Reading /sys/class/power_supply/BAT0/status takes a tenth of a second.

user0984379 · 2025-08-16 22:00:14

I am currently writing a script that reads the contents of /sys/class/power_supply/BAT0/status and puts it into a variable.

When the battery is charging and the output of /sys/class/power_supply/BAT0/status is either "Full" or "Charging" the time to complete the operation is normal.
However, when the battery is discharging and you read from the file /sys/class/power_supply/BAT0/status ; the operation takes an unusually long time to complete. (Especially considering how simple the operation is.)

Output:

$ echo $battery_status

$ time { battery_status=$(< /sys/class/power_supply/BAT0/status); }

real    0m0.000s
user    0m0.000s
sys     0m0.000s
$ echo $battery_status
Full
$ time { battery_status=$(< /sys/class/power_supply/BAT0/status); }

real    0m0.103s
user    0m0.001s
sys     0m0.000s
$ echo $battery_status
Discharging

I've also tried just catting the file by itself without using command substitution and the results are still the same.
Any ideas on why this is occurring?

Succulent of your garden · 2025-08-17 01:11:45

Could be many things. Sometimes by hardware if you disconnect the battery charger the system starts to works in a battery saving performance. As you can see here https://wiki.archlinux.org/title/Laptop#Battery_state , here https://wiki.archlinux.org/title/Power_management and here https://wiki.archlinux.org/title/CPU_frequency_scaling you can do some tweakings.

You can check the arch wiki if your notebook/laptop does have something. For example here is the asus one: https://wiki.archlinux.org/title/Laptop/ASUS where you can see common errors and among other things. Try to find if your device is in the arch wiki.

Maybe if your are using #!/bin/bash instead of #!/bin/sh that does some impact. To be honest I'm not so sure if you have by default shell bash this would affect or not. But it is a topic where you can earn some improvement. Bash is the slowest shell in Linux. But I don't think this creates your main issue, just a performance tweak that you can do.

If you are interested in only know the percentage of your battery, here is what I do:

#!/bin/sh
output="$(upower -i /org/freedesktop/UPower/devices/battery_BAT1 | grep percentage | awk '{print $2}')"
echo "^c#FFE7C7^ ${output}"

This scripts works among others, which let's me know the current temperature of the system among other things. Does this works faster ? maybe you should change the "battery_BAT1" for the one that your system have. I think sometimes that changes in the path.

Last edited by Succulent of your garden (2025-08-17 01:12:36)

seth · 2025-08-17 09:37:57

Especially considering how simple the operation is.

sysfs isn't like cat'ting files, you're polling a device through a kernel module.
Check whether there're dmesg errors but most likely the battery is just slow to respond?

time acpi

Arch Linux

#1 2025-08-16 22:00:14

Reading /sys/class/power_supply/BAT0/status takes a tenth of a second.

#2 2025-08-17 01:11:45

Re: Reading /sys/class/power_supply/BAT0/status takes a tenth of a second.

#3 2025-08-17 09:37:57

Re: Reading /sys/class/power_supply/BAT0/status takes a tenth of a second.

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