[Samsung Galaxy S9+ Questions & Answers] logan893:
Update with USB tester and comparison with a Samsung Galaxy S5 Neo.

TL;DR: 2800 mWh is the new 3500 mWh

First, S5 Neo.

I have an old S5 Neo, last of its era of Samsung phones with easily replaceable battery. I purchased a new battery for this phone from iFixit in the same order as the S9+ replacement battery.

S5 Neo's battery is rated at 4.4 V (3.85 V nominal) with a 2800 mAh capacity, or 10780 mWh. Depleting the battery and making sure the phone would no longer switch on with the power button, I charged it from 0% to 100%.

After slowly charging for 3 hours at just below 4.8 V and 750 mA for most of the time, the USB tester showed 10780 mWh, and the phone showed 92% charge. At the 3h 30min mark, shortly after reaching 100% (I didn't catch it perfectly), USB tester showed current had dropped to 350 mA and total charge delivered was just over 12000 mWh. After another 10-15 minutes the charge current dropped to zero and total power delivered showed 12326 mWh.

The S5 Neo phone, using the app GSam Battery Monitor while the phone is otherwise idle, it shows an internal battery voltage reading of 4.38 V when fully charged, still plugged in and trickle charging. Once it's saturated (charging icon disappears) the voltage drops to 4.33 V.

Going by the USB tester results and progress shown by the phone, and with the assumption that the battery is able to be charged to its full rated capacity with this old phone, the battery charged to capacity with an efficiency between 87.5% and 92%.

I also captured a few mid-way readings. Here is the complete list of my captured readings including comparison to rated capacity assuming perfect efficiency (with extrapolated total charge for levels below 100%):

100% (charging current at 0.0 A)	12326 mWh	~114% of rated capacity
100% (still charging)	12062 mWh	~112% of rated capacity
92%	10780 mWh	11717, ~109% of rated capacity
81%	9291 mWh	11470, ~106% of rated capacity
62%	6440 mWh	10387, ~96% of rated capacity
33%	3160 mWh	9575, ~89% of rated capacity
19%	1868 mWh	9831, ~91% of rated capacity

The extrapolated charge to reach max rated capacity is increasing as charge level goes up, which I think is kind of expected. Conversely, the efficiency is a little lower than I would have expected. Still, the total power required to reach 100% exceeds the rated capacity by a fair margin. Better still, the phone itself reports a battery voltage near the battery's rated 4.4 V.

Second, S9+.

As mentioned in the original post, the internal battery voltage max reading is 4.22 V, and the capacity seems to not reach the expected level. Checking the Battery status option of Diagnostics in the Samsung Members app to check the battery, it shows it is in "weak" condition. Resetting all the battery related readings using USSC codes (see battery calibration) doesn't seem to change anything.

The battery is rated at 4.4 V (3.85 V nominal) with a 3500 mA capacity, or 13475 mWh. With fast charging disabled, and after draining the phone until it powered off and would no longer power back on, I started charging via the USB tester. I'm using a Samsung travel charger this time, which delivers a little more current than when I charged the S5 Neo.

Charging starts out at just below 5 V and 1.5 A. Current quickly drops to 1.172 A, and somewhere between 33% and 50% it drops to 1.072 A. This current is stable until beyond 81% and then starts to gradually decrease throughput the rest of the charging cycle. (Actual charging current is slightly lower with screen off vs screen showing current charge level.) This I believe is indicative of the battery capacity (or perhaps rather the phone's perception or expectation of the battery capacity) is below its typical levels. It could also be a difference in how the S9+ charges compared with the S5 Neo, or perhaps less likely a difference between the chargers used, I can't really know for sure with only my two samples. In comparison, the S5 Neo kept charging at essentially the same current level between 0% and 92%.

Again, here are a few mid-way readings, as well as the extrapolated capacity assuming perfect efficiency:

N/A	13475 mWh	never reached
100% (charge current at 0.0 A)	12341 mWh	~92% of rated capacity
100% (first reached)	11982 mWh	~89% of rated capacity
92%	11398 mWh	12389, ~92% of rated capacity
81%	10053 mWh	12411, ~92% of rated capacity
62%	7646 mWh	12332, ~92% of rated capacity
50%	6174 mWh	12348, ~92% of rated capacity
33%	4100 mWh	12424, ~92% of rated capacity

Oddly consistent, this ratio between charged power and percentage charge, at around 92% of rated capacity throughout almost all of the charge cycle.

The total power required to reach 100% is very disappointing. Even assuming perfect efficiency, it is well below the battery's rated capacity. Assuming similar efficiency as the S5 Neo at around 90% translates to roughly 80% capacity at full charge. This mimics closely the estimates from AccuBattery, which shows 78% capacity after 12 "full" cycles.

To reach a "saturated" 100% requires equally much power for the S5 Neo as for the S9+, even though the S9+ should have a battery with 25% more capacity.

Now that I have externally validated that my 4-year-old S9+ is only ever able to charge my fresh newly replaced battery to 80% of its rated capacity, I suppose the optimistic view is that it's now a forced charge limit for substantially increased battery longevity. While this is functionality I do want, it is something I'd prefer to have the option to use, and with the ability to top up to 100% when necessary.

If I have the opportunity, I may drop by a Samsung service center and ask if there's anything I can do to reset the battery status, or retrain the phone's perception of battery capacity.

Thank you for coming to my TED talk, and have a great day.