It may be hard to believe, but a good argument can be made that the iPhone is the world's fastest wireless charging phone. When one considers that the iPhone has the lowest wireless charging power capability of other flagship smartphones, this doesn't seem possible.
Xiaomi claims to have the world's highest power wireless charging phone at 20W; the Mi9. They also claim to soon be releasing an even higher 30W version. Conversely, Apple’s maximum wireless charging power is only 7.5W. That is a mere 38% of the Xiaomi wattage!
I think reasonable people would assume a nearly three-to-one power advantage will leave the much weaker iPhone charging at a turtle’s pace. If it didn’t, what would be the point behind a higher wattage charger? Of course, there would be no point.
Wireless charging power, just like milli-amp-hour ratings (mAh) for batteries, is a term that is abused. Those terms translate into essentially nothing when comparing smartphone brands. Users don’t experience mAh or charging speed or even the battery charge percentage displayed at the top bar of most smartphones. Rather every smartphone user experiences how long they can use their phone after it has been charged for a certain time, say, 1 hour. I call this the roundtrip charging time for a phone. And this is where the iPhone shines, even at only 7.5W.
Smartphone wirelessly charging in landscape mode
Time for an Experiment
Consider the following experiment.
Two people take their fully discharged phones, place them on their wireless charger for 1 hour, and watch a video while the phone charges. At the end of 1 hour, both users pick up their phones and continue to use them until they need to be recharged again. That is the roundtrip experience. A longer use time is better.
I ran such an experiment using a 7.5W capable iPhone Xs and a 20W capable Xiaomi Mi9; a phone claimed by Xiaomi to be capable of charging at the world’s highest wireless power levels. The Xs was charged on my company's prototype charger and the Mi9 was charged on Xiaomi's proprietary 20W charger. The results of that test are shown in the graph below*.
Mi9 versus Xs roundtrip charging
The green curve represents the iPhone Xs charge and discharge state of its battery over time and the red curve represents the same for the Mi9. I’ll go through the various points on the curve in a moment, but the only important number is the roundtrip time. This time occurs at the point where both curves cross the Time axis at 0% battery charge and the respective phone shuts down. For the Xs that is slightly over 4.5 hours whereas the Mi9 roundtrip time is less than 4.25 hours.
To affect the discharge, I turned on each phone’s video recorder and let it run until the phones turned off. The interpretation of the experiment is that I can record video 20 minutes longer after an 80 minute wireless charge with the 7.5W Xs relative to the 20W Mi9.
While the 20-minute difference might seem somewhat trivial here, that 20 minutes is relative to using a phone for video recording; a substantial power burden for a phone. The usage time difference would be much greater for other typical use cases such as watching a video or simply using one's phone throughout a normal day. It very well could mean not having to recharge your phone throughout your whole day versus stopping for a charge in the early afternoon.
If I had conducted the above simple demonstration in front of an audience without using charts, and then asked which phone had the higher power charger, without a doubt everyone would pick the phone that performed the longest. After all, that’s what people have experienced with wired charging for over one decade; higher wattage chargers invariably charge batteries faster.
I like using the Xiaomi example because it is an excellent demonstration of a general weakness in all wireless charging phones I have investigated, not just those manufactured by Xiaomi. When it comes to wireless charging, don't place much value in claimed charging power.
It is marketing noise to describe the charging capability of a phone in terms of wireless charger wattages, speeds of wireless charging, and milli-amp-hour (mAh) ratings. Those terms are exploited because reasonable people have been taught that in technology, bigger numbers mean better performance. 64-bit computers are always better than 32-bit computers. 128GB of storage is two times better than 64GB. 8K video is better than 4K. The list is endless.
But here we have the situation where a phone charged with 170% more power and 24% more battery capacity (3300 mAh vs. 2660 mAh) operates for 10% less time.
Why is this so? Well, it takes a couple of charts and some discussion to explain this, but the value of the explanation will empower you to make wiser decisions when shopping for your next phone or a charger for your current phone, even if that phone is not an iPhone.
The Answer Lies in the Details
It is clear, from the first chart above, that the Mi9 does reach a slightly higher battery charge than the iPhone for a given amount of charging time. And it does so despite its larger battery. That is no small feat. But that advantage is lost during discharge because the iPhone is more power efficient than the Mi9.
It is the iPhone's superior power efficiency that creates the crossover of the discharge curves in the above chart. But that iPhone advantage could be overcome if the nearly three to one advantage in Mi9 charging wattage materialized into equivalently faster charging of a 24% larger battery. Clearly, that is not happening in this experiment. Upon closer inspection of the chart, one can see why.
Let’s look at the previous chart in more detail. For convenience, that chart is repeated below.
Mi9 versus Xs roundtrip charging (repeated)
To the left of the dashed vertical line at 1.3 hours are the charging curves, and to the right are the discharging curves. The 1.3-hour point was chosen because this corresponds to about an 80% battery charge on the Mi9.
The green iPhone charging curve is essentially a straight line whose first point is at about 3% and whose end point is about 72%. The end point time is the same for both phones at 1.3 hours.
Conversely, the red Mi9 charging curve has two break points; one at 0.05 hours and the second at 0.4 hours. These points coincide with drops in the charging rate.
If the Mi9 charging line had been straight starting at zero minutes, the battery charge percentage would have been racing up to 100% at rocket speed, consistent with 20W charging. The Mi9 power inefficiency weakness would have been overcome.
I call the time interval starting at zero minutes until the second breakpoint of the Mi9 curve the kickstart period. This is when the Mi9 battery accumulates substantial charge in a relatively small period of time. The kickstart period lasts until the phone starts to overheat.
Batteries don’t like heat. If you’re unlucky, they explode. More likely, they just age quicker.
To keep the batteries from exploding and maintaining a long life, a phone’s computer tells a wireless charger to slow down. Slow way down. And in the case of the Mi9, this appears to happen twice.
After the second slow down, the rate of charging for the Mi9 and the Xs are essentially the same. The lines are parallel.
Charging can be terminated at any time point while the charging lines are parallel and the outcome would always be the same; the 7.5W iPhone would always have more use time till the phone shutdown. The higher 20W charging process shows essentially no value for most of the charging time for the use case in the experiment. If that was the typical use case, the extra expense of a high watt wireless charger could not be justified.
The Rest of the Story
At this point, I need to clear things up a bit.
The chart I showed was based upon using the phone while charging. I did this for two reasons.
First, that is a typical use case when a battery runs out. Someone might be watching a video, making a phone call, or in some other way using the screen. And in those use cases the phone is heated from both charging and operating at the same time.
Secondly, the demonstration shows two heating events; the break points in the charging curve that occur when attempting to charge at such high-power levels.
A more favorable result for the Mi9 is possible if the phone is not in use, and cool, before placing the phone on the charger. The cool requirement turns out to be very important, which I’ll explain later why.
Look at the chart below for charging while the phone is off.
Mi9 versus Xs charging with cooled down phones that are turned off
In the case where the phone is cool and turned off while charging, the red Mi9 charging curve shows only one break in the charging line rather than two breaks. Consequently, the charging rate, while still not equivalent to 20W, is nonetheless faster over a much longer time period than shown in the previous chart.
In the above situation, the roundtrip advantage is again 20 minutes of recording time, but for the Mi9 this time rather than the Xs. Under this scenario, the Mi9’s higher wattage shows a 14% advantage.
But what about that cool requirement? Well, I’ve got another chart for that.
Phone Temperature Dependent Charging Rates
Look at the chart on the right that shows only the charging characteristics for the Mi9 phone, and for various levels of phone warmth.
Temperature dependent charging rates for Mi9 with 20W charger
The green curve represents the same phone-off and cool conditions of the previous chart. That curve is repeated here for reference.
The blue curve is what one observes if the phone wasn’t provided any time to cool prior to charging. For example, if one were playing a video game when the phone dies, or right before it dies, and then placing the phone on a charger with the phone off. Here one can see that it takes some time for the phone to cool down before fast charging begins; perhaps five to six minutes.
I observed during my experiments that this five to six minutes of time delay seems to be somewhat variable and based upon the phone temperature immediately before the phone was placed on the charger. Nevertheless, the more important observation is that the valuable 20W kickstart didn't happen. This was a consistent observation.
Without the kickstart, the 22% battery charge advantage is substantially reduced. This can be seen on the chart by looking at the percent charge difference between the blue and green curves, and at any point in time during the entire charging period. The original 20-minute kickstart advantage has substantially disappeared.
The orange curve shows a more drastic response to a warm phone. For this test, a video game was played for 8 minutes while charging was ongoing. After the 8 minutes, the phone was turned off and cooling began. It took well over one hour for the phone to cool to the point where charging resumed at a reasonable rate. The rate for the first 1.4 hours was less than what one would expect from a 5W wired charger.
Admittedly, playing a game can make a phone quite warm. But then so can having your phone lay on a table outside by the pool on a warm summer day or resting on the dashboard of your car on the way home from work. The orange curve is not an unrealistic use case.
There is one other variable in the experiment that I didn't discuss. Namely, both phones were inside their respective cases. The iPhone was encased in an OtterBox Defender and the Mi9 was encased in a Spigen Rugged Armor. I chose to use cases for my experiments because over 85% of smartphone users have their phones inside a case.
Cases can sometimes provide an advantage to wireless charging time. Having the Spigen Rugged Armor case on the Mi9 provided up to a 4% advantage in charging when the Mi9 phone was cool. I attributed that to the delayed heating of the Mi9 phone back surface by the Xiaomi 20W charger. Effectively, the Spigen case acted like an insulator which helped elongate the kickstart period.
The 4% benefit vanished as the battery charge approached 100%. But for quick charges of 50% or less, this was a real advantage for the Mi9, and that advantage is captured in the results I've shown in this blog. So, to a certain extent, the charts I've shown in this blog are best case for the Mi9, but not typical.
It is unnecessary to belabor this point. The thesis of my blog is simply that a higher wattage charger does not translate to an equivalently faster charging rate under any condition. Furthermore, it is factors other than charger power that more greatly impact a user's experience with a phone that is charged wirelessly.
The Mi9 is Not Alone
The Mi9 is a fine phone. At half the price of an iPhone Xs, it is somewhat unfair to compare these two flagship phones. On the other hand, claims of being the world's best at something deserves investigation. In fact, it is because Xiaomi makes the claim of the highest power (hence, best?), and iPhone the lowest, that it is necessary to use these two extreme examples for the study.
Samsung is another company that heavily promotes the benefits of their phone's high wattage charging ability. Their latest S10 phone is one example. I ran experiments with the Galaxy S10 which incorporates their new 12W Fast Charge 2.0 technology.
I've included below one chart that was produced under the same conditions as the Mi9. Here, the 12W capable S10 was charged on the new Samsung 12W Fast Charge 2.0 charger dock while a video was playing. Despite the 4.5W advantage relative to the iPhone Xs, the charging rate is much slower.
S10 versus Xs charging while phones are active
I also ran tests with Samsung's previous 9W technology inside the S9. In all but one experiment out of the 12 chargers I tested, the S9 didn't charge while in use. Curiously, it was a non-Samsung charger that was able to charge the battery while the phone was active.
With the S10, charging always happened while using the phone, just at a slow pace.
Despite Samsung's newest Fast Charge 2.0 technology, Samsung goes to great lengths on their product page to temper charging expectations. I've shown throughout this blog why such disclaimers are necessary and why one should be cautious with their expectations when spending hard earned money on high wattage wireless chargers or phones. I had to pay $80 plus shipping for the Fast Charge 2.0 charger. Ouch!
Summarily, from a user experience perspective rather than through charts with numbers and marketing specifications, the iPhone certainly appears to have superior wireless charging capability built inside the phone. Of course, to experience those capabilities requires a high performing charger, not a highest wattage charger. My company has been developing such a charger, and our prototype was used to produce the iPhone Xs results shown in this blog.
This blog is one of a five-part series. Links to the other blogs in the series can be found here. Please subscribe to our Newsletter and you will be one of the first to know when more blogs and vlogs become available. Thanks. Scott
*Note: To simplify data collection, the company commissioned the development of iPhone and Android apps. The iPhone version of the app is available on the App Store (+12C Pro). The Android version hasn’t been published simply because of the expense in preparing the app for general use. But if there is interest, take a look at the online videos for the iPhone version and post a comment in the comments section for this blog. With sufficient interest, I can have it published on Google Play.