The real story behind MagSafe, USB-C PD, and why you need a 20W AC charger

Posted:
in General Discussion edited October 2020
If you want to charge your iPhone 12 using MagSafe, Apple recommends using certain 20W AC adapters -- and they don't need to come from Apple, despite what you may have heard. We explain what the requirements are, and why an older 18W adapter just won't cut it.

The MagSafe Charger can charge your iPhone 12 up to 15W
The MagSafe Charger can charge your iPhone 12 up to 15W


Apple released the iPhone 12 lineup with an all-new accessory and charging system called MagSafe. The name was previously used to refer to a magnetic quick-release cable for Apple's portable Mac line, but had since been lost when Apple shifted to USB-C.

What is MagSafe?

MagSafe in this case, refers to the magnetic capability of the accessories more so than the safety the accessories provide like it did prior. One accessory released is the MagSafe Charger, which is a large charging puck that attaches to the rear of the iPhone 12 for wireless charging. Unlike other wireless chargers, it magnetically grips the iPhone when in use which means you can pick up and use the iPhone while charging.

The large surface area coupled with Apple's own proprietary design allows for up to 15W of delivered power to a device when charging when using the MagSafe charger vs only 7.5W on Qi chargers. The ability to charge at twice the speed of Qi is a major benefit to using MagSafe, but it does come with some caveats.

Buckle up. This is going to get a little more technical than we normally get. If this stops you from reading further, here's the takeaway: if you don't want to buy Apple's USB-C AC adapter, any USB-PD 3.0 one with equivalent specs will do to get that faster charging speed, and a lesser one, won't.

USB-C Power Delivery

USB PD is a specification for handling higher power on USB and allows a range of devices to charge quickly over a USB connection. It facilitates negotiation between two devices so they can determine how much power can be pulled from the charger. Power Delivery offers multiple power profiles from 5V to 20V, including a crucial 2.22 amp power profile. More on this in a bit.

The USB PD 3.0 spec was released in mid 2019 and the rollout of the technology takes time. Adapters purchased before 2020 may not have the spec, and thus, will not and cannot meet the charging requirements for MagSafe. Apple chose this spec for a reason, it is a more intelligent standard that understands more about the device it is charging.

A USB PD 2.0 device would negotiate power based on power requirements alone. The USB PD 3.0 spec allows the adapter to gain more information from the device while charging, like temperature and charging malfunction.

The USB PD 3.0 spec also includes more intelligence for controlling amperage and voltage while charging. When adjusting voltage or amperage it can move by as little as 20mV or 50mA steps, which is constantly negotiated every 10 seconds by the source.

So while Apple could have gone with an older spec for its 15W charging requirements, it decided to engineer a safer and more efficient solution by relying on the newer spec. USB PD 3.0 is backwards compatible though, which is why you will still get 10W out of older chargers, even if its rated for 96W.

If the MagSafe puck and the AC adapter can't negotiate the 9V and 2.22 amps, USB Power Delivery will default to the highest common mutually compatible voltage and amperage that it can, not to exceed that 9V or 2.22 amps. This will vary, depending on what AC adapter you have, given that the MagSafe puck can handle that 9V and 2.22 amps.

USB power profiles

Monotonic Incremental Power Rule (Image Credit:Texas Instruments)
Monotonic Incremental Power Rule (Image Credit:Texas Instruments)


The PD 3.0 spec includes specific requirements for available voltage rails for a given power rating. Any adapter above 7.5W will include only 5V delivered. Greater than 15W will utilize both 5V and 9V.

For even more power and variability greater than 27W adapters will have 5V, 9V, and 15V rails, and finally any adapter greater than 45W will have 5V, 9V, 15V, and 20V power options.

Other voltages can be offered for negotiation, but cannot exceed the highest required voltage "rail" in the adapter. Once a device is connected, it negotiates with the adapter for the best combination of voltage rails to achieve maximum efficiency while charging.

Adapters can offer any amperage up to a 5A rating and will determine the best amperage to use when negotiating. Modern devices choose a variable voltage and constant amperage for better temperature control. Only the 20V rail uses the 5A rating to achieve the 100W power rating for something like a MacBook Pro, while the rest rely on 1.5A to 3A ratings for constant current.

Like we said, Apple uses 9V and 2.2A for its MagSafe Charger to get to 20W. You can only get this combination in USB PD 3.0 20W or higher wattage adapters -- and Apple is not alone in providing that combination. A higher wattage alone won't work.

That 15W wireless charging in the MagSafe adapter

Apple recommends using a 20W adapter, but doesn't specify what spec is needed. Conveniently, the one with the correct spec is one they recently started selling and include with the 2020 10.2-inch iPad and iPad Air 4. There are 18W USB-C adapters available, but as we've discussed, they do not meet the minimum requirements for charging at 15W.

Apple says the MagSafe Charger will charge with any adapter greater than 12W, but at a reduced rate. Tests show around 10W or less when used with adapters below 20W. When the MagSafe Charger negotiates for power it specifically looks for the 9V x 2.22A supply, which is only present in 20W PD 3.0 adapters -- and only some with a greater power potential, given that it specifically needs that 9V x 2.22A supply. This is why it defaults to the 5V x 2A supply when using the 18W adapter, and will do the same with most -- but not all -- existing 60W USB-C power adapters.

Why can't a 15W adapter get 15W out of the MagSafe charger?

Apple's 20W adapter next to Anker's 20W adapter
Apple's 20W adapter next to Anker's 20W adapter


Even accounting for USB PD 3.0 requirements mentioned above, no system will ever be 100% efficient. Even if we ignored the industry specifications and built a 15W adapter for the 15W wireless charger, it wouldn't meet the minimum requirements to charge at full speed.

No system is inherently perfect, and every system will lose efficiency in charging based on multiple factors:
  • Length of cable used
  • Size of cable used
  • Conductor used (Copper)
  • Surface area of connected charger
  • Heat generation
These characteristics affect regular charging over a cable plugged directly to a device, so a 15W adapter connected to a suitable cable will still only charge less than 15W due to efficiency losses. In fact, the heat given off by your iPhone when charging, wired or wireless, is generated by the chemical reaction of charging the battery, which represents a loss in charging efficiency.

There is already some loss in efficiency in charging with a direct cable, just because of the resistance of the cable itself. A short and thick cable will have less resistance than a long and thin cable. The more resistance a cable has, the more energy is converted to heat. This is referred to as copper loss, and is inherent to any electrical system just as friction is inherent to a mechanical system.

When passing current through a charging coil, multiple losses are happening. First is copper loss, due to the thickness of the coil you are introducing more resistance, thus more heat. The second loss is from producing a magnetic field within the charging coil called eddy current loss, which also introduces heat to the system.

The heat generated from wireless charging due to how energy is passed between charging coils represents the sheer inefficiency of wireless charging. While more convenient, assuming a constant rate of battery charge, you are using more power per minute when using MagSafe or Qi. Alternatively, using the same charger with the same power per minute, you will deliver less power to the battery wirelessly charging than when using a cable.

What MagSafe, Qi, and USB-C PD practically means for buyers

If you purchase a MagSafe Charger, be aware of the power adapter you use with it. Purchasing the Apple 20W adapter with the demanded specs for the full 15W charging from quality manufacturers will get you the desired results. Be sure to follow Apple's recommendations though, as 20W with USB PD 3.0 is the minimum in order to charge with MagSafe at full efficiency.

Some speculate that Apple has introduced an artificial limitation to force users to buy new power adapters. Other than the minimum spec that has been a factor in tech purchases since the dawn of personal computing, there is no secret sauce.

Between inefficiency and energy loss when wireless charging, voltage profile requirements, and intelligent charging systems, Apple's minimum requirement fits within industry standards. But, Apple's isn't the only charger that will, and we'll be discussing which will work, and which won't, soon.
jdb8167Alex1Nrazorpit
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Comments

  • Reply 1 of 43
    thttht Posts: 5,619member
    Nice article!

    It's odd how tech media covers topics they have little interest in explaining what is going on. Pleasant to see one.
    StrangeDayskuduGG1webweaselsteve_jobsdoozydozenAlex1Nrazorpit
  • Reply 2 of 43
    So, if we plug the MagSafe into any particular charger how do we know what is being delivered?  Is there any display on the iPhone that tells us what we are getting?
    svanstromGeorgeBMac
  • Reply 3 of 43
    Wesley HilliardWesley Hilliard Posts: 243member, administrator, moderator, editor
    jagnut said:
    So, if we plug the MagSafe into any particular charger how do we know what is being delivered?  Is there any display on the iPhone that tells us what we are getting?
    Nope. This information is obtained by connecting a meter to the charger. Apple doesn’t provide a way to do this in the operating system. You’ll just need to be aware of what the adapter is capable of, if you care. Most people won’t notice much difference between 10W, 12W, and 15W when charging over long periods of time. Knowing how to maximize charging is useful, however, since nerds will likely care most.
    bikerdudejdb8167doozydozentwokatmew
  • Reply 4 of 43
    StrangeDaysStrangeDays Posts: 13,058member
    The tech pundits are trying to turn this into another controversy, which is stupid. the reality is super boring power protocols, amps, efficiency, etc. Most people who use charging pads aren’t in a rush anyway or they’d plug in. It’s fine and over time the better chargers & protocols will be common. 
    JFC_PAAlex1Ntwokatmew
  • Reply 5 of 43
    thttht Posts: 5,619member
    jagnut said:
    So, if we plug the MagSafe into any particular charger how do we know what is being delivered?  Is there any display on the iPhone that tells us what we are getting?
    There are many apps that will provide you this info. Just search for charging, amperes, battery, etc, in the App Store.


    llamaAlex1Ntwokatmew
  • Reply 6 of 43
    Mike WuertheleMike Wuerthele Posts: 6,919administrator
    tht said:
    jagnut said:
    So, if we plug the MagSafe into any particular charger how do we know what is being delivered?  Is there any display on the iPhone that tells us what we are getting?
    There are many apps that will provide you this info. Just search for charging, amperes, battery, etc, in the App Store.


    The apps aren't the greatest because of polling speeds, delays, or incomplete data sets, making them unsuitable for all but the most casual observations.

    Fine for Jagnut's observations, probably, with qualitative "close enough" measurements. Not good with unreliable information to hang a video or similar premise on.
    Alex1NGeorgeBMactwokatmew
  • Reply 7 of 43
    Please post link on this under all bush... YouTube videos slaming Magsafe....
  • Reply 8 of 43
    "When the MagSafe Charger negotiates for power it specifically looks for the 9V x 2.22A supply, which is only present in 20W PD 3.0 adapters"

    This is incorrect. With the rules used by PD 3.0, the amperage is only the *maximum* it is able to provide. 9V/3A, which is a standard profile in any charger able to provide 27W or higher, isn't required to only provide 3A, it can also provide less than that, anywhere from 0 to 3. Which means it also provides 2.2a if requested by the device. This is shown in the graph you inserted in the USB Power Profiles section.

    Edit: I worded this poorly, my intent is to clarify that 20w exactly is not required and that higher is okay too as long as it supports 9V/3A (which is standard). I've removed mention of PD 2.0 since I'm not trying to say that PD 3.0 is not required.
    edited October 2020 jdb8167svanstromdarkvadergatorguytwokatmew
  • Reply 9 of 43
    MplsPMplsP Posts: 4,003member
    Nice article - it explains a lot. It still doesn’t explain why the MagSafe connector couldn’t use the older 2.0 protocol to get higher power delivery. Strictly limiting bit to the 3.0 protocol that’s less than a year old means relatively few people are going to have a compatible charger. If it’s not clearly documented, many more people will never know why they are not reaching the full potential of their device. 

    Thanks for including the info about power loss and inefficiencies. A 25% power loss is a huge amount of waste and one of the reasons I really don’t care for wireless charging. 

    Alex1Nsvanstromdarkvader
  • Reply 10 of 43
    GG1GG1 Posts: 483member
    MplsP said:
    Nice article - it explains a lot. It still doesn’t explain why the MagSafe connector couldn’t use the older 2.0 protocol to get higher power delivery. Strictly limiting bit to the 3.0 protocol that’s less than a year old means relatively few people are going to have a compatible charger. If it’s not clearly documented, many more people will never know why they are not reaching the full potential of their device. 

    Thanks for including the info about power loss and inefficiencies. A 25% power loss is a huge amount of waste and one of the reasons I really don’t care for wireless charging. 

    I suspect the power loss will increase as the distance between the phone's coils and the puck increases, such as with a thick case.
    Alex1N
  • Reply 11 of 43
    Mike WuertheleMike Wuerthele Posts: 6,919administrator
    Kamirose said:
    "When the MagSafe Charger negotiates for power it specifically looks for the 9V x 2.22A supply, which is only present in 20W PD 3.0 adapters"

    This is incorrect. USB hasn't used profiles since PD 1.0. With the rules used by PD 2.0 and PD 3.0, the amperage is only the *maximum* it is able to provide. 9V/3A, which is a standard profile in any charger able to provide 27W or higher, isn't required to only provide 3A, it can also provide less than that, anywhere from 0 to 3. Which means it also provides 2.2a if requested by the device. This is shown in the graph you inserted in the USB Power Profiles section.
    The USB-IF and power regulatory chip manufacturers disagree with you on profiles existing.

    The combination, and steady maintenance, of 9v 2.22A is ONLY in PD 3.0. 9V and 3A is absolutely in 2.0, and 9v 2.22A is hittable in PD 2.0 only as a transient in response to external conditions like heat and whatnot as regulated by the regulatory circuitry and handshake. It isn't maintainable. The spec documents for PD 1 through 3 are clear on this.

    The bottom line is, a USB PD 2.0 adapter won't do it. A USB PD 3.0 will, and you don't need Apple's specifically.
    edited October 2020 viclauyycAlex1N
  • Reply 12 of 43
    GG1GG1 Posts: 483member
    tht said:
    Nice article!

    It's odd how tech media covers topics they have little interest in explaining what is going on. Pleasant to see one.
    I agree. Now these small power bricks are getting smarter each iteration.
  • Reply 13 of 43
    sflocalsflocal Posts: 6,123member
    I don't have a degree in electrical engineering, but even I knew why one needed a 20w input to power a 15w MagSafe.  This is the problem with idiots on social media.  Just because they have a web presence, they feel they're in a position to pass their stupidity as fact.

    Unless superconducting materials are perfected to operate at room temperature, we will NEVER have 100% power delivery from source to endpoint.  

    Jeez.. this is irritating.  
    viclauyycRayz2016jdb8167doozydozenAlex1Nrazorpittwokatmew
  • Reply 14 of 43
    Kamirose said:
    "When the MagSafe Charger negotiates for power it specifically looks for the 9V x 2.22A supply, which is only present in 20W PD 3.0 adapters"

    This is incorrect. USB hasn't used profiles since PD 1.0. With the rules used by PD 2.0 and PD 3.0, the amperage is only the *maximum* it is able to provide. 9V/3A, which is a standard profile in any charger able to provide 27W or higher, isn't required to only provide 3A, it can also provide less than that, anywhere from 0 to 3. Which means it also provides 2.2a if requested by the device. This is shown in the graph you inserted in the USB Power Profiles section.
    The USB-IF and power regulatory chip manufacturers disagree with you on profiles existing.

    The combination, and steady maintenance, of 9v 2.22A is ONLY in PD 3.0. 9V and 3A is absolutely in 2.0, and 9v 2.22A is hittable in PD 2.0 only as a transient in response to external conditions like heat and whatnot as regulated by the regulatory circuitry and handshake. It isn't maintainable. The spec documents for PD 1 through 3 are clear on this.

    The bottom line is, a USB PD 2.0 adapter won't do it. A USB PD 3.0 will, and you don't need Apple's specifically.
    My post was not about PD 2.0 vs PD 3.0. It was about requiring an exact 20w brick to get full power. Sure, you may need PD 3.0 for magsafe, but you do not require a 20w brick - you require 20w or higher with a 9V rail and 2.2a or higher because with PD/PPS the amperage the charger supplies can dynamically change according to the requirements of the device being charged.  9V/3A is standard in any 27w or higher charging brick and will provide full power to magsafe (provided it has the correct PD standard).
    jdb8167Alex1Nsvanstromtwokatmew
  • Reply 15 of 43
    Mike WuertheleMike Wuerthele Posts: 6,919administrator
    Kamirose said:
    Kamirose said:
    "When the MagSafe Charger negotiates for power it specifically looks for the 9V x 2.22A supply, which is only present in 20W PD 3.0 adapters"

    This is incorrect. USB hasn't used profiles since PD 1.0. With the rules used by PD 2.0 and PD 3.0, the amperage is only the *maximum* it is able to provide. 9V/3A, which is a standard profile in any charger able to provide 27W or higher, isn't required to only provide 3A, it can also provide less than that, anywhere from 0 to 3. Which means it also provides 2.2a if requested by the device. This is shown in the graph you inserted in the USB Power Profiles section.
    The USB-IF and power regulatory chip manufacturers disagree with you on profiles existing.

    The combination, and steady maintenance, of 9v 2.22A is ONLY in PD 3.0. 9V and 3A is absolutely in 2.0, and 9v 2.22A is hittable in PD 2.0 only as a transient in response to external conditions like heat and whatnot as regulated by the regulatory circuitry and handshake. It isn't maintainable. The spec documents for PD 1 through 3 are clear on this.

    The bottom line is, a USB PD 2.0 adapter won't do it. A USB PD 3.0 will, and you don't need Apple's specifically.
    My post was not about PD 2.0 vs PD 3.0. It was about requiring an exact 20w brick to get full power. Sure, you may need PD 3.0 for magsafe, but you do not require a 20w brick - you require 20w or higher with a 9V rail and 2.2a or higher because with PD/PPS the amperage the charger supplies can dynamically change according to the requirements of the device being charged.  9V/3A is standard in any 27w or higher charging brick and will provide full power to magsafe (provided it has the correct PD standard).
    I don't believe we said that a 20W was the only option, but I see where you might have gotten that from the text you included, which I will clarify when I am done here. 9V/3A will work assuming you have a PD 3.0 charger. Most 27W USB-C chargers on the market right now are NOT 3.0. You can have that 27W as a 2.0, but you won't get there.

    Also FTA: "as 20W with USB PD 3.0 is the minimum in order to charge with MagSafe at full efficiency."
    Kamirosejdb8167doozydozenAlex1N
  • Reply 16 of 43
    jagnut said:
    So, if we plug the MagSafe into any particular charger how do we know what is being delivered?  Is there any display on the iPhone that tells us what we are getting?
    I bought an USB A amp/voltage meter off eBay or Amazon, $20. So u can check the power output. However, you will also need an USB load tester, $10-40, to simulate the load, as the power draw from a smart phone is not always the same. 
    Alex1Ntwokatmew
  • Reply 17 of 43
    Kamirose said:
    Kamirose said:
    "When the MagSafe Charger negotiates for power it specifically looks for the 9V x 2.22A supply, which is only present in 20W PD 3.0 adapters"

    This is incorrect. USB hasn't used profiles since PD 1.0. With the rules used by PD 2.0 and PD 3.0, the amperage is only the *maximum* it is able to provide. 9V/3A, which is a standard profile in any charger able to provide 27W or higher, isn't required to only provide 3A, it can also provide less than that, anywhere from 0 to 3. Which means it also provides 2.2a if requested by the device. This is shown in the graph you inserted in the USB Power Profiles section.
    The USB-IF and power regulatory chip manufacturers disagree with you on profiles existing.

    The combination, and steady maintenance, of 9v 2.22A is ONLY in PD 3.0. 9V and 3A is absolutely in 2.0, and 9v 2.22A is hittable in PD 2.0 only as a transient in response to external conditions like heat and whatnot as regulated by the regulatory circuitry and handshake. It isn't maintainable. The spec documents for PD 1 through 3 are clear on this.

    The bottom line is, a USB PD 2.0 adapter won't do it. A USB PD 3.0 will, and you don't need Apple's specifically.
    My post was not about PD 2.0 vs PD 3.0. It was about requiring an exact 20w brick to get full power. Sure, you may need PD 3.0 for magsafe, but you do not require a 20w brick - you require 20w or higher with a 9V rail and 2.2a or higher because with PD/PPS the amperage the charger supplies can dynamically change according to the requirements of the device being charged.  9V/3A is standard in any 27w or higher charging brick and will provide full power to magsafe (provided it has the correct PD standard).
    I don't believe we said that a 20W was the only option, but I see where you might have gotten that from the text you included, which I will clarify when I am done here. 9V/3A will work assuming you have a PD 3.0 charger. Most 27W USB-C chargers on the market right now are NOT 3.0. You can have that 27W as a 2.0, but you won't get there.

    Also FTA: "as 20W with USB PD 3.0 is the minimum in order to charge with MagSafe at full efficiency."
    Yeah, reading back my first post was poorly worded. I've edited it to clarify. :)
    jdb8167Alex1N
  • Reply 18 of 43
    According to Mark of the Mark's Tech YouTube channel, his RavPower 60w PD 3.0 charger isn't getting the the full 15 watts.
  • Reply 19 of 43
    Mike WuertheleMike Wuerthele Posts: 6,919administrator
    According to Mark of the Mark's Tech YouTube channel, his RavPower 60w PD 3.0 charger isn't getting the the full 15 watts.
    Well, here's the problem. A great many of the chargers that advertise USB PD 3.0, actually aren't 3.0. They're 2.0 with some out of spec enhancements and the manufacturer calls them 3.0 because they're compatible. This is why we didn't post a list of compatible ones in this post, because this is going to be a bit challenging to work out which is, and which isn't.

    Which model of RavPower?
    webweaseljdb8167Alex1NrazorpitGeorgeBMac
  • Reply 20 of 43
    citpekscitpeks Posts: 251member
    "if you don't want to buy Apple's USB-C AC adapter, any USB-PD 3.0 one will do to get that faster charging speed, and a lesser one, won't."

    Is that really the case?  Apple's 96W adapter, released in 2019 with the MBP 16", has been tested to result in only a 10W level with the MagSafe.  Presumably, it's PD 3.0 compliant.

    I think there is a distinction to be made -- 1) an adapter must be PD 3.0 spec, and 2) it must be able to successfully negotiate the new/oddball (for now) 2.22A/20W profile that Apple has created.  The latter is far from certain, even with a PD 3.0 adapter.  PD 3.0 introduced better handshaking and accounts for more variables, but when the source and the sink don't agree, it will default to the next lowest step.

    Another distinction that hasn't been noted in this new "-gate" controversy, and that's what it is -- is that these quirks apply not only to the MagSafe, but to the iPhone 12 as well.  That's made clear in Apple's support documents.  Everyone is testing against MagSafe, but that introduces another variable in the wireless coupling.

    What people should be testing against is the iPhone 12 itself, where the only negotiation is between the adapter and the phone itself, which eliminates all the potential quirks that exist in the wireless coupling negotiation as well.

    On the surface, non-proprietary standards like USB PD should prevent a lot of issues.  But underneath the surface, there are plenty of deviations and proprietary games being played, which only get noticed in situations like this.  Apple is not alone in playing these games.

    Many fail to see the bigger picture that the MagSafe and iPhone 12 have new requirements, and it will take time for 3rd parties to adjust and adapt with new products to match.  But they will, and eventually the issue will fade, just like it did with Apple 2.4A, which saw scant support when it first appeared, but eventually became ubiquitous, despite not being an official USB spec.
    Alex1Nsvanstrom
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