I have the same issue… Phone becomes hot (above the battery) when I use the Internet (3G, 4G, wifi…) and drains very quickly.
But my phone is completely new !!
I did kickstart the battery, checked the SIM cards, the SD card, followed the battery guide, but nothing changes.
What should I do ? A 550€ phone which claimes to be “fair” but does now work properly is not quite “fair”…
How did you check them? How old are they?
I have one of my former operator (which doesn’t work anymore since I changed my operator some days after getting the fairphone) which is 2 years old, and did perfectly work in the previous phone, and another one from the new operator, which is totally new, that works perfectely in the previous phone I have kept.
With each one of the 2 SIM cards the fairphone overheats, but another phone works perfectly, so I think my SIM cards are not the problem.
I also tried with one card only in slot 1, with one card only in slot 2 : the phone still overheats and the battery drains quickly.
For last couple of months CPU area of my phone got really hot when watching video, playing game or even some other kinds of apps. I think it was since some Fairphone open update (don’t remember the version).
After last update (Fairphone open 18.01.1) it got lot better. It never gets so hot and battery lasts longer.
A post was merged into an existing topic: 
Battery has short live, phone is overheating - La batterie se décharge vite, le téléphone chauffe trop
This week I suddenly had issues with short battery life. After using CPU Monitor Advanced Lite, an app which I can recommend if you want to find which app is using excessive CPU, I found that it was caused by F-Droid. The CPU usage was so few that the battery settings didn’t show F-Droid at all, but it caused frequent wake-ups at such a rate that I couldn’t make the end of the day on one charge which typically works fine even though my battery is slightly old and has occasional hiccups.
I don’t know why F-Droid had started using CPU excessively but after killing the process this morning, it stopped doing so.
Apps that you rarely use can cause a lot of trouble without you noticing! Last year, it was GMail, now it was F-Droid.
In the settings of the F-Droid app I have the updates, automatic update interval to a week.
It could be that the setting on your phone was hourly.
It’s set to daily. F-Droid did not appear to be using the network either when it woke up the CPU - usually it takes a while to download the package lists, so that should’ve been visible in the CPU monitor.
Anyway after killing its process the CPU usage vanished and I can’t reproduce it anymore. It was just a suggestion: check the hidden CPU usage of your apps when your battery is draining!
Hi. My fairphone 2 get suddenly really hot and drained battery in 2 hours.
I finally found the solution with FAQ of the website (Troubleshoot your Fairphone 2 issue --> Improve and monitor your battery life)
I restart on my phone on safe mode. Phone wasn t hot anymore, seems that it was an application problem.
Then I cleared all cache of the apps (Settings - Storage - tap Cached data - Clear cached data)
Since phone is working normally (hope for a long time 
I only now happened to stumble upon a possible malware problem, that can be responsible for battery draining as well as overheating the battery.
There are apps, that start mininig cryptocurrency (mostly monero) in the background. At a Kaspersky-Lab on battery caught fire, destroing the phone.
Mainly this threat comes by as an antivirus- or adult-content app. Pretending to be an antivirus app is a tad extra devious in my opinion. 
More info:
Might at least be worth looking into, if your phone suddenly unexpectedly starts draining the battery and turning hot.
Not a major factor for the extreme cases, but the Facebook app is very demanding on system resources and can cause overheating, reduce battery life and impair speed. I got rid of it, it’s better to use the web version instead.
FYI there are a few dedicated apps on F-droid that wrap the mobile web version and apply ad-blocking and custom CSS to it
My phone hasn’t been working properly after the latest Update here in Feb. 2018. It gets really hot in the top part and the battery drains within a few hours. And will only charge fully when it’s off. If I leave it on in the charger - then it just stays on the same % - or even loose battery.
I have both tried to manually re-install the update (twice) and all the “battery boost” tips found in here. But nothing seems to work. It doesn’t seem as if the battery is even fully drained because 1. when I plug it in, the red light doesn’t blink (i.e. it isn’t fully empty) and 2. I can for the most part just turn it back on and then it has between 1%-40% battery still. But then the smallest usage can make it die again - and sometimes not. It’s really frustrating. Can anyone help?
Thank you,
Stephanie
Have you tried the battery reset from the #batteryguide?
Have you tried different cables and plugs?
Have you tried charging without the SIM card(s) (often the culprit for heat and drain)?
Hi Paula,
Yes, I’ve tried everything from the battery guide.
No - maybe that’s an idea. But they worked before? Should that make a difference?
No - I just ordered a new today so let’s see in 5-8 workdays!
Thank you 
Did you try setting USB debugging to on permanently in the developer options menu? This solved my problem.
Hi! I am sorry, I am not sure what you mean - but it sounds promising.
What is the “developers options menu”?
Go to Settings -> About phone and tap on the build number 10 times. Then the developer options will appear in settings.
Another thing that I just discovered: cable quality can cause charging misbehavior. Check this link for more info.
Wow, what? How does that work? Is there a topic where this is explained in more detail?