I believe limiting charging to a maximum of 80% should extend the liftime of the battery - please correct me if this wrong.
I cannot find an option to do this - please tell me if there is. I can do this manually by stopping the charge early but I would appreciate an option to do this automatically.
I agree a battery between 45% and 90% is what I aim for but there is no automatic option that I have discovered.
Further there is no idea what 100% means on the Fairphone system ~ it could be in fact 90% though I doubt it.
There is an app at F-Droid: Caution, the APP requires root access!
Battery Charge Limit
https://f-droid.org/de/packages/com.slash.batterychargelimit/
By the way, the Fairphone shows the green notification LED at 90% charge level.
Looks like you need to root the device to use it 
Any app that does this will require root (I personally use AccA with the Acc magisk module).
What you can do is set an alarm when the phone reaches 80%, so that you unplug it. AccuBattery on the play store and BatteryBot Pro on F-Droid can do that.
Personally I find BatteryBot Pro by far the simplest yet most intuitive and useful one, compared to AccuBattery (also pro version), Volle Accu&Diefstal, and Battery 100% Alarm ( Full Battery Alarm).
I’ve been using AccuBattery (non-pro) on my FP3 without any problems for nearly two years.
I have things set up so I have a flashing red light to warn me to charge it when it gets down to 50%, and a sound to alert me to disconnect the charger when it’s back up to 80%.
I didn’t need to root my phone to use it.
And I rarely open the app so the adverts don’t trouble me.
Signed up just to give those interested in this topic more information. According to Battery University’s BU-808 every reduction of 0.1V doubles the number of charging cycles you can get out of the battery while every 0.7V is 10% of your battery capacity. The maximum benefit, making the battery last for the maximum number of cycles, is about 60% max charge according to them, roughly a 2.8x lifespan increase. On a related note, for maximum longevity in satellites NASA charges the battery to 50%, because if you thought most phone manufacturer’s batteries were difficult to replace at least you’ve never had to launch a rocket.
My understanding is that it’s all about incremental gains on every charge (or rather, avoiding incremental losses on every charge). For example, if you keep it between 60% and 25% most of the time, but whenever going on a long trip you charge it all the way to 100%, that’s OK - you’re not straining the battery most of the time, but when you need it the capacity is there for you to use. Ideally, manufacturers would allow us to set the charge, because allowing your phone to sit on the charger at 50% basically all day (i.e. NOT cycling) while at your work desk or whatever is the healthiest possible thing for it.
For a manufacturer like Fairphone to not have this, when procuring the raw materials for building batteries is a big environmental concern, seems like a pretty obvious miss to me. I really hope they add it.
At 90% charge the battery always is being slowly charged, to increase longevity of the battery.
True but not the point being made, it’s the degree of charge not just the rate that ‘allegedly’ is damaging
Also not true, at least on my phone, it’s the same all the way. It may be that I don’t use a Q3 compatible charger, I charge from a 12v to 5v Belkin car type converter that can produce 2.4A but the phone only takes about 1 A from zero to 100
It doesn’t use QC for the last 10%.
Li-On and Li-Po batteries can perfectly handle first 90% with QC and last 10% w/o. They’re made for that.
On the long term, always charging them to maximum is nevertheless damaging them.
And there’s not such thing as 100% it’s just applied force, pressure on the battery.
100% of what? It’s stated capacity? A battery can be pushed over that. Though the charging algorithm should limit it, why trust the algorithm when you can try and avoid 4.*+V by stopping at 90%?
The thing is: they already don’t get charged to 100%, even when the device states 100%. The 90% is actually 80%, and the 100% is actually 90%. The relevant fact is that batteries degrade over time. They are, in essence, not good for the environment compared to physical/wired equivalent device. Does that mean we should only use physical? Of course not. Its just each has its pros and cons, and a replaceable battery is a good way to increase longevity of the device (esp user replaceable). If you want to use your Fairphone for 5 year as daily driver (main smartphone), it won’t last 5 years with the same battery unless you barely use it (screen time and brightness are biggest culprit of battery usage), but on average users do use their smartphones quite a bit every day. So for average users the batteries won’t last for 5 years either way.
Well you are really miring the pool : each battery has recommended max voltage, when
a) the energy used to increase it’s charge is disproportionate to the energy required to force the charge and the energy goes more into heat.
b) The heat is damaging
On my FP3 the max voltage is 4.4 and the phone is calibrated to that as a ‘max’
Fairphone could set 4.2v as the max and calibrate to that that but that would mean less usability for calls etc.
So Fairphone want to make the battery usage max to make the phone more attractive and then sell another battery.
That’s business
Of course Fairphone can say that 4.4v is recommended by the battery manufactures 
Most people may swap phones in two or three years so what incentive do the manufacturers have of extending the battery life.
Given the battery trouble with FP2 I would suggest Fairphone ensure the bloody thing Just Works ™. Which they did with FP3, including with QC.
Also, the smartphone does not continuously charge when it is full. It drains a while, and then it starts to charge again. If you don’t want it to drain so quickly, stop with cloud syncs and all that (‘and all that’ → read: Google phoning home). For example, if you put it on airplane mode during the night or when its otherwise not in use, I’m sure that’s going to help with the battery and longevity.
On top of that, modern smartphones shut down before the battery is drained, to protect the battery. So when it says 0%, its not actually at 0% either.
All of this is explained here in this article https://www.digikey.com/en/articles/a-designer-guide-fast-lithium-ion-battery-charging its not unique to any Fairphone. What is unique -sadly- is that you don’t have to open up a glued device to replace the battery. But the devices you got which are glued, can be taken care of if you follow an iFixit guide.
On top of that, rooting your device comes with serious side effects.
Possibly serious, but maybe seriously beneficial, else why root?
Didn’t get the connection to battery, should I have followed the link ?
Just to be clear, the main factors in the number of cycles you can get from your battery are:
So. The user can kind of control each of these. Temperature is a little tough in some ways. Delta SoC and C-rate will depend on how much you’re using the phone and what you’re running. Mean SoC is a pretty big factor and one that the phone charger controls, it would be nice if the user had control of it.
Basically, what it means is that if your typical usage is starting the day at 100% charge and ending the day at 50%, your battery will last nearly twice as many cycles (say, 4 years instead of 2 years) if you limit it to 70% charge and finish the day at 20% instead. Bumping it back to 100% on days when you know you might need it (e.g. traveling) would give you the full benefit of the li-ion’s energy density when you need it but not degrade the battery when you don’t.
While the user can kind of control the other factors, this is the easiest one by far to control as long as you can anticipate your needs, and it’s a big factor in battery longevity (i.e. not outweighed by the other 3). Why not let the user manage it? Especially for a company with such an environmental focus?