To deliver power via wireless charging requires that large currents circulate through coils that get hot. This is just like what happens to the coils of an inductive cooktop in your kitchen. Of course, in the kitchen, converting coil current into heat is the objective. A wireless charger on the other hand needs to transfer coil current to a battery without getting the phone or charger hot.
Heat is always related to power loss. Heat during charging is not your friend. Your battery ages more quickly and computing speed slows down. The phone’s software manages the phone’s resources as the temperature rises by lowering performance, including wireless charging performance.
There is only one thing that a phone manufacturer can do to maximize the wireless charging rate of a Qi** compatible phone. They need to minimize the heating of both the charging coil inside the phone and the electronics associated with those coils.
Bigger Coils, Lower Heat
To minimize heating of the coils, both the phone and charger would need to use more efficient coils. To keep the higher current from leaking and creating havoc throughout the phone, thicker magnetic shields should also be necessary. But more efficient coils and magnetic shields have one primary problem. They are thicker.
With modern phones, coil thickness is not an option one wants to play with. Nor is the thickness of a magnetic shield. The public has been trained that somewhere near 8mm, and preferably less, is the preferred phone thickness.
In the absence of a thickness limit, there are some impressive wireless charging coils possible, hence faster possible charging rates. Those coils are built with layers of super thin wires.
A large part of my work this past year has been studying the Xiaomi Mi9 phone. Xiaomi claims that the Mi9 has the world's highest power wireless charging capability at 20W. Recently I have learned of a newer Mi9-class phone that purports to charge at the even higher wattage of 30W. I have not yet had a chance to study that phone which is to be released in China at the end of October 2019.
Nevertheless, one might be led to believe that the Mi9 should also be the thickest phone according to the claims I make above. As it turns out, the Mi9 is 7.61 mm thick. This is impressive given that there should have been a thickness increase to affect the 20W charging.
Bigger Coils Have Other Issues
The magnetic shield thickness and achievable coil currents are indirectly controlled via regulations established by governmental regulatory agencies. In the United States (U.S.), it is the Federal Communications Commission (FCC) that establishes compliance requirements that then effectively force certain design decisions for all wireless charging systems. All smartphones and chargers sold in the U.S must meet the regulatory requirements imposed by the FCC.
To demonstrate FCC compliance, products are assigned a unique number recorded with the FCC. This number is printed on the labels for those products and can be verified online. Trying to find the location for the number is easy; generally on the bottom side of the charger. Reading it is virtually impossible without a magnifying glass.
Europe and China have similar organizations with their own regulations and unique markings. Herein lies potentially a first explanation for how the 20W charging is possible.
The Mi9 is not marketed to the US, so there is no FCC ID number. But there is a CE mark. Conversely, the 20W wireless charger that is designed to go with the Mi9 has neither a CE or FCC mark. Since it is the charger that sources the 20W charging power, the Mi9 itself has less strict compliance requirements. Therefore, a thicker magnetic shield in the Mi9 would be unnecessary for CE compliance. Perhaps there are still some safety issues to manage magnetic field leakage, but that would not require nearly the thickness increase as would be the case for CE compliance.
I was able to run my 20W experiments because I bought the Mi9 products from the wild west of shopping, better known as Amazon. Clearly these products can not be legally sold in the US, and I suppose I shouldn't be using them either. But I'm only doing experiments with one device, and there is generally a waiver for experiments.
Summarily, I suspect the Mi9 by itself would meet FCC requirements, which suggests that there is little need for much thicker magnetic shielding.
Nevertheless, I can point to one fact that suggests I am correct about the coil assembly area increase.
The size of the Xiaomi Mi9 phone is 89,400 cubic mm whereas the Galaxy S10+ is 91,100 cubic mm. In other words, the Mi 9 is only 1.8% smaller than the Galaxy phone. But the battery in the Xiaomi phone is 20% smaller than the S10+ (3300 mAh versus 4100 mAh). Therefore, it seems to me, the sacrificing of battery capacity affords Xiaomi the necessary space to use a larger wireless coil assembly.
While I didn't tear down the Mi9, there is at least one online teardown report that asserts the charging coil assembly is in fact substantially larger than would be required for sub-20W charging.
But so what? Even at 20 W, there is no 2.7:1 charging speed improvement.
Sleeping Phones Account for Some Difference
There are several vlogs on YouTube that demonstrate accelerated charging times with the 20 W capable Xiaomi Mi9 phones and Samsung’s 12 W capable S10 with Fast Charge 2.0 technology. These comparisons were all done with the phones asleep while all of the comparison data I have presented so far dealt with tests while the phone is active. Perhaps the advantage lies there.
As I described in a previous blog, a dock is my preferred charging stand because I prefer that the phone be useable while charging. Perhaps that usage is video calling or movie watching. Perhaps it is watching text scroll by on a message thread. With the power of today’s smartphones and Bluetooth game controllers, one can play high performance games such as Fortnite and PUBG on their phone while streaming the screen to a TV; games consume huge amounts of power, hence the phone can get very hot.
Nevertheless, and in fairness to Xiaomi, I did check charging speed under best case conditions while the phones slept. The chart below shows the performance comparison [Note: Data collection was relative to a 4% start].
While sleeping, the Mi9 phone charges to 80% in about 1 hour whereas the Xs charges to 80% in about 1.5 hours. This is a 1.5:1 advantage over the iPhone. Still not the 2.7:1 that one would expect though. For reasons stated in an earlier blog about roundtrip deficiency, I didn’t include the battery size advantage of the iPhone. But even then, adding in that factor doesn’t explain the difference.
Mi9 versus Xs Charging While Sleeping
Best case testing though doesn't tell the full story. Many times people connect their charger just as the phone hits near 0%. At that point the phone is heated. Perhaps one was watching a movie or playing a video game. Perhaps one just arrived home on a hot day where the phone was just sitting idle inside a car. The chart below shows how best case performance can change dramatically under everyday practical situations.
Mi9 sleep charging under various starting temperatures
The chart shows the charging performance under three different phone operating scenarios.
The green curve is a repeat of the idealized case where the phone had settled to indoor room temperature (i.e. about 77F) prior to sleep mode charging. This is the reference curve.
The blue curve shows charging under the condition that the phone was in use prior to placing the phone on the 20W charger. The phone was then put to sleep. Consequently, the phone was moderately warm with little cooling time before sleep mode charging began. Also very important, the charger surface was cool since the charger was off until the phone was set down on the charger.
The orange curve shows the charging performance when the phone was placed in sleep mode immediately after gaming for about 8 minutes, and while the phone laid on the charger. Unlike the blue curve conditions, here the charger had also been operational while the phone was gaming.
The immediate observation looking at the curves is that the 20W kickstart never shows up when the phone is already warm, even under moderate warming conditions. And for extreme heating conditions, it takes a very long time for the phone/charger combination to cool down before charging resumes at a sub-20W charging rate.
I haven’t discussed the Samsung Galaxy phone series much in this regard. According to Samsung, their Galaxy S10 supports 12W charging using their new Fast Charge 2.0 technology and charger. I've seen numerous 10W chargers for the Galaxy S9, but apparently Samsung's charger for the S9 is only 9W capable, which I suppose means that the phone is only 9W capable.
Without spending another 1,000 words going through the details, let me simply say that I wasn’t able to charge the Galaxy S9 in a reasonable amount of time while it was active. Look at the chart below.
The charging rate at the beginning was respectable, but after some time heating, the charging rate dropped precipitously. It appears the charging time would have been about 20 hours had the video kept playing.
During the test, the video I had playing stopped a little after 4 hours, whereupon things cooled down and moderate charging resumed. Thirty (30) minutes after the video stopped I happened to check on the status. Noticing the video had stopped, I restarted the video at about 5 hours into the tests. As the chart shows, shortly thereafter charging effectively stopped once the phone reheated.
Galaxy S9 Charging Across Several Different Conditions
This was a common experience with both S9s I tested.
Samsung apparently worked to rectify this when they went from their 9W design to their newer 12W design inside the S10. Nevertheless, as the chart below shows, it still took up to 270 minutes (about 4.5 hours) to charge while active, whereas the iPhone Xs took only 133 min using our prototype charger.
iPhone Xs vs. Galaxy S10 charging while active
But to be fair again, and similar to the Mi9, the S10 has a 3400 mAh battery versus the iPhone Xs’ battery of 2660mA. So, there should be an additional factor of improvement when that is taken into consideration. Yet on the other hand, there is still the question about roundtrip time despite having a 28% higher capacity battery.
As it turns out, the larger batteries of the Galaxy phones provide no roundtrip advantage. Whereas the iPhone video recording time from 100% to 0% was 267 minutes, the S10 recorded for 250 minutes. Ignoring the small difference in recording times, the S10's 28% larger capacity battery should have brought a significant advantage, but it didn't.
When Samsung discusses wireless charging specifications for their latest 12W Fast Charge 2.0 technology, they bury numerous disclaimers in fine print that no doubt cover these types of situations.
The Data Doesn't Lie*
I titled this blog “It’s All About Heat”. You can immediately see why if you refer back to the chart above for Mi9 idealized charging at 20 W.
After only 8 minutes of charging, the rate of charging drops precipitously. This is evident by the slope change in percent/hour at 8 minutes. I call this the kickstart charging interval.
Had the slope not changed so radically, the ratio very well could have been 2.7:1. Mind you, I also had the phone placed in airplane mode during the sleep mode test, so even the cellular modem was not active.
As the picture below shows, the Xiaomi 20W charger definitely starts at 20W. Of course it doesn't stay there for very long, which explains all of the shortcomings.
Xiaomi 20W charger AC input power immediately after Mi9 charging starts
While not shown in the above charts, there is a similar event that occurs with the S10 at about that same 8 minute delay.
Perhaps it is because all of the performance advantages disappear when the phone is in use that explains why I could not find a 20 W Xiaomi charging dock. There is virtually no benefit to a $50 dock if you can’t derive a $50 charging advantage.
Actually, it costs more to manufacture a dock charger. I paid $80 for Samsung’s newest Fast Charge 2.0 dock.
Charger manufacturers attempt to solve charging related heating issues by employing cooling fans inside their chargers. This does help some.
While the Samsung 9W disk charger has a fan, there is a much larger fan inside the Fast Charge 2.0 charger. Furthermore, the airflow of the 12W charger is routed across the region nearest the phone's charging coils rather than near the charger's coils, as is the case with the 9W charger. Maybe that is how Samsung overcame the S9 hurdle of no charging while active, even though the reward included over 4 hours of charging time.
Fans present another host of issues that also need to be considered. This is one of the topics covered in the next blog of the series where I write about the not-so-standard, wireless charging standard called Qi**.
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.