Wireless Charging: Trading Efficiency for Convenience

Wireless charging is a common sight these days. You’ll find the technology in earbud cases, cell phones, and even your bedside table. The benefit of wireless power is obvious: The convenience of freeing ourselves from all those pesky cords and various connector types. But that might be the only benefit.

What about the tradeoffs? Is there any truth to claims that wireless charging overheats your phone, or damages your battery? How efficient is wireless charging when compared to wired charging? And are all wireless chargers made equal?

The Basics

Let’s start at the beginning. How does wireless charging work? Whether we’re talking about the charging pads many of us have on our bedside tables or the latest phones that incorporate “reverse charging”, the fundamentals are the same. 

As with wired connections, a wireless charger needs to transfer energy to your phone’s battery. That’s easy enough with a cable connection, but what happens when your charger is separated from the phone by plastic, glass, and a layer of highly reluctant air? Reluctance is an engineering term by the way, I’m not suggesting air is lazy.

The trick is to manipulate the electrons in the wireless charger’s coil (what we’ll call the transmitting coil) by pushing them in one direction and then the other to create an alternating current (AC). The AC current inside the transmitting coil then creates an alternating magnetic field (check out Faraday’s Law of Electromagnetic Induction to understand why this happens). 

Each back and forth of the current is measured in hertz (Hz) and wireless charging coils generally operate in the 130-140 kilohertz (kHz) range. That means the current moves back and forth in the coil around 140,000 times per second.

There’s a corresponding coil on our target device (which we’ll call the receiving coil). When placed close to the transmitting coil, the alternating magnetic field induces an alternating current in the receiving coil. This AC current is then converted to direct current (DC) using a full bridge rectifier and some other circuitry. And hey presto, we have power! 

What about wired connections? That’s much simpler. The AC current from the wall socket is converted to DC by your power brick and flows through the copper wire straight into your phone. Your phone then decides how quickly it wants to draw that current based on the safe limits set by the manufacturer.

Wired Charging Efficiency

Due to the cruel realities of our physical realm, all energy transfer will suffer some loss. Various factors play a part in the efficiency loss including:

• Switching and transformer losses in the power brick circuitry
• Resistive heating (aka Joule heating)
• Energy draw by internal components in charging bricks, IC’s in cables, and the phone’s own internal components

So if we take a wired charging setup as an example, the power brick will convert the AC from the mains into DC for our device. The AC to DC conversion losses generate heat, which is bad news for electronics. Heat is such an issue that engineers spend lots of time trying to minimize it. In fact, as our electronics get warmer, the internal resistance of the copper wires and traces also increase. So not only is the heat considered wasted energy, it also actively makes the process of transferring electrons less efficient (thereby generating even more heat!). 

Modern GaN chargers utilize gallium nitride semiconductors which have a higher electron mobility, larger band gap, and better thermal conductivity. All this means a GaN charger will heat up less and dissipate heat faster. It’s cooler than silicon-based chargers (literally and figuratively!).

Let’s break this down into a few easily digestible stages.

Stage 1

Our charging brick takes the AC current from the mains and converts it to DC, by far the most common input for our home electronics. We’ll lose some energy in the form of dissipated heat as the electrons make their way across the circuits. We’ll also lose some energy to the IC’s and other components that draw energy to enable the charger to function.

Stage 2

The DC current from the GaN charger flows through the copper wire in the charging cable to our device. Electrons don’t have a driver’s license or if they do, they were issued in California. This means they won’t necessarily move in a straight line. They’ll collide with other particles, the edges of the copper surface, and any impurities that may be present in the copper cable. Each time an electron bounces off something, it loses energy and generates more heat. But as far as our tests are concerned, the amount of energy lost in this way is relatively small.

Stage 3

The electrons reach another set of IC’s on the other end of our USB-C cable as they enter the cell phone. These IC’s will draw some energy to function, while diverting the correct amount of current to the battery.

Stage 4

The stream of electrons finally arrives at the battery. As ions are exchanged and the chemical reactions take place inside the battery, we see the greatest loss of energy throughout the entire process thus far. Heat is generated as more collisions take place and chemical reactions between the anode and cathode of the battery gradually increase the potential difference between the two poles. The more current we push into the battery, the more energy we lose as heat. 

Stage 5

It’s very rare that we unplug our phones the very moment the battery hits 100% charge. It’s ok if you sit over your phone till it ticks over to that last percent, we’re not judging, but most of us charge our phones overnight while we sleep. This means your phone will continue to draw power after it charges to 100%. 

Thankfully, modern chargers are smart enough to shut off completely once you unplug your device so the charging brick won’t continue to draw power from the mains when not in use.

In a polymer encased nutshell, this is how most people charge a cell phone battery.

Wireless Charging Efficiency

Let’s take a look at the process of wireless charging. It has all the same inefficiencies of wired charging, there’s a power brick and some copper wiring, but it also introduces an additional loss through the wireless charging coils. Again, let’s break this down into stages.

Stage 1

Our charging brick takes the AC current in the wall and converts it to DC, just like in our wired example. No changes there.

Stage 2

The DC current then flows through the copper cable to the wireless charging pad suffering all the same energy losses as our wired example.

Stage 3

After powering some IC’s, our excitable electrons arrive at the transmitting coil in the wireless charging puck. Unlike our wired example, our wireless charging coil needs AC current to create a magnetic field that can transfer energy. So we’ve converted AC from the mains to DC in our GaN charger, and now back to AC to power the transmitting coil. That’s a fair amount of energy we’ve just lost throughout that process.

Stage 4

Once the coil has its AC supply, that current moves back and forth at around 140,000 cycles per second. Operating at such high frequencies introduces additional losses caused by the Skin Effect. When a current alternates in a conductor at high frequency, the electrons vacate the center of the conductor (the copper wire) and only utilize the outer surface. This effectively reduces the conductor’s surface area, packing the electrons ever closer and increasing the number of collisions which occur. More collisions means more energy wasted as heat.

Stage 5

The energy transferred to the receiving coil will also oscillate at a high frequency meaning that the Skin Effect will apply here too. It also means that the receiving coil is generating an AC current and before charging can begin, that current needs to be converted to DC. That means that from the wall to the battery, we’ve converted:

1. AC to DC
2. DC to AC
3. AC to magnetic flux
4. Magnetic flux to AC
5. And finally AC back to DC. 

Each conversion represents further energy loss.

Stage 6

The stream of electrons arrive at the battery and the charging process begins, just like in our wired example.

Stage 7

Unlike our wired example, wireless charging pads will continue to draw energy after you remove your phone. Typically around 0.2W but that figure fluctuates depending on the charger.

Additional Loss of Energy

Most wireless chargers are round pucks that allow you to place your peripheral anywhere on the pad. That means there’s a high chance that you’re not perfectly aligning the receiving coil with the transmitting coil, reducing the overall efficiency of the energy transfer between coils. The Qi2 standard addresses this with a circular magnetic ring around each coil (think MagSafe design) which takes care of this problem. All other chargers, or smaller devices like AirPods, will suck up more power purely because the coils can’t achieve optimal alignment.

The distance between the transmitting coil and receiving coil also impacts how efficient the charging experience will be. The further apart the coils are, the more energy is needed to charge the device. So if you have a case on your phone, the extra millimeter or three will result in a decrease in efficiency.

Lastly, unlike modern power bricks that shut off when not in use, a wireless charger has to “probe” constantly for the presence of a wirelessly chargeable device. This constant probing consumes energy, and how much depends on the charging pad in question. Of the ones we tested, we saw a continuous average power draw of around 0.2W (excluding our worst performer, more on that later). The average user will leave their wireless charger plugged into mains in a fixed position, say on the bedside table or on a desk, which means the charger will draw power throughout the day as the wireless pad continuously pings for a device.

So the theory is clear. Wireless charging should definitely be less efficient than wired charging. But by how much? And are batteries being damaged in the process?

Let’s get to testing and find out.

The Baseline

As avid fans of the scientific method, we’re going to do our best to account for as many factors as we can reasonably control. That means using the same cables, charging bricks, phones, and measuring equipment throughout each test. Having said that, there’s always going to be a margin of error that we won’t be able to account for (such as ambient room temperature differences throughout the day). 

In order for these test results to have any meaning, we’ll compare wireless charging to a stable and proven alternative: A wired charge of an iPhone 15 Pro using iFixit’s 65W GaN charger and iFixit’s 240W rated USB-C to USB-C cable. We’ve already tested our charger and cable against a 70W Apple GaN charger and found them to operate at roughly the same efficiency (ours was actually slightly more efficient but who’s keeping score?).

We’re keen to get accurate temperature measurements from the battery itself and not just the surface of the phone. Our resident engineer, Arthur Shi, set us up with a handy dandy temperature probe that slides through one of the P2 screw holes at the bottom of our iPhone 15 Pro and gets taped snugly against the lithium polymer battery pack. We can even close the device back up to get readings that are as close to real world usage as possible.

For power draw, we’re using a watt meter at the mains, to which our iFixit GaN charger is plugged into, and a second watt meter between the GaN charger and the iPhone 15 Pro (either as a cable direct to the phone or a cable to the charging puck).

Last but not least, we’re monitoring and recording temperature and power draw at five minute intervals and recording the results.

So without further ado, here’s the data from the wired test which will comprise our baseline for everything else.

The graph shows that over two hours and 10 minutes, the phone charged from 0% to 100% and had a peak power draw of 20W at the 30% charge mark. The battery temperature also peaks shortly after at 30°C and begins to decline back down as the battery cell fills and the power draw from the mains also declines. The total energy used to fully charge a 12.7Wh battery was 18.25Wh, which means there was a 5.55Wh or 35.9% loss of energy. That’s a lot of waste, and it’s also our best case scenario.

While this data is informative, it cannot be taken as an absolute measure of charging efficiency. Most people charge their phones overnight, which means the phone remains plugged in long after it fully charges.

Assuming a 7 hour sleep cycle for the average wage slave, the phone will continue to draw around 1.1W from the mains for another four hours and fifty minutes before it’s unplugged. The good news is that modern charging bricks are smart enough to shut down completely if there is no draw, so once the phone is unplugged there is nothing being drawn from the mains.

So over a 24-hour period, an iPhone 15 Pro will draw something in the region of 23.6Wh when charged with a USB-C cable. Since wired charging is the most efficient means of charging available to us, we’re not going to include any energy used to get the battery to 100% in our calculations for avoidable waste. Anything after 100% charge, that is to say any value above our baseline of 18.25Wh per day, will be included as waste.

Assuming we’re charging the phone from empty to 100% without adaptive charging and at the fastest speeds available to the phone (i.e. worst case scenario), the potential maximum waste for a wired charge over a day, would be 5.3Wh x 365 which comes to 1.94kWh over a year. That’s the equivalent of leaving a 10W LED lamp on for eight days straight.

Candidate One: Apple’s 15W MagSafe Wireless Charger

Apple’s MagSafe wireless charging pad is our first test case because I suspect it’s going to do pretty well compared to the other wireless chargers we’re looking at. It has a USB-C connector and has magnets integrated around the coil which allows it to latch on to an iPhone 12 or newer device. Apple actually contributed to the Qi2 standard and any newer Qi2 compliant device with the MagSafe style magnets would be compatible with this charger.

It also charges at 15W, so long as you’re using a 20W or higher charging brick. Let’s see how it compares to a wired charge.

As expected, Apple’s MagSafe charger did relatively well in the 0-100% charge scenario but as the graph shows, there were some key differences when compared to our baseline wired test. First off, we can see the power draw ramps up faster and earlier in the charge process but also ramps down a little quicker later on. This results in a total energy use of 23.33Wh. That’s a 24.4% increase in energy consumption when compared to our wired test and represents a 59% loss of energy to charge a 12.7Wh battery. 

In addition to the extra 5.08Wh required to get to a full charge, taking our 7 hour sleep cycle scenario means the wireless charger continues to draw 1.5W from mains for another 4 hours and 55 minutes which uses another 7.4Wh. We’re now at 30.73Wh, and we’re not done yet. 

Once you remove your phone from the wireless charging station, the station continues to draw power to “probe” for the presence of a device on the pad that may need to be charged. These probe signals are sent out often enough that we see energy fluctuations from the mains that average to around 0.2W, even though we’re not charging anything. So over the 17 hours that our phone isn’t being charged, the device itself draws 3.4Wh. This gives us a total draw of 34.13Wh per day, or 12.36kWh per year. That’s 36.48% more energy used when compared to a wired charge.

Since we’re using 15.88Wh of energy above our baseline of 18.25Wh, this means that a potential 5.8kWh a year is being wasted. Remember that 10W LED light? You’re now leaving it on for 24 days straight!

And that’s still not the whole story. The higher power draw results in higher battery temperatures, nearly reaching 40°C in our tests, while the battery temperature climb down is also extended. We’ll revisit the battery temperature problem later. For now, note that the battery is getting hotter.

For those of you that are curious, you can take a look at the innards of a MagSafe wireless puck in our iPhone 12 teardown analysis.

Candidate Two: Amazon Basics 15W Wireless Charger

I don’t know who’s design Amazon ripped off or who’s business they drove into the ground to make this, but it is a cheap and cheerful 15W USB-C charger that is Qi certified. That means the transmitting coil doesn’t have the alignment magnets of Qi2 certified devices that lock the transmitting and receiving coils in the most optimal positions. 

This is our worst case scenario test. How inefficient can wireless charging be when the coils are misaligned?

The answer is: Very inefficient. It took nearly four hours to charge the iPhone 15 Pro when it was placed off center and the battery temperatures remained above 40C for most of the charge cycle, which is bad news for battery longevity.

We ran this same test with an iPhone 15 Pro fitted with a non-MagSafe compatible case, but the results were surprisingly similar.

Drawing a hefty 33.93Wh for a 0-100% charge, another 14Wh for the three hours the phone would remain on the charging puck to complete our 7 hour sleep cycle, and 10Wh in standby mode while the charger is unused, the Amazon Basics charger consumes 57.93Wh per day and 21.15kWh per year. 

Using our 18.25Wh baseline, that’s 104.2% more energy used to charge the device. That translates to 14.48kWh per year being wasted. We’re now leaving our 10W LED bulb on for more than 60 days straight. By this point every dad in the country would have turned the thermostat down to 62F.

Bonus Candidate: Tesla’s Wireless Charging Platform

There’s not much information on the Tesla Wireless Charging Platform though we’re told it’s compatible with Qi certified devices. We’re also told that “charging performance may vary”. Very cryptic. Let’s see if this charging pad sticks the landing like a Falcon 9 or if it’s going to crash and burn like Starship.

I’ve not even started the test and the end result is already a foregone conclusion. At idle and without any device attached, the Tesla charger draws a constant 1.4W from mains! Compare that to the 0.2W drawn by the Apple MagSafe charger.

Placing a phone on the charge pad doesn’t improve the situation. As we’ve already seen in teardowns of the device, the Tesla charger relies on multiple transmitting coils that overlap each other.

This means that regardless of where you place your device, several coils will always be misaligned with the receiving coil. This is an incredibly inefficient design and the test results show this.

Tesla’s wireless charger performs nearly as well as a poorly positioned phone on a non-MagSafe charger. The disparity in efficiency is even more glaring when compared to a properly aligned transmitting and receiving coil.

There is no best case scenario for the Tesla Charging Platform. It’s the absolute worst case scenario for wireless charging wasting 66.3Wh a day which is a whopping 113.7% more than a wired charge would consume. 

The 17.54kWh wasted every year is the equivalent of leaving a 10W LED light on for 73 days straight. At this point there’s no point turning the thermostat down any further because Santa’s definitely bringing you coal for Christmas. Which is great because you can finally turn that light off and warm yourself up.

Pretty poor showing from Tesla. I’m not sure how Elon plans to save the planet with this kind of wasteful energy consumption.

Conclusion

While not performed under strict laboratory conditions, the data we’ve gathered still provides us with a very good idea of what works and what doesn’t. Wired charging is the most efficient way to charge anything and that is an indisputable fact. We can also show that though wireless charging is inefficient, there are grades of inefficiency among wireless chargers too. Qi2 and MagSafe chargers clearly offer the best efficiency while the first generation Qi chargers and poorly designed products can consume more than twice as much power as a wired charging setup.

Tesla’s Wireless Charging Platform deserves special mention for their dumpster fire of a design. This charger has the singular honor of being ludicrously power hungry when not being used and absurdly inefficient no matter how you place your phone on the stand.

Our tests also suggest that your battery may be better off kicking the wireless habit.

While the back glass of the iPhone was warm to the touch during our charging tests, the battery didn’t exceed 30°C on a wired charge or 45°C (which is a warm day in Texas!) in the worst wireless scenario. Most of the waste heat is likely dissipated through the phone frame. This is a good thing, because according to Battery University, temperatures above 30°C are considered “elevated temperatures”, causing the battery to degrade if the temperature is maintained over a long period of time. Go over 60°C, and lithium-ion batteries begin to break down rapidly and irreversibly. 

Thankfully, none of the charging temperatures get anywhere near 60°C. The charger and phone have temperature sensors and we did see indications that some throttling was occurring to avoid excessively high temperatures.

WIth that said, our wired charge was the only test scenario where the battery momentarily touched 30°C while all our wireless charging test scenarios saw the battery warming well above 30°C. You can expect to see reduced battery longevity in the case of the Tesla charger or any situation where the charging coils are improperly aligned.

The tl;dr: is that wired charging is by far the most efficient way to charge any device. 

Finally, I can say without a shadow of a doubt that the Tesla charger is by far one of the most inefficient designs I’ve seen. Not only is it poorly designed, maximizing waste for the sake of extending wireless reach across a surface, it’s also so inefficient in transferring energy that the prolonged excess heat generated by the charging battery will degrade the operating life of the battery cells.

If you do go wireless, choose wisely. Qi2 and MagSafe certified devices would be your best bet in terms of efficiency. Combine that with an energy saving power strip and you’ll limit the energy lost while the charger is not in use.