Apple MagSafe Charging Speed Tested

Why USB-C Power Delivery?

USB-C Power Delivery (USB PD) chargers take advantage of extra control channel signals within a USB-C connector/cable, which are only accessible by using a USB-C connector and appropriate cable. These control signals allow a charger to advertise its capabilities to a device and the device to tell the charger what it will accept. Other info changes hands between the charger and device, ACK’s, vendor data etc but that's for another day. 

USB PD 2.0 also allows for a number of different voltages and current limits to be advertised to the connected device: 5/9/15/20V, 2/3/5Amp etc. The advertised voltages depend on the power available from the charger. This power is calculated by multiplying the charger Voltage by the charger Current in Amps. For example, a charger offering 9V and 3Amps would be offering 27Watts (9x3=27).

• One charger offering more than 15Watts will advertise 5V and 9V.
• Another charger offering more than 27Watts will advertise 5V, 9V and 15V.
• A final charger offering more than 45Watts will advertise 5V, 9V, 15V and 20V.

In the case of a MagSafe Charger, it is looking for 9V at 2.22Amps which works out to be 9*2.22=20Watts. This 2.22Amps should be regarded as the minimum current that a compatible charger should offer at 9V.

All USB-C cables, according to the USB-C spec should be able to handle at least 3Amps. 5Amp cables require additional electronics in the form of an e-Marker and are designed for devices requiring over 60Watts such as some laptops.

If you’ve got time on your hands and want to learn more about this then here’s a link to a presentation on USB Power Delivery from USB.org.

What does MagSafe use: USB PD 2.0 or USB PD 3.0? 

During testing I captured the data packets between the USB-C charger and the MagSafe Charger. These data packets were only relating to USB Power Delivery (USB PD) spec version 2.0, rather than USB PD 3.0.

Is there a problem with this? No, although I expect to see Apple switch to USB PD 3.0 across the board in the future so they can leverage the increased functionality within the USB PD 3.0 spec. The 2018 MacBook Pro ‘talks’ USB PD 3.0.

USB PD 2.0 will work with a USB PD 3.0 charger since USB PD 3.0 is backwards compatible. Which means there are additional features baked-into USB PD 3.0, which can offer benefits: 

• PPS (Programmable Power Supply). This allows the charger and device to negotiate a voltage between 3.3V and 21V in 20mV steps. The device current limit can also be negotiated in 50mA steps. This means if the internal device charging electronics work most efficiently (read: less heat generated) at 4.4V then the device can request 4.4V.
• C-Auth (USB-C Authentication). This allows a charger or device to authenticate with each other, which is in addition to authenticating the e-marker in the cable between them. This means it could be possible in the future that certain chargers could be ‘blacklisted’ or certain chargers may only work with certain devices. For example, an Apple device may only authenticate with Apple chargers or approved OEM chargers.

Wireless charging and the environment

Wireless charging will never equal the efficiency/speed of wired charging. As a result it's the least friendly to the environment both in terms of quantity of materials used and energy wasted as heat. Calculating the inefficiencies of wireless charging and multiplying this by the number of mobile device users in the world gives a very daunting figure in terms of wasted power, albeit a worst case scenario.

My view is that MagSafe Charger will not replace having a Lightning or USB-C connector on an Apple product. If it did then ultimately, over time each and every mobile Apple product would utilise it and each and every user would waste some additional Watt-Hours each time they charge their phone. This kinda goes against the world efforts to save energy.

The tests   

How the test data was collected

The data was collected using a commonly used USB PD Analyser which measures Vbus, Vconn, CC1 and CC2 voltage and current as well as capturing and decoding the USB PD data packets between the USB-C charger and MagSafe Charger.

I haven't published the data packets between the USB-C charger and MagSafe Charger as it doesn’t help present the data which the article is about: How quickly does the MagSafe Charger charge an iPhone 12 from 0-100% with various USB-C chargers.

Collecting charging data was a long process!
Firstly I have to drive the battery level of the test iPhone 12 down to the point where it doesn’t boot. This takes many hours of streaming video, sound and flashlight!

Once the battery is flat, I charge it and record various parameters during the full charging run from zero to 100%. Each of the tests runs for at least 4hrs and create a CSV file with close to 2,000,000 rows! Way in excess of the amount of data which Apple Numbers and Microsoft Excel can import. Thankfully the  DataGraph for MacOS app helped dealing with huge CSV files.

Analysing the charging test results

Initial thoughts on the data

I'm presenting basic comparison data to give a little more insight into what happens to the charging current when a MagSafe Charger is used with an Apple or non-Apple USB-C Power Delivery charger and an iPhone 12.

I don’t have detailed data regarding the decisions made within the iPhone and/or the MagSafe Charger when it comes to manipulating the battery charging speed although Apple do offer the caveat that charging speed may vary depending on System Activity and Temperature: https://support.apple.com/en-gb/HT211829

These caveats from Apple make perfect sense from a technical point of view for the following reasons:

• Li-Ion batteries can be irreparably damaged if charged above or below a specific temperature range. Apple provides guidance on charging Li-Ion batteries on their website and they recommend to not charge the battery when device is operated within an ambient temperature above 35℃: https://www.apple.com/uk/batteries/maximizing-performance/
• The MagSafe Charger is not made of magical superconductors and does have inefficiencies. These inefficiencies manifest themselves as heat and the generator of this heat is attached, in close proximity, to the rear of the iPhone 12. Think of the MagSafe Charger as a little heater held in place with integrated magnetics next to your battery. This, or any method of charging, will always generate heat, this is just a law of physics.
• System Activity generates heat. Ever played an iOS game and felt your device get a little warmer than normal? That’s due to a lot of system activity. More activity and information pushed around, more heat - again, this is just physics and can’t be avoided.
• Charging a battery of any type causes the battery, and its charging electronics, to heat up.
• Things that are hot take time to cool, things that are cool take time to heat up. Changes to the charging rate, up or down, will not cause an immediate change in internal temperature.
• Given the choice of a 22℃ office or a 35℃ summer day, your electronics would choose the cooler environment. Prolonged use at elevated temperatures can have a detrimental affect on electronics systems and the temperature of the iPhone case doesn’t necessarily give the user any indication of how hot a chip on the internal PCB is! The internal temperature of the iPhone could be a lot higher than what you feel on the outside - especially if you use a case.
• Prematurely aged batteries, due to being charged at elevated temperature and within a warranty period, would cost the OEM money to replace. It’s absolutely expected that an OEM like Apple will do whatever they can to prevent in-warranty failure and potential brand-damage.

What I think is going on: 

My educated guess is that there are some clever Apple algorithms which take all of the previous factors into account and either reduce or increase the charging speed in response to these environmental and system factors between the iPhone and MagSafe. I suspect these algorithms may maintain a balance between charging as fast as possible whilst maintaining battery health. As a result, I believe there is a big emphasis on temperature.