In recent years, fast charging technologies have revolutionized: from the first breakthroughs in 20 W and 30 W chargers to today’s standard 40 W to 60 W technologies. But 2020 has brought us even further: 120 W are now being launched. The promise is to get our phone batteries from empty to full in minutes. However, there are still questions about the impact on battery life.
To see in more detail how charging works with 120 W and how fast it actually is, we put the Xiaomi Mi 10 Ultra and its 120 W plug through their paces.
More about chargers: How fast it really works
Charging efficiency
First, let’s check that 120W is actually getting into the phone from the mains. After all, 120W is more than a typical PC, so let’s hope that power isn’t wasted. Interestingly, the 120 W charger from Xiaomi “only” delivers 80 W for the Mi 10 Ultra. Maybe 120W is possible in Xiaomi’s lab, but I couldn’t pick up anything near that level in the real world. Fortunately, the charger doesn’t waste the missing 40 watts.
Xiaomi’s charger delivers closer to 80W to the phone than to 120W.
The power drawn by the wall starts at 92.3W and quickly drops to 86.4W for the majority of the charging process. Just 6.3W (7.3%) of the power wasted on the cord and charger is actually better than any of mine tested chargers with lower power. At least as a percentage of the overall performance. This is likely due to the more efficient GaN technology used in the Xiaomi charger.
| charger | Wasted strength | Electricity pulled from the wall | Power supply on the handset |
|---|---|---|---|
| Xiaomi 100W | 6.3 W (7.3%) | 86.4 W. | 80.1W |
| Huawei 40W | 5.5 W (19.2%) | 28.6 W. | 23.1 W. |
| Poco 27W | 5.6 W (17.7%) | 31.6 W. | 26W |
| Samsung 25W | 3.6 W (15.1%) | 23.8 W. | 20.2 W. |
| Google 18W | 2.8 W (16.4%) | 17.1 W. | 14.3 W. |
For example, Huawei’s 40W charger wastes less than 5.5W, but that’s actually 19% of total power consumption. Likewise, the Poco F2 Pro’s 27W charger wastes 5.6W – a significant 17% of the power drawn from the wall before it reaches the phone. Samsung and even Google chargers are similarly wasteful.
The bottom line is that charging with Xiaomi’s 120W plug is no worse for your electricity bill than any other charger on the market. In fact, it’s more efficient than many in-box chargers. I am pleasantly surprised at how efficient the GaN charger compares to older and slower chargers on the market.
How fast does 120 W (80 W) charge?
Note: Since the Xiaomi charger does not actually offer 120 W, I refer to the charging power in diagrams and analyzes from now on as 80 W in order to better reflect the power actually recorded in our tests. In order to get comparison points for different charging speeds, I charged the Xiaomi Mi 10 Ultra with different wattages from the Xiaomi charger and other USB PD chargers and tracked the time until it was fully tracked.
For starters, even 80 W instead of the full 120 W record an impressively fast charging time. It only takes 21 minutes to completely empty, or just three minutes to reach a 25% charge. This is quite a feat as the phone has two 2,250 mAh batteries with a total capacity of 4,500 mAh.
Somewhat surprising is that charging at 50W is practically just as fast. It only takes 29 minutes and a little over five minutes to reach 25% charge. Minutes of difference mean the 120W slogan is more of a marketing aspect than a revolutionary change in our charging habits. It will be very difficult to distinguish Xiaomi’s technology from 60W and other very fast chargers on the market without taking such accurate measurements.
The charging time improves from 18 W to 50 W much more than from 50 W to 80 W.
Charging the Xiaomi Mi 10 Ultra with a conventional 18 W charger takes significantly longer. 68 minutes too full is still okay by modern standards, but significantly slower than the other two tests. The most interesting thing is the decreasing loading time for an increase in performance. The 32 W distance between 18 W and 50 W results in a 39-minute improvement in charging time, which is clearly noticeable. Another 30 W more juice up to 80 W only improves the full charge by a rather meaningless 8 minutes.
Continue reading: It’s 2020 and USB-C is still a mess
Temperature and battery condition
Super fast charge times are great, but it’s not a good thing if it negatively impacts long-term battery life. Temperature is a major factor in battery life. We therefore kept an eye on the software’s battery temperature display during our tests.
The nature of this battery sensor data may not be entirely correct. However, since we use the same smartphone in all tests, this is still a good reference point for relative comparisons. We also started the phones slightly warmer than room temperature to simulate charging shortly after use.
The Xiaomi Mi 10 Ultra charges at 80W and reaches a peak of 43.8 ° C. Worryingly, this would be even higher if the phone could draw the full 120W. This is certainly a warning sign of the longevity of the battery, as the temperatures should ideally remain below 40 ° C. Since the phone only takes 21 minutes to charge, the battery is not exposed to high temperatures for very long. However, enough time is spent well above 40 ° C to be cause for concern. Note that the temperatures will start to drop again as soon as the battery reaches 70% and presumably leaves the constant current phase of the charging process.
Charging at 50 W measured a peak temperature of 39.1 ° C, which is 4.7 ° C cooler than charging at 80 W and stays below our red line. This temperature peak also only occurs towards the end of the constant current cycle, while the 80 W mode starts up much faster. Most of the charging occurs below 37 ° C, which is not bad. Given that 50W only charges 8 minutes slower than 80W, this seems like a worthwhile compromise.
A temperature jump of 5 ° C and 30 W more power to be charged just 8 minutes faster are a bad compromise.
If you step down to 18W, the sensor won’t break even 30 ° C. When charging at this lower wattage, it will take much longer for heat to build up. This means that the battery cools down faster. At 14.7 ° C cooler than 80 W, the temperature difference is day and night. But there is also a noticeable compromise on loading time. A sweet spot for temperature and charging time is likely around 30-40W.
It’s also worth noting that the 120W option pulls more power into the battery and emphasizes its C-rate. This is the other important half of the battery life equation. Between this and the higher temperatures, I definitely have concerns about how Xiaomi’s 120W implementation will affect battery capacity in the long run. Charging at lower power levels is the safest way to ensure the battery will last for many years.
Is it worth charging with 120 W?
120W has its uses and disadvantages. At least Xiaomi’s 120 W implementation gives cause for concern about much higher charging temperatures. Especially since it is only a few minutes faster than charging with a cooler of 50W.
Personally, I would only use 120W for the occasional quick charge. Literally for a minute or two. Temperatures won’t rise too high during a two minute charge and you get a 20% battery. That’s pretty useful. However, with stressful C-rates and high temperatures building up over a full charge cycle, I would stick to something slower for more than a few minutes.
120W charging is best for a very quick charge rather than a full charge.
I would love to see how Xiaomi implements temperature throttling to keep the battery cooler. This would retain the benefits of fast burst charging while ensuring better battery health over the long term. The real benefits of fast charging are fast recharging that will help you at the end of the day. There’s no point in burning a battery to shave just minutes before a full charge.
I’m less cynical about 120W charging than when I first announced the idea, but there is clearly still a lot of work to be done to mitigate the battery life issues. Although other companies like Oppo claim to monitor battery temperatures, I would be very interested to compare implementations.
After all, there’s a reason Xiaomi included this feature in its ultra-premium Mi 10 Ultra. New techniques for making batteries and a dual-cell setup, improved charging circuits, and an efficient GaN charger that can handle 120W are not cheap. For now, at least, Xiaomi’s 120W charging is more of an experimental feature than something primed for prime time.