Initial code

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});
+}