More detailed info from https://batteryuniversity.com/
Note the dark blue 100 ~ 50% and
and the light blue 75 ~ 25%
both reduce the ‘measured’ capacity by 50% but the later actually causes more stress.
This is not the same as capacity and discharge cycles but it’s the stress that lowers the discharge cycles.
So from the graph you can see 75% to 65% is optimal
The point is the 100% and not really the 50%, this is an example.
“Far less” is an exaggeration. Power consumption is voltage times current, comparing 4.4V to 4.0V is only a 10% difference. While 10% is noticeable, it is not “far less” in my opinion. Also, I do not believe that phone manufacturers are unaware of this difference. Can you prove that they do not take this into account when giving you the percentage of battery life remaining? My phone does record current usage, are you certain it does not use that value to help inform its battery life percentage? I believe it does at least one of those two things, but I cannot prove it - but still, even if they do not, it is no more than a 10% difference in power supplied in your example.
Your image didn’t come through correctly, but I think you’re looking at the “Number of DST cycles” vs. “1C Capacity Retention” graph, correct? I think you may have misread the graph, or perhaps your explanation of what you saw wasn’t clear. The latter case, 75-25%, is the less stressful - hence the better capacity retention over time (higher on the graph).
Also, looking at this graph, the blips in the light blue and orange lines make me think that they did not run the test with e.g. 10 batteries and average it out; it looks like they tested one battery for each type of run. So, upon closer inspection of this graph, I’m not sure how much I trust it even though I referred to it earlier.
Optimal in terms of the number of cycles you can get from it, and only on the items they tested, absolutely. However, had they tested from 60%-50% I’m reasonably certain from the other information in that article that the results would have been even better. It’s also important to note that you are using only 10% of the battery’s capacity, which isn’t exactly a great real-world scenario.
Lastly, it is clear from that graph that the biggest driving factor is the number of charge/discharge cycles. I would like an option to not charge the battery when the phone is plugged in. That is, it should just run the phone from the charger as much as possible and the only interaction with the battery should be to pull from it during peak current draws (e.g. cell radio transmits). That way I can keep the phone plugged in at work, and it maybe discharges 5% during the work day instead of 30%, allowing me to choose an even lower overnight “full” charge (say, 60% instead of 75%) and still have plenty of capacity remaining at the end of the day without feeling like I’m burning out my battery.
To summarize, what I would like to see is:
Seems reasonable to me too 
Yes, this would be the ideal solution. It’s exactly what I do have with my laptop which has software for managing battery charge. I can restrict the charging level to 60, 80 or 100%; usually held at 60%. The app presumably controls some hardware-specific code and circuits, since it will hold the charge at 60% even when the device is completely switched off but still plugged in to mains supply.
For all my other Li-ion devices I simply use a timer switch that cuts off after 40’ or whatever. When away from home I sometimes set the clock timer of the phone to go off after the required time to remind me to stop charging. Not very sophisticated but effective 
@AlbertHall
There are other topics where this issue has been addressed, the only viable option is to root the phone and download an app.
Because this topic mentions 80% it may appear a different issue, it isn’t, maybe this topic can be merged with
Another issue is that Fairphone have, no doubt, taken advice from the battery manufactures, and the charge rate is set for optimal user experience ??. The battery is guaranteed for a year.
If you root the phone and mess with the settings Fairphone have no way of knowing what you may do and the warranty would be void.
Two years now for the Fairphone 3/3+ … Fairphone 3 Warranty - What is a warranty and how does it work?, and for the Fairphone 4 … Our Warranties for the Fairphone Devices and their accompanying accessories and spare parts - Fairphone.
The only option that is viable in the current phone, sure. But there is absolutely no reason it couldn’t be built into the phone. As @OldRoutard mentions, his laptop has this, we shouldn’t be required to root the phone to achieve what we’re looking to do. If a rooted app can do it, then the OS could do it natively if a manufacturer such as Fairphone cared to do so.
Don’t forget that the manufacturers recommend full charge, and that full charge wears the battery out faster. What happens when the battery wears out? You buy another one, and the battery manufacturer gets your money.
Taking a less cynical approach, it does make sense for the phone manufacturer to have the default be a full charge for most users - it would be frustrating for most users if their phone didn’t make it through a whole day without charging unless they changed a setting. But those of us who care about battery waste and reducing the cost of ownership of our devices should be allowed to choose differently.
There’s a debate about what a full charge is. The 100% may only be 90% and when then phone cuts out there may be 20% left.
Fairphone have their business to look after so they must come to a compromise.
As for accessing the charge rate it is an Android issue not a Fairphone issue. To be able to sell a phone with Android via Google Fairphone have to comply with 500,000 tests.
You want to manage the battery outside warranty that’s fine, but not otherwise. 
I see this as irrelevant. The percentages technically don’t matter, it’s the battery voltage that is important, and that’s hidden from me. However, if I’m picking the lowest maximum charge I can, while finishing the day at the lowest remaining percentage I’m comfortable with, it doesn’t matter what the battery voltage is in the background - I did my best and will see improved battery longevity.
I’m not an expert in this area. But, considering the number of manufacturers out there who load a custom Android version on their devices because Android is open source, I don’t understand your assertion that Fairphone cannot solve this if they cared to. However, considering that they didn’t realize FPOOS for FP3 it is unlikely that they will integrate a feature like this. But my point is that they could, I shouldn’t have to root and void the warranty for something that increases the phone’s ecological sustainability since that’s what FP is all about.
The battery voltage is shown by dialling *#*#66#*#* then Service tests > Test single > Battery status test
Once you have established the percentage show for the voltage you want you can then use the notification bar to show the percentage.
You can then set an alarm for that percentage if you like.
By the way, there is no indication of what voltage the battery would be at when it is ‘fully’ charged.
There is no definition of full and no relative voltage.
The battery manufactures state 4.4V on the battery but unless you ask them, in China’ what that refers to there is no indication if that is 90% or 100%.
There is no info on how the battery is made.
The charging mechanics only allow the 4.4V
Well if don’t like Fairphone’s lack of conforming to your designs it would be better to contact them and discuss it with them.
Maybe then you could come back here and let the forum know of your conversation.
UPDATE:
There’s another issue to do with your statements
a)
b)
If you have come across any of those you refer to in b) above that give you access to a) above you could also inform Fairphone and post the info here.
if not then why would you think Fairphone would chose to do so?
Why did Fairphone choose to allow you to repair your device without voiding the warranty? I know of no other manufacturer who does that, they want you to buy a new phone every 2 years.
Fairphone did so because keeping your phone longer is more sustainable for the environment. It makes sense to me that they would also want you to keep your battery longer, as that is also more sustainable for the environment.
Well ask them! Maybe they have what they consider to be the best charging algorithm for the avarage user, they can’t tweak an OS for each customer and get Google certification and keep the warranty.
You really could to better by asking them than by using the forum to gather just more user ideas ??
How about this?
Buy one and report!
Dude, that’s awesome. I don’t like that I would have yet another hardware piece to keep track of, but the algorithms described on that page are exactly what’s needed to prolong longevity. Why this hasn’t been built into phone OSs is beyond me, but at least this is a relatively universal solution.
Not sure yet if I’ll be buying one to test out or the 4 pack, but I can tell you that I’ll be giving this guy some cash. I’m probably going to look into replicating this with an Arduino and a relay for my wife’s and kids’ laptops, I do not enjoy ripping the battery apart to replace dead 18650 cells and trying to cram it all back together.
Not convinced that it does what it says and wouldn’t even try it if was free with my breakfast cereal.
Can you name any technical reasons why? Or is it just your feeling?
If you can’t work it out from the site that sells it I will not go into details. Just look at the graph they use from batteryuniversity it says more that enough, it says nothing and is irrelevant a distraction and . . . .
So you have no solid arguments.
Which graph do you mean? I cannot find it on their website…