[Samsung Galaxy S21 Ultra] enigmaamit:
My thoughts on the charging situation with the S21 Ultra

In the newer S21 series (as with the Note 20 Ultra), 45W super fast charging 2.0 support has been dropped (I suspect it has something to do with the findings stemming out of the Note 7 battery fiasco investigation). From the reports I've read, they seem to have invested a lot of money and resources in studying battery safety after that embarassment, and have a new lab dedicated to studying battery and charging safety.

Max supported input wattage on the S21 series is 25W (so called superfast charging) and then there is the regular 15W "fast" charging. There's also 9W and 15W wireless charging (latter only available vis proprietary Samsung wireless charger)

There isn't a very big difference in the charging times between the OEM 15W and 25W Samsung adapters - there's a differnce yes, but its not a huge margin - definitely not what a 66% higher wattage charger should provide (25W over 15W).

Heating during cable charging is present on the S21U both with the 15W and the 25W charger, way more than what I have experienced with Warp charge on my OnePlus phone. This is despite the much higher wattage used by OnePlus, thanks to the VOOC implementation from Oppo, with lower voltages and higher amperages, and letting the charging brick do most of the thermal dissipation rather than the phone. Samsung on the other hand uses USB PD 3.0 + PPS standards.

If you're using a wireless charger without a built in fan, you get an even hotter phone than with the 25W cable charging, despite a lesser charge throughput. It's objectively worse for battery longevity because the battery is kept at the higher temperatures much longer due to the slower charging speed. Heat and Li batteries don't mix well.

From my limited experience with various charging standards and different phone brands, my conclusions about the S21 ultra are as follows:

1. Regular cable charging works just fine on the S21U. I use regular QC 2.0 chargers lying around at home and they work fine for charging up the phone without wearing out the battery. I plug in the phone whenever i am not using the phone, like for 15 to 20 mins at a time and most battery experts say that multiple small top up charges are better than a high stress pedal-to-the-metal full charge.

2. 15W is the fast charger of choice that I would charge the S21 Ultra with, if I needed a quicker boost on a nearly flat battery. 25W performance doesn't justify the extra cost of the brick. If you already have a 25W charger, then use that - what I'm saying is, there's no point buying one thinking it will charge 66% faster than the 15W charger.

3. I avoid Wireless charging on the S21U with third party Qi chargers (these only charge at 10W or lower). The phone really heats up depending on the charger - and then safety protocols slow down the charging speed even further on an already inefficient power delivery system. I would either use the OEM Samsung fast wireless charger (15W) with built in cooling, or none at all. This ensures minimum charge time to full and active fan cooling - least thermal stress is what I'd look for here.

4. My phone's battery charge level usually ranges from 20% to 90% - i try not to let it run down to flat or charge up all the way to 100%. If it does so occasionally, it's fine - the phone can handle it. I just try not to let it happen all the time. Have you noticed how fast the battery falls from 100 to 99? and then to 95... but it takes way longer for it to drop after that? 100% charge is a very unstable state for the battery 85-90% is the sweet spot. And around 50% is the most stable state. This is why Li Ion batteries are shipped at near 50% charge by most smartphone manufacturers to provide longest shelf life (the company has no way of knowing how long the phone is going to sit on the shelves before being sold)

5. I don't charge my phone overnight. I know that good chargers stop supplying power once battery reports 100% charge, but it starts charging up again when battery drops to 99%, charges it back to 100 in a few mins, then it cuts out and battery drops again and the cycle continues many times during the night till you take your phone off the charger. We know 0% and 100% battery level are the highest stress states for a battery - yet we leave the charger connected overnight, maintaining the battery at 100% for 6 to 8 hours at high electrochemical stress level.

All this is inconsequential if you change phones every year. You can do anything with your phone if you aren't planning to keep it for very long. I personally use my phones for 2-3 years and like to keep the battery is as good shape as possible. The above charging discipline has helped me maintain good battery health on all my devices for an average of 2.5 years each.

Some readers may not agree with my assessment and recommendations. Please feel free to disregard them and follow whatever has been working for you over the years. My use case and lifestyle may be very different from yours and consequently your charging habits will vary. That is fine.

This is not a directive from me to anyone, nor am i a battery expert. These are just my observations and advice for people that may be looking for it. If you've already figured out your best cahrging protocol, I'm happy it works for you.