Guide to charge limiting your Fairphone 5

ChromiumOS-Guy

I AM NOT RESPONSIBLE FOR ANY DAMAGE WHATSOEVER TO THE EXTENT LAW PERMITS.

EDIT: there is a edge case that lets the battery continue charging it seems easy to fix so I'll do so tomorrow, for now just use slow chargers. (or make sure to unplug/replug the phone after it stops charging from there it will work fine)

I'll start by saying I have no idea if this works for Focal but I haven't tried so it may work.
also this is a proof of concept you may want to refine this by using a udev rule or whatever else this is just how I got it working the first time so its a known good.

essentially there is something called user_fcc my take is that the fcc part is "full charge capacity" you can essentially tell the device that it actually has a smaller/bigger battery with this, I did tests on this however and either its using different units then charge_full (which is how you get the battery capacity) or its something else entirely, the point is that if set to 0 battery stops charging if set to big number like 10^6 battery charges again.

so... heres my setup I did for the proof of concept.

first start by running:

sudo mount -o remount,rw /

this is to remount read-only root as read/write.

make a file and name it charge-control.sh and put it in the /usr/bin directory ( /usr/bin/charge-control.sh)
change THRESHOLD to whatever percent you want to stop charging at, I put it on 80%

#!/usr/bin/env bash

# Path to capacity file (adjust if needed)
CAP_FILE="/sys/class/power_supply/battery/capacity"
UFCC_FILE="/sys/class/power_supply/battery/user_fcc"
FULL_CAP_FILE="/sys/class/power_supply/battery/charge_full"

# Threshold to compare against (integer 0-100)
THRESHOLD=80

# Commands to run
ON_GE="echo 0 > ${UFCC_FILE}" #echo Battery >= threshold: do X here
ON_LT="echo `cat ${FULL_CAP_FILE}` > ${UFCC_FILE}" #echo Battery < threshold: do Y here

# Trim whitespace and ensure integer
capacity=$(tr -d ' \t\n\r' < "$CAP_FILE")
if ! [[ $capacity =~ ^[0-9]+$ ]]; then
  echo "Error: capacity value is not an integer: '$capacity'" >&2
  exit 3
fi

# Compare as integers
if (( capacity >= THRESHOLD )); then
  eval "$ON_GE"
else
  eval "$ON_LT"
fi

exit 0

make sure you set executable permissions sudo chmod +x /usr/bin/charge-control.sh

now we make a system service to start this script, make a file called charge-control.service at /etc/systemd/system (/etc/systemd/system/charge-control.service)
and put this into it

[Unit]
Description=Battery charge control script

[Service]
Type=oneshot
ExecStart=/usr/bin/charge_control.sh
User=root

now we create a timer to run this every minute because we want it to enable/disable battery charging without us running the service manually.
so create a file named charge-control.timer in the same directory (/etc/systemd/system/charge-control.timer)
and put this into it

[Unit]
Description=Run charge_control every 1 minute

[Timer]
OnBootSec=1min
OnUnitActiveSec=1min
Persistent=false

[Install]
WantedBy=timers.target

then enable the timer by running these commands:

sudo systemctl daemon-reload
sudo systemctl enable --now charge_control.timer

you may also check that it is working with these commands if you want

sudo systemctl status charge_control.timer
sudo systemctl list-timers --all | grep charge_control
sudo journalctl -u charge_control.service

hope this helps y'all manage you're battery health better.

gpatel-fr

@ChromiumOS-Guy

thanks, may I ask why you are remounting the root fs r/w ? It's possible to create a systemd unit without it.

ChromiumOS-Guy

@gpatel-fr don't want the bash script in the user directory.
this is because I mess with the user directory a lot, I didn't change it for the guide because this is a proof of concept and if I have a known good I will use it.

I opened an Issue on this for indicator-upower fork, paulcarroty (the developer) is aware.
and I am mostly counting on him to make this into something useful because he already has an app for it.

gpatel-fr

@ChromiumOS-Guy

I have looked at the kernel sources, the 'user_fcc' is a limit value for the charging current (in micro amperes). If you set it at 0, well, current_now goes to 0 (logical) and the battery charging indicator disappears. If you set it to infinity, well, the result is unclear to me. It would seem logical that the maximum amount that can be tolerated by the battery is set if user_fcc is higher.
Since the user_fcc value is aimed at setting an artificial low value when people are connecting a high powered charged (fast charger), I'd be wary of setting a very high value myself. It could work or maybe have unfortunate consequences. Setting it to 0 seems safe though.