Disclaimer: Other manufacturers might also have this flaw, but I have the experience with Samsung.
Fast charging is such a practical feature I have always appreciated. I just cherished how fast the battery percentage increased within minutes. It fascinated me so much that I explored the technical details of Qualcomm QuickCharge and MediaTek Pump Express (e.g. voltage negotiations) and USB cable resistances. I experimented smartphones and power banks using bench power supplies. And I love 5A USB-C cables due to their low electrical resistance.
Back to topic:
I am not at all implying that Samsung is bad, I am just describing a flaw that defeats much of the usability of fast charging.
The flaw is:
Fast charging is ineffective while the mobile phone is in use!
The maximum charging current for Samsung flagship phones until 2016 (after 2016: untested yet) is 1.8A, the maximum voltage is 10.0V since the Galaxy Note 4 in late 2014 (6.0V earlier), of which fast charging uses 9V 1.67A. (Yes, I know, mention voltage first. But current came to my mind earlier this time. To all bench power supply manufacturers: Please put voltage up or left (first), and current below or right next to it! Not the other way around!)
Now here’s the problem:
According to my tests, no matter the input voltage, while the device is being used (i.e. not in stand-by mode or turned off entirely), the total input power gets reduced to 6 watts! (e.g. 5V 1.2A; 9V 0.67A; 6V 1A, latter tested using bench power supply and compensated for cable resistance losses)
Device power consumption + battery charging speed = total input power.
This means: If the mobile phone needs 3 watts momentarily, only 3 remaining watts of the 6 power enter the battery! 3 watts instead of 15 watts (!!!). Considering that, since 2015, Samsung forces deactivating power saving mode while charging, which is OK when charging at home, where relatively abundant power is available. Lesser OK when charging from a power bank, but for a 100-Watt-hour powerbank, it should not be an issue in everyday life.
The purpose of fast charging gets defeated while the device is being used!
(Edit 2019–07–21: 6 Watts is the same total input power wattage that the Galaxy S4 and Note 3 from 2013 support while the device is in use. These 2013 flagships support a maximum input current of 1.7A (only while not in use) and a maximum input voltage of 6.0 V. The Galaxy S5 from early 2014 is untested yet.)
Why? To prevent overheating? So why does the UleFone Armor 2 stay pretty cool during it’s fast charge during usage? (more about that later in this post)
Why not allow full-throttle charging (9.9V 1.8A maximum total input power while device in use) while it is not too hot? (i.e. 35 °C, where the charging speed would have been reduced to 5W anyway?)
Samsung could have technically given users the luxury of unrestricted charging speeds during usage. And for heat: Users have the option to deactivate fast charging in the settings, if it bothers them, which I would never do because I adore fast charging so much, that I even own multiple multi-output fast chargers for the automobile power outlet and a fast-charging powerbank! I don’t mind that bit of additional heat for watching the battery percentage increase rapidly and being few seconds away from the next higher battery percentage number, a luxury iPhone users see for the first time years after Android users, like so many other things (e.g. 2160p video recording).
Trivia: Their charging speed of 15 W has shockingly stayed the same from Note 4 to Note 9 and S6 to S9. As far as I know, the same EP-TA20 wall adapter from 2014 had been still used in late 2018 for the Note 8! 15W was excellent in 2014 and not bad in 2018, but at least a few additional watts during these years would have been appropriated.
(At least, they don’t fool their users by putting a tiny, weak 5W (5V 1A) charger into the delivered package of a device that might cost beyond €1000 or $1000 respectively, even as of 2019. Yes, I am looking at you, Apple!)
How to do it right:
While some mobile phones maintain the total input power (device power consumption + battery charging speed = total input power), others do what laptops have been doing for decades: Use the spare current of the power supply / phone charger.
One phone that does it right, for example, is an UleFone Armor 2, of which the battery charging speed is limited to 10W (despite of the advertised and technically possible 18W), but while the phone itself is being operated, the battery will still be charged with the entire 10W, and the power required to power the mobile phone components themselves will be drawn from the power supply in addition, which makes charging speed independent from usage, which is among the things I appreciate most about that phone.
And that’s how laptop computers have been doing it ever since. (Provided that the connected power supply has a sufficient maximum wattage.)
But it is time to see it a bit positively: If that fast charging flaw did not exist, just like if global warming did not exist, most would likely have taken it for granted. Grant-taking instead of appreciation.
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Similar: https://medium.com/@sha8/samsung-powershare-avoid-using-it-716d035638a6