Battery

Battery

Print battery information

Module type	battery
Default order	44 (only used by --gen-config)
Module source	src/modules/battery/battery.c
Detection source	src/detection/battery/

Prints every battery the system reports, plus the AC/USB/wireless power source state, the remaining time and (optionally) the battery temperature. One line is printed per battery.

The default key is Battery (<model name>), or plain Battery when the model name is unknown. A custom key is itself a format string and may use {index}, {name}, {icon} and {module-name}; {index} is 0 for a single battery and 1, 2, … when several batteries are present.

Default output looks like:

Battery (LION-4C20): 100% [AC Connected] - 31.2°C

Platform support

Platform	Implementation	Notes
Linux	battery_linux.c	/sys/class/power_supply/* (sysfs)
Android	battery_android.c	/sys/class/power_supply/* plus Android-specific sysfs nodes
macOS	battery_apple.c	IOKit AppleSmartBattery registry entry (+ SMC for temperature)
Windows	battery_windows.c	WMI battery classes (batclass.h)
FreeBSD / MidnightBSD / DragonFly	battery_bsd.c	hw.acpi.battery.units sysctl + ACPI_IOCTL_BATTERY ioctl on /dev/acpi
NetBSD	battery_nbsd.c	/dev/sysmon + proplib acpibat* dictionaries
OpenBSD	battery_obsd.c	APM ioctl on /dev/apm
Solaris / illumos	battery_sunos.c	kstat
Haiku	battery_haiku.c	/dev/power/acpi_battery/ + acpi_battery_info ioctl
GNU/Hurd	battery_nosupport.c	Reports Not supported on this platform

Configuration

Key	Type	Default	Description
temp	boolean | object	false	Detect and display the battery temperature. Also accepts a color-range object ({ "green": 60, "yellow": 80 }) to color the value.
percent	object	{ "green": 50, "yellow": 20, "type": 0 }	Color thresholds for the capacity output.
key	string	module name	Module key. A single space hides it.
keyColor	color	–	Overrides display.color.keys.
keyWidth	integer	–	Overrides display.key.width.
keyIcon	string	built-in glyph	The icon printed when display.key.type includes the icon bit. Set it to any glyph you like, or to "" to print none.
outputColor	color	–	Overrides display.color.output.
format	string	–	Custom output format (see below).
condition	object	–	Show the module only if the conditions match.

The default percent uses green: 50 > yellow: 20, i.e. the inverted interpretation: 50–100 % is green, 20–50 % is yellow and 0–20 % is red. The exact semantics of the two thresholds are documented in Configuration.

Format string

Run fastfetch -h battery-format for the authoritative list.

Variable	Description
{manufacturer}	Battery manufacturer
{name}	Battery model name (also available in the key format)
{technology}	Battery technology (e.g. Lithium)
{capacity}	Capacity percentage, formatted as a number
{capacity-bar}	Capacity percentage, formatted as a bar
{status}	Status list, e.g. AC Connected, Charging
{temperature}	Temperature, formatted with the display.temp settings
{cycle-count}	Cycle count
{serial}	Serial number
{manufacture-date}	Manufacture date, YYYY-MM-DD
{time-days} / {time-hours} / {time-minutes} / {time-seconds}	Remaining time, split into components
{time-formatted}	Remaining time, formatted by display.duration

JSON output

{
    "type": "battery",
    "result": [
        {
            "capacity": 100.0,
            "manufacturer": "Apple Inc.",
            "manufactureDate": "",
            "modelName": "LION-4C20",
            "technology": "Lithium",
            "serial": "0123456789ABCDEF",
            "temperature": 31.15,
            "cycleCount": 42,
            "timeRemaining": null,
            "status": ["AC Connected"]
        }
    ]
}

temperature and timeRemaining are null when unknown.

Examples

{
    "type": "battery",
    "key": "Battery",
    "temp": true,
    "format": "{capacity} {status} ({time-formatted})"
}

{
    "type": "battery",
    "format": "{name}: {capacity-bar} {capacity} - {temperature}",
    "percent": { "type": 3, "green": 30, "yellow": 15 },
    "temp": { "green": 45, "yellow": 55 }
}

Pitfalls

• {temperature} is empty unless temp is true. Temperature detection is opt-in on every platform, because it either costs an extra SMC/kstat round-trip or needs privileges.
• {time-formatted} / {time-*} are only filled while discharging. While charging or on AC the remaining time is unknown (-1) and the variables expand to 0.
• Reading capacity can be slow on some Linux laptops — the source carries an explicit "this is expensive" note; the value is read once per battery per run.
• The percentage is not the "battery health". It is current / max, so a worn battery still reports 100 % when fully charged.
• OpenBSD reports very little. {manufacturer}, {name} and {technology} are always empty and temp has no effect.
• Multiple batteries print multiple lines. The default key does not contain an index, so two identical models produce two identical keys. Use a custom key such as "Battery {index}" to tell them apart.
• AC Connected is a status flag, not a separate row. On Linux it is inferred from the Mains power supply, and it is applied to all detected batteries.
• The default percent thresholds are inverted (green > yellow): high capacity is green, low capacity is red. Setting green below yellow flips the meaning to "low value is good".

Implementation

Linux and Android

Enumerates /sys/class/power_supply/ with readdir + openat(O_PATH | O_DIRECTORY) so the directory handle stays valid while reading. For each entry the following files are read relative to that directory handle:

• type — must be Battery; an entry of type Mains is not a battery, but its online file is used to mark every battery as AC Connected.
• present — skipped if 0.
• scope — skipped if it is Device (these are USB gadget/battery-charger devices, not batteries).
• capacity — the only mandatory file; the entry is discarded if it cannot be read.
• manufacturer, model_name, technology, serial_number, capacity_level, cycle_count, status, manufacture_{year,month,day}.
• temp — only read when temp is true; the sysfs value is in tenths of a degree, so it is divided by 10.

status maps to flags: Discharging / Charging / Unknown; capacity_level: Critical adds Critical. Remaining time is taken from time_to_empty_now when present, otherwise it is derived from charge_now * 3600 / |current_now|.

An Asahi-Linux machine exposes the battery as macsmc-battery, which has no manufacturer file, so Apple Inc. is hard-coded for that ID.

The sysfs ABI is documented at https://www.kernel.org/doc/Documentation/ABI/testing/sysfs-class-power.

macOS

IOServiceGetMatchingServices(IOServiceMatching("AppleSmartBattery")) followed by IORegistryEntryCreateCFProperties for every matching registry entry:

• MaxCapacity / CurrentCapacity → capacity (as a percentage).
• DeviceName, Serial, Manufacturer, CycleCount.
• ExternalConnected → AC Connected, otherwise Discharging plus AvgTimeToEmpty (minutes → seconds; 0xFFFF and negative values mean "unknown").
• IsCharging → Charging, AtCriticalLevel → Critical.
• built-in → fills in Apple Inc. / Lithium / Built-in when the keys are missing.
• ManufactureDate is a packed SBDS value (5 bits day, 4 bits month, 7 bits year since 1800). Apple Silicon instead stores it as a string inside the BatteryData dictionary, which is parsed as YY MM DD with an 8-year offset.
• Temperature comes from kIOPMPSBatteryTemperatureKey (tenths of Kelvin) and falls back to the SMC TB0T-style sensor via ffDetectSmcTemps().

Windows

Enumerates battery devices and queries the four WMI battery classes declared in batclass.h: BATTERY_STATIC_DATA_WMI_GUID (manufacturer, model, serial, chemistry, manufacture date), BATTERY_STATUS_WMI_GUID (power state, remaining capacity, charge rate), BATTERY_RUNTIME_WMI_GUID (EstimatedRuntime) and BATTERY_FULL_CHARGED_CAPACITY_WMI_GUID (full-charged capacity, used as the percentage base). Sentinel values such as BATTERY_UNKNOWN_CAPACITY / BATTERY_UNKNOWN_TIME are treated as "unknown" rather than converted.

BSD family

• FreeBSD: sysctlbyname("hw.acpi.battery.units") gives the count, then ioctl(fd, ACPI_IOCTL_BATTERY, &battio) on /dev/acpi yields battinfo (capacity, state, minutes left) and bif/bix (OEM info, model, type, serial, cycle count). bix is preferred over bif when the ACPI extended battery info is available.
• NetBSD: opens /dev/sysmon and walks the proplib device tree, keeping only keys that start with acpibat. Capacity is charge / capacity * 100; remaining time is charge / discharge_rate * 3600.
• OpenBSD: ioctl(fd, APM_IOC_GETPOWER, &info) on /dev/apm. Only capacity, state and minutes left are available — manufacturer, model and technology are always empty, and the temperature is never reported.

Solaris / illumos / Haiku

Solaris reads the battery kstat module through libkstat. Haiku opens /dev/power/acpi_battery/, calls the acpi_battery_info and acpi_extended_battery_info ioctls and computes capacity = basic.capacity * 100 / extended.last_full_charge.