Initial code

.gitignore

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+/.zig-cache
+/zig-out

build.zig

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+const std = @import("std");
+
+pub fn build(b: *std.Build) void {
+	const target = b.standardTargetOptions(.{});
+	const optimize = b.standardOptimizeOption(.{});
+
+	const exe = b.addExecutable(.{
+		.name = "powerbottom",
+		.root_source_file = b.path("src/main.zig"),
+		.target = target,
+		.optimize = optimize,
+	});
+
+	b.installArtifact(exe);
+	const run_cmd = b.addRunArtifact(exe);
+	run_cmd.step.dependOn(b.getInstallStep());
+
+	// This allows the user to pass arguments to the application in the build
+	// command itself, like this: `zig build run -- arg1 arg2 etc`
+	if (b.args) |args| {
+		run_cmd.addArgs(args);
+	}
+
+	// This creates a build step. It will be visible in the `zig build --help` menu,
+	// and can be selected like this: `zig build run`
+	// This will evaluate the `run` step rather than the default, which is "install".
+	const run_step = b.step("run", "Run the app");
+	run_step.dependOn(&run_cmd.step);
+}

build.zig.zon

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+.{
+    // This is the default name used by packages depending on this one. For
+    // example, when a user runs `zig fetch --save <url>`, this field is used
+    // as the key in the `dependencies` table. Although the user can choose a
+    // different name, most users will stick with this provided value.
+    //
+    // It is redundant to include "zig" in this name because it is already
+    // within the Zig package namespace.
+    .name = "powerbottom",
+
+    // This is a [Semantic Version](https://semver.org/).
+    // In a future version of Zig it will be used for package deduplication.
+    .version = "0.0.0",
+
+    // This field is optional.
+    // This is currently advisory only; Zig does not yet do anything
+    // with this value.
+    //.minimum_zig_version = "0.11.0",
+
+    // This field is optional.
+    // Each dependency must either provide a `url` and `hash`, or a `path`.
+    // `zig build --fetch` can be used to fetch all dependencies of a package, recursively.
+    // Once all dependencies are fetched, `zig build` no longer requires
+    // internet connectivity.
+    .dependencies = .{
+        // See `zig fetch --save <url>` for a command-line interface for adding dependencies.
+        //.example = .{
+        //    // When updating this field to a new URL, be sure to delete the corresponding
+        //    // `hash`, otherwise you are communicating that you expect to find the old hash at
+        //    // the new URL.
+        //    .url = "https://example.com/foo.tar.gz",
+        //
+        //    // This is computed from the file contents of the directory of files that is
+        //    // obtained after fetching `url` and applying the inclusion rules given by
+        //    // `paths`.
+        //    //
+        //    // This field is the source of truth; packages do not come from a `url`; they
+        //    // come from a `hash`. `url` is just one of many possible mirrors for how to
+        //    // obtain a package matching this `hash`.
+        //    //
+        //    // Uses the [multihash](https://multiformats.io/multihash/) format.
+        //    .hash = "...",
+        //
+        //    // When this is provided, the package is found in a directory relative to the
+        //    // build root. In this case the package's hash is irrelevant and therefore not
+        //    // computed. This field and `url` are mutually exclusive.
+        //    .path = "foo",
+
+        //    // When this is set to `true`, a package is declared to be lazily
+        //    // fetched. This makes the dependency only get fetched if it is
+        //    // actually used.
+        //    .lazy = false,
+        //},
+    },
+
+    // Specifies the set of files and directories that are included in this package.
+    // Only files and directories listed here are included in the `hash` that
+    // is computed for this package. Only files listed here will remain on disk
+    // when using the zig package manager. As a rule of thumb, one should list
+    // files required for compilation plus any license(s).
+    // Paths are relative to the build root. Use the empty string (`""`) to refer to
+    // the build root itself.
+    // A directory listed here means that all files within, recursively, are included.
+    .paths = .{
+        "build.zig",
+        "build.zig.zon",
+        "src",
+        // For example...
+        //"LICENSE",
+        //"README.md",
+    },
+}

src/batteryinfo.zig

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+const std = @import("std");
+
+const _maxRead = 4096;
+
+pub const BatteryInfo = struct {
+	Manufacturer: []u8,
+	Model: []u8,
+	Technology: []u8,
+	CellSeriesCount: u32 = 0,
+	Capacity: u32 = 0,
+	CapacityDesign: u32 = 0,
+	Vnow: f32 = 0,
+	Vmin: f32 = 0,
+	ChargeNow: u32 = 0,
+	ChargeFull: u32 = 0,
+	ChargeFullPercentageDesign: u32 = 0,
+	ChargeFullDesign: u32 = 0,
+	_allocator: std.mem.Allocator,
+
+	pub fn deinit(self: *const BatteryInfo) void {
+		self._allocator.free(self.Manufacturer);
+		self._allocator.free(self.Model);
+		self._allocator.free(self.Technology);
+	}
+};
+
+fn _readInfo(allocator: std.mem.Allocator, dir: std.fs.Dir, path: []const u8) ![]u8 {
+	const start = try dir.readFileAlloc(allocator, path, _maxRead);
+        defer allocator.free(start);
+        const trimmed = std.mem.trim(u8, start, "\r\n\t ");
+        return allocator.dupe(u8, trimmed);
+}
+
+fn _readInfoU32(allocator: std.mem.Allocator, dir: std.fs.Dir, path: []const u8) !u32 {
+	const info = try _readInfo(allocator, dir, path);
+	defer allocator.free(info);
+	return std.fmt.parseInt(u32, info, 10);
+}
+
+pub fn GetBatteryInfoAlloc(allocator: std.mem.Allocator, dir: std.fs.Dir) !BatteryInfo {
+	const manufacturer = try _readInfo(allocator, dir, "manufacturer");
+	const model = try _readInfo(allocator, dir, "model_name");
+	const technology = try _readInfo(allocator, dir, "technology");
+
+	var info = BatteryInfo{
+		.Manufacturer = manufacturer,
+		.Model = model,
+		.Technology = technology,
+		._allocator = allocator,
+	};
+
+	info.Capacity = try _readInfoU32(allocator, dir, "capacity");
+	info.ChargeNow = try _readInfoU32(allocator, dir, "energy_now") / 1000;
+	info.ChargeFull = try _readInfoU32(allocator, dir, "energy_full") / 1000;
+	info.ChargeFullDesign = try _readInfoU32(allocator, dir, "energy_full_design") / 1000;
+
+	if(info.ChargeFullDesign != 0) {
+		info.ChargeFullPercentageDesign = 100 * info.ChargeFull / info.ChargeFullDesign;
+		info.CapacityDesign = 100 * info.ChargeNow / info.ChargeFullDesign;
+	}
+
+	const Vnow = try _readInfoU32(allocator, dir, "voltage_now") / 1000;
+	const Vmin = try _readInfoU32(allocator, dir, "voltage_min_design") / 1000;
+
+	info.Vnow = @as(f32, @floatFromInt(Vnow)) / 1000;
+	info.Vmin = @as(f32, @floatFromInt(Vmin)) / 1000;
+
+	info.CellSeriesCount = @intFromFloat(@ceil(info.Vmin / 3.7));
+
+	return info;
+}

src/main.zig

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+const std = @import("std");
+const batinfo = @import("batteryinfo.zig");
+
+pub fn main() !void {
+	var buffer: [65535]u8 = undefined;
+	var fba = std.heap.FixedBufferAllocator.init(&buffer);
+	const allocator = fba.allocator();
+
+	const powerSupply = try std.fs.cwd().openDir("/sys/class/power_supply", .{});
+	var i:u32 = 0;
+	while(true) : (i += 1) {
+		const name = try std.fmt.allocPrint(allocator, "BAT{}", .{i});
+		defer allocator.free(name);
+		const batDir = powerSupply.openDir(name, .{}) catch |err| switch(err) {
+			error.FileNotFound => {return;},
+			else => {return err;},
+		};
+		const battery = try batinfo.GetBatteryInfoAlloc(allocator, batDir);
+		defer battery.deinit();
+		try printBattery(battery);
+	}
+}
+
+fn printBattery(battery: batinfo.BatteryInfo) !void {
+	const stdout = std.io.getStdOut().writer();
+	try stdout.print("Battery {s} {s}\n", .{battery.Manufacturer, battery.Model});
+	try stdout.print("\tTechnology:           {s}\n", .{battery.Technology});
+	try stdout.print("\tCell series count:    {}s\n\n", .{battery.CellSeriesCount});
+	try stdout.print("\tCapacity:             {}%\n", .{battery.Capacity});
+	try stdout.print("\tCapacity (design):    {}%\n", .{battery.CapacityDesign});
+	try stdout.print("\tVmin:                 {d:.2}V  Per cell: {d:.2}V\n", .{battery.Vmin, battery.Vmin/@as(f32, @floatFromInt(battery.CellSeriesCount))});
+	try stdout.print("\tVnow:                 {d:.2}V  Per cell: {d:.2}V\n", .{battery.Vnow, battery.Vnow/@as(f32, @floatFromInt(battery.CellSeriesCount))});
+	try stdout.print("\tCharge now:           {}mAh\n", .{battery.ChargeNow});
+	try stdout.print("\tCharge full:          {}mAh\n", .{battery.ChargeFull});
+	try stdout.print("\tCharge full (design): {}mAh\n\n", .{battery.ChargeFullDesign});
+}