Laptops and Mobile Devices

How laptops differ from desktops

A laptop (notebook) is a portable computer that combines the screen, keyboard, pointing device, battery and all the components of a desktop in one folding case. The parts do the same jobs as in a desktop — see What's Inside a Computer — but three constraints shape everything inside:

• Integration. There is no room for standard expansion cards or large connectors. Components are small, many are soldered to the mainboard, and parts are often specific to one model. A replacement keyboard or screen from a different model rarely fits.
• Power. Everything must run from a battery for hours. Laptop processors are designed for a lower power draw and slow down aggressively to save energy; the screen is usually the largest single consumer.
• Heat. A thin case leaves little room for cooling. Heat pipes carry heat from the central processing unit (CPU) and graphics processor to a small fan and heat sink at the edge. When the vents clog with dust, the processor throttles — it slows down to protect itself — and the laptop feels slow and loud.

Tablets and smartphones take this further: almost everything is integrated into one system on a chip (SoC), the battery is glued in, and the only parts a technician normally replaces are the screen assembly, the battery and sometimes a port or camera.

Laptop components: what can be replaced

What can be upgraded or replaced differs per model, so the first step is always the manufacturer's documentation. As a rule of thumb:

Working inside a laptop

Laptop repairs fail more often from the process than from the replacement part. Good practice:

• Get the service manual. Many manufacturers publish service manuals or repair guides per model, with the order of disassembly, the screw types and the part numbers. Without it you risk breaking hidden clips.
• Document as you go. Take photos before you disconnect each cable, and note how cables are routed.
• Organize the screws. Screws of different lengths look alike. Keep them in the order they came out — on a magnetic mat or a tray with compartments, or taped to a sketch. A screw that is too long, driven into the wrong hole, can pierce the mainboard or the battery underneath.
• Disconnect the battery first, after shutting the laptop down fully (not sleep or hibernation), and unplug the power adapter. The battery powers the board even when the laptop seems off.
• Prevent electrostatic discharge (ESD). Use an antistatic wrist strap and mat; see Safety and Handling.
• Use the right tools. Plastic spudgers and opening picks for clips and cases; the correct screwdriver bit (small Phillips and Torx are common) to avoid stripping heads. Flat ribbon cables often sit in zero insertion force (ZIF) connectors: lift the small latch first, then the cable slides out without force.

Batteries

Laptops, tablets and phones run on a lithium-ion battery (Li-ion): a rechargeable battery in which lithium ions move between the two electrodes as it charges and discharges. It stores a lot of energy for its weight and has no "memory effect", which is why it replaced older nickel-based batteries. Many devices use the lithium-ion polymer variant in a flat pouch, which fits thin cases.

Capacity: watt-hours versus milliampere-hours

Battery capacity is given in two ways. Milliampere-hours (mAh) say how much charge the battery holds, but not at which voltage. The energy — what actually decides how long a device runs — is in watt-hours (Wh):

Wh = mAh × V / 1000

A single lithium-ion cell has a nominal voltage of about 3.6–3.85 volts (V); laptop batteries put three or four cells in series for around 11–15 V. That is why a laptop battery with about the same mAh number as a phone holds about three times the energy:

def watt_hours(mah, volts):
    """Energy in watt-hours from capacity in milliampere-hours and voltage."""
    return mah * volts / 1000

batteries = [
    ("Phone", 4_500, 3.87),
    ("Tablet", 8_000, 3.85),
    ("Laptop", 4_800, 11.55),
    ("Power bank", 20_000, 3.7),
]
for name, mah, volts in batteries:
    wh = watt_hours(mah, volts)
    print(f"{name:<11}{mah:>7,} mAh at {volts:5.2f} V = {wh:5.1f} Wh")

bank_wh = watt_hours(20_000, 3.7)
print(f"Power bank at 5 V USB output: at most {bank_wh / 5 * 1000:,.0f} mAh, before losses")

laptop_wh = watt_hours(4_800, 11.55)
print(f"Laptop at an average 9 W: about {laptop_wh / 9:.1f} hours")

Phone        4,500 mAh at  3.87 V =  17.4 Wh
Tablet       8,000 mAh at  3.85 V =  30.8 Wh
Laptop       4,800 mAh at 11.55 V =  55.4 Wh
Power bank  20,000 mAh at  3.70 V =  74.0 Wh
Power bank at 5 V USB output: at most 14,800 mAh, before losses
Laptop at an average 9 W: about 6.2 hours

Two practical consequences. A power bank's mAh rating is for its internal cells at about 3.7 V, so it delivers far fewer mAh at the 5 V of a Universal Serial Bus (USB) port — and conversion losses take a bit more. And airline rules are written in watt-hours: under the widely used International Air Transport Association (IATA) rules, batteries up to 100 Wh are allowed; 100–160 Wh need airline approval; over 160 Wh are forbidden for passengers. Spare batteries and power banks must always be in carry-on, never in checked baggage, and many airlines now restrict using power banks in flight. The Wh value is printed on the battery.

Charging and battery health

A lithium-ion battery charges fast at first and slows down near the end: the charger first delivers a constant current, then holds a constant voltage while the current tapers off. That is why the last 20 percent takes relatively long. The battery management circuit in the device controls this; it also stops charging when the battery is full, so leaving a laptop plugged in does not "overcharge" it.

Every battery wears. Capacity drops with the number of charge cycles and with age, and faster when the battery is kept hot or at 100 percent for long periods. Many laptops and phones therefore offer a battery care setting that stops charging at around 80 percent for devices that are almost always plugged in. The operating system (OS) can tell you how worn a battery is — on Windows, powercfg /batteryreport writes a report as a web page with the design capacity and the current full charge capacity; on Linux, upower shows the same (illustrative output):

upower -i /org/freedesktop/UPower/devices/battery_BAT0

  native-path:          BAT0
  power supply:         yes
…
  battery
    present:             yes
    rechargeable:        yes
    state:               discharging
    energy:              38.52 Wh
    energy-full:         44.1 Wh
    energy-full-design:  57 Wh
    energy-rate:         9.84 W
    voltage:             11.9 V
    time to empty:       3.9 hours
    percentage:          87%
    capacity:            77.3684%
    technology:          lithium-ion

energy-full-design is what the battery held when new, energy-full what it holds now. Their ratio is the battery's health:

design_wh = 57.0   # energy-full-design
full_wh = 44.1     # energy-full: what a full charge holds today

health = full_wh / design_wh
print(f"Battery health: {health:.0%} of design capacity")
print(f"Lost: {design_wh - full_wh:.1f} Wh")

Battery health: 77% of design capacity
Lost: 12.9 Wh

A battery at 77 percent still works, but the user gets about three quarters of the original runtime. Many organizations replace batteries when health drops below a threshold they set, often around 80 percent or when the OS reports the battery needs service.

Swollen batteries

An aging or damaged lithium-ion battery can produce gas inside its pouch and swell. Signs: a bottom cover that bulges or no longer sits flat, a touchpad that lifts or no longer clicks, a phone screen that separates from its frame. A swollen battery can catch fire if it is punctured, bent or charged further.

Docking stations and port replicators

At a desk, a laptop user wants a large monitor, a full keyboard and mouse, wired networking and power — ideally without plugging in five cables every morning. Two kinds of devices provide that:

• A port replicator simply copies the laptop's own ports to a box on the desk: the cables stay connected to the replicator, and the laptop connects to it with one connector. It adds no new functions.
• A docking station does that and adds functions the laptop does not have on its own: charging, extra display outputs, a wired network port, more USB ports, audio, and in larger models even expansion slots or drive bays.

Older business laptops had a proprietary docking connector on the bottom. Today most docks connect through one USB-C or Thunderbolt cable (see Ports, Connectors and Cables), which carries data, video and power at once, so the distinction between the two terms has blurred. When choosing a dock, check that its charging power in watts meets what the laptop needs, and that the laptop's port supports the number and resolution of monitors the user wants — a dock cannot add display capability the laptop's port does not provide.

Laptop displays and touch

The display assembly in the lid holds more than the screen. Next to the panel — liquid crystal display (LCD) with a light-emitting diode (LED) backlight, or organic LED (OLED); see Displays — it usually contains the webcam, microphones and the Wi-Fi antennas. The antennas sit in the lid because it is the highest point when the laptop is open, away from the metal of the base. That is why Wi-Fi can become weak after a screen replacement when an antenna cable was pinched or left disconnected. Very old LCD panels were lit by a cold cathode fluorescent lamp (CCFL) that needed an inverter board for its high voltage; LED backlights do not.

A touchscreen adds a digitizer: the transparent layer that detects touch and converts it into coordinates for the operating system. Most are capacitive: they sense the small change in an electric field when a finger comes close, which is why they do not react to ordinary gloves. Devices with an active pen use an additional digitizer that tracks the pen's position and pressure. In phones and tablets the digitizer is often laminated to the cover glass and the display panel, so a cracked glass means replacing the whole screen assembly. A digitizer that does not respond in one area, or registers touches by itself ("ghost touches"), needs calibration or replacement.

Smartphones, tablets and connectivity

Smartphones and tablets are small, sealed computers running a mobile OS. Nearly all use processors based on the ARM architecture (see CPUs), integrated with graphics, memory control and radios in one SoC. Storage is soldered, so the capacity chosen at purchase is final, apart from a microSD slot on some models. Wearables such as smartwatches and fitness trackers are even smaller and depend on a paired phone for much of their function.

Connectivity

• Cellular: the mobile network of a carrier. 4G (fourth generation, Long Term Evolution or LTE) and 5G (fifth generation) provide data and, on phones, calls. The subscriber is identified by a subscriber identity module (SIM): a small card, or an eSIM built into the device and provisioned by downloading a profile.
• Wi-Fi: the local wireless network; see Wireless Networking.
• Bluetooth: headsets, watches, car kits and keyboards; pairing works as described on Peripherals.
• NFC: see below.
• Location: satellite navigation such as the Global Positioning System (GPS), combined with Wi-Fi and cellular signals.

Near-field communication (NFC) is a wireless technology with a range of only a few centimeters. Two devices must almost touch, which is exactly the point: it is used for contactless payments, access badges, transit cards and tap-to-pair with Bluetooth accessories. The very short range makes it hard to eavesdrop on or to trigger by accident.

A phone can share its cellular data connection with other devices: as a Wi-Fi hotspot, or by tethering over USB or Bluetooth. Hotspot use drains the battery quickly, and some mobile plans limit it. Airplane mode switches off all radios at once; Wi-Fi and Bluetooth can then be turned back on individually. A surprising number of "no internet" tickets from mobile users end with airplane mode.

Accessories and synchronization

Common accessories are chargers and cables (mostly USB-C with USB Power Delivery for fast charging), wireless chargers, protective cases and screen protectors, styluses, keyboard covers for tablets, headsets, and external battery packs. For chargers, use the manufacturer's or a certified model: a cheap charger may be slow, or unsafe.

Synchronization keeps email, contacts, calendars, photos and files the same on all of a user's devices. Today that almost always happens through cloud accounts — a personal account or the organization's email and collaboration service — instead of by connecting the phone to a computer. When a user says contacts or mail "disappeared" from a new phone, check first that the right account was added and that sync is enabled for that data type.

Organizations manage company phones and tablets with mobile device management (MDM): devices are enrolled in a central system that pushes settings, Wi-Fi profiles, email accounts and apps, enforces a passcode and encryption, and can wipe a lost device remotely. The security side is covered in the Security topic.

Common laptop problems

No display

First rule out the simple causes: brightness turned all the way down, the laptop asleep, or a display-switching key combination (a function key with a monitor symbol, or Win+P on Windows) that sent the picture to a projector. Then connect an external monitor:

• External works, internal stays black: the graphics are fine; the problem is the internal panel, its cable or its backlight.
• Internal image very dim but visible when you shine a flashlight on it: the backlight has failed (on very old models, the inverter).
• Nothing on either screen, while fans and lights come on: suspect the mainboard, memory or firmware; see Hardware Troubleshooting.

Battery not charging

• Check the adapter: its light, the cable, the connector, and the wattage. A USB-C charger with too little power charges very slowly or not at all under load; many laptops warn about a "slow charger".
• Check for a battery care or charge-limit setting that stops charging at, for example, 80 percent — it is working as designed.
• Check the battery health report; a worn-out battery may refuse to charge or drop from 30 percent to zero suddenly.
• Try another outlet and, if possible, a known-good adapter of the same model before replacing the battery or the charging port.

Overheating

Symptoms: a loud fan, a hot base, sudden slowdowns (thermal throttling), or shutdowns under load. Common causes are dust in the fan and heat sink, and use on a bed or cushion that blocks the vents. Clean the vents with compressed air in short bursts — hold the fan blades still so they do not overspin — and advise a hard, flat surface. On older laptops, replacing dried-out thermal paste between the processor and the heat sink can help; check a background process at 100 percent CPU before opening anything.

Keyboard and touchpad issues

• Some keys type numbers: on small laptops without a separate numeric keypad, Num Lock can turn part of the letter keys into a keypad. Toggle Num Lock (often Fn+another key).
• Keys stuck or not working: often dirt or a spill. Test with an external keyboard to separate hardware from software; a liquid spill needs immediate power-off and inspection, because it can reach the mainboard.
• Touchpad does nothing: check the touchpad key or setting before suspecting hardware; see Peripherals.
• No Wi-Fi at all: airplane mode, a function key or physical switch for the wireless radio, or antenna cables after a repair.