion manufacturers generally recommend charging at < 0.8C for optimum battery lifespan. (The other answers are correct in that the BMS in the phone will not accept charge current in excess of the design spec, but 20W is now an allowed option, at least with an official 20W Apple charger).

For comparison, a Tesla Model Y has a 75kWh battery pack. The peak supercharger power is 150kW which is a C rate of 2 (150 / 75), so scaled for battery size, the 20W charger is roughly like supercharging your phone. It's pretty well accepted that while supercharging your Tesla is safe and not catastrophically destructive to your battery pack, it also isn't ideal for long-term battery health.

Will your phone, or your Tesla explode? No, not unless something else is wrong. But is it ideal for your long-term battery capacity? No.

My recommendation is that if you need to charge fast, go for it. But if you're going to have your phone on the charger overnight anyway, why not use a slower charger in that location and save your battery a little stress?

  • Interesting (+1), but the iOS devices charging firmware is smart, and delivers more or less watts depending on how the battery is empty.
    – Déjà vu
    Dec 7, 2020 at 16:43
  • Right, but this doesn't change the physics that faster charging is harder on the batteries. They're making a calculated tradeoff that making people happier with faster charging is worth more long-term battery degradation. Like the Tesla supercharger, I'm happy to have the option, but I try to use it only when necessary. Dec 7, 2020 at 20:10

As several have explained, so called "chargers" are actually just charging power supplies; it is up to the charge controller inside the device to chose the appropriate charging rate, bounded of course by what the charging supply can provide.

As a result, the one argument why using a lower wattage external supply might be preferable, is if one felt that the on-boarding charging logic tended to prioritize convenience in the form of speed, over longevity. If one believes that, then using a lower wattage source would constrain what the on-board charger controller could do; in effect you'd "game" it and force it to charge at a maximum rate of your preference, even if it were designed to "think" that a higher rate were permissible in a given case.

Related to this, and perhaps more meaningful, there are those who have rigged charging supplies to cut off when the device battery has reached some fraction of total specified charge short of completion. There is some real reason to believe that the upper part of the charging curve (and storage at the upper part of the allowed state of charge) is more detrimental to long-term service life of lithium ion batteries than the middle part. So if something like 70% charge would see a user through a given day, it's not a crazy idea. At the same time, deep discharge isn't great either. Someone going this route probably wants to carry a portable powerbank as backup.


You know how a wall outlet works? The outlet in the wall that is able to run a garage door opener, a vacuum cleaner, hair dryer, coffee pot, or toaster doesn't make your 15 watt LED lamp overheat and explode. That outlet is able to supply 1800 watts but your little 20 watt cell phone charger isn't melting down. That's because the wall outlet supplies a consistent voltage and the devices plugged in are made to draw no more power than they are designed to take.

A USB power supply does the same thing, it supplies a consistent voltage and the phone is made to draw no more current than it needs. There's an additional level of safety in that that every smart phone has a battery charge management system. They monitor the battery temperature and voltage to make sure it doesn't explode.

Now things happen and you will see people that blow up their phones. Most of this is from improper battery replacements or severe damage to the phone. Sometimes it is from a cheap charger or damaged charger. Buy quality products and maintain them well and the chances of a phone exploding is small.

Buy a quality charger of any wattage and the phone will be safe. Apple has made improvements to their iOS battery management to improve battery life, one thing they did was to lower the rate of charge to the battery as it reaches full capacity. They have the phone "learn" your routine and slow down charging when it thinks you are asleep.

I have chargers from a number of vendors and they have all treated me and my phones well. If you are concerned about quality of a high wattage charger then look for the USB "trident" symbol, UL and CE approval, USB-PD (indicating USB power delivery support), and other markings that indicate someone checked it over for safety. There's no law against building a cheap power supply but it is illegal to use these markings without permission. It's too expensive to fake these markings because the people that own them have a lot of money to chase violators down and take them to court.

  • There's no design required to restrict the current drawn, it's a product of the load resistance. Voltage = current x resistance, which can become current = Voltage / resistance. A 25 Watt light bulb has no sophisticated design, but draws only 25W because of its resistance.
    – Pedro
    Dec 12, 2020 at 3:52
  • That resistance of the 25 watt bulb was designed in so that it would draw only 25 watts. The people designing them use the equations you describe to guide their design. Not sophisticated but still designed.
    – MacGuffin
    Dec 12, 2020 at 5:47
  • That's putting the cart before the horse. Current is a direct product of resistance & Voltage. You can't arbitrarily set or change current in a load. It is always a result of the other factors. Choosing a suitable resistance is such a simple thing that it hardly qualifies as either design or engineering. On the supply side things are quite different.
    – Pedro
    Dec 12, 2020 at 6:37

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