Do newer Samsung phones still have the ultra power saving mode? It used to convert the screen to greyscale, throttle CPU, disable all background services and apps, and useable only for phone calls, sms, and basic web browsing. It could last over a week.

> It could last over a week

Install LineageOS but skip "GAPPS" (Google apps), don't install nosy background-active spyware like Facebook and its ilk and you should be able to easily go beyond 1 week on a device with a 4500mAH battery with "light usage". in full colour, with normal functionality. Standby time of up to a month (I got 21 days on a Motorola Defy with a 1700mAh battery), "normal" charge interval of twice per week with normal use. I would not buy a device which needs daily charging.

Yes they still do, without the greyscale as standard nowadays though. Its pretty great, you can "promote" 1-2 normal apps to work in that mode too. I used it heavily on trips where I only needed the camera and gmaps anyway - my trusty S10 lasted for days thanks to it.

My current phone is also an S20 FE (SD865). Comparing it to my previous phone, a Oneplus 6 (SD845), both get almost identical amounts of screen-on time while browsing the web at 150 nits, despite the S20 FE having a 4500mAh battery and the OP6 a 3300mAh battery. The SD835/845 era was a high water mark for performance per watt and so far the situation is only getting worse. The 888 and 8 gen 1 are both hot power hogs and the exynos 2200 is a garbage fire.

My original Galaxy S in 2010 would last 2-3 days pretty regularly...

Are you still using it as your daily phone? If yes, which custom rom/android version are you running? and is it still usable?

I was using it as my only phone with the stock OS up until mid 2018 when it started having what looked like hardware issues - like, sometimes the phone wouldn't ring but I got the missed call notification seconds later. It still had around 1 - 1.5 days of battery life at the time, with the original battery.