awesome_conf/battery/init.lua

local wibox = require("wibox")
local awful = require("awful")

battery_widget = wibox.widget.textbox()
battery_widget:set_align("right")

function update_battery(widget)
  local cmd = "acpi | awk '{if($1 == \"Battery\"){print substr( $4, 1, length($4) - 1 )}}'"
  local fd = io.popen(cmd)
  local status = fd:read("*l")
  fd:close()
  widget:set_markup("B: "..status)
end

update_battery(battery_widget)

mytimer = timer({ timeout = 10 })
mytimer:connect_signal("timeout", function () update_battery(battery_widget) end)
mytimer:start()