Wirebook › Bonus lesson
Why does a big laptop need a big charger?
Plug a laptop into a little phone charger and it grumbles "slow charger". Plug a phone into a huge laptop charger and nothing bad happens at all. That's because a charger isn't a hose that blasts power into whatever you connect: it's more like a café. The gadget reads the menu, orders what it can eat, and the cable decides how big a plate can get through. Here you can plug pretend gadgets into pretend chargers, listen in as they agree on the power, then play Feed the laptop: get eight hungry gadgets charged in time.
Watts: how much power, how fast
A watt measures how fast energy flows. It's volts (how hard the electricity is pushed) times amps (how much of it flows): 20 volts at 3 amps is 60 watts. Batteries are measured in watt-hours, so the sums are friendly: a laptop battery that holds about 50 watt-hours needs about an hour at 50 watts to fill from empty, or five hours at 10 watts. A phone holds around 15 watt-hours, which is why it fills in under an hour from a small brick.
That's the whole reason a big laptop comes with a big charger. A 20 W phone charger plugged into a laptop isn't harmful, it's just a small tap filling a big bath: the laptop says "slow charger", fills very slowly, and if you're busy on it, it might only slow down the battery going down rather than fill it.
The gadget orders from the menu
USB-C chargers use a rulebook called USB Power Delivery (PD). When you plug in, everything starts at a gentle 5 volts. Then the charger sends its menu: "I can do 5, 9, 15 or 20 volts, up to 100 watts." The gadget picks what it can safely take, say "9 volts, 25 watts please" for a phone, and only then does the charger switch up. A gadget only ever gets what it asks for, so a 100 W charger gives a phone exactly the same 25 W as a 25 W charger would.
And it can't overfill the battery either. Inside every phone and laptop is a small battery chip that watches each cell. It slows charging down after about 80% and stops completely at 100%, then the gadget just runs off the charger. Leaving it plugged in overnight is safe; many phones and laptops even have a setting to stop at 80% to keep the battery healthy for longer.
The cable has a say too
Most USB-C cables can carry 3 amps, which is 60 watts at 20 volts. Anything above that needs a 5 A e-marked cable, with a tiny chip in the plug that tells the charger "I can handle 5 amps" (100 W, or 240 W on the newest ones). If the chip isn't there, the charger plays safe and stays at 60 W, which is why a gaming laptop on an ordinary cable can still lose battery while it's plugged in. And a USB-A to USB-C cable (the flat rectangle on one end) has no wire for the menu conversation at all, so it stays at 5 volts: fine for a slow phone charge, but most laptops ignore it.
Where the watts go missing
- What you're doing. A big game can use 75 watts by itself. Power for the game comes first; the battery only gets what's left.
- Hubs, docks and monitors. Charging "through" a USB-C hub or a monitor means the hub keeps some power, often 10 to 20 W, for itself and whatever's plugged into it.
- Two-port chargers. The big number on the box is the total. Plug in a phone too and the charger splits it between the ports.
- The wrong port. Many laptops only charge from one or two of their USB-C ports, marked with a lightning bolt, battery or plug icon.
The real danger: fake chargers, not big ones
Big chargers from proper makers are safe for small gadgets because of that menu conversation. The chargers to worry about are fakes and no-name bargains. Inside a real charger there's a careful gap and extra parts between the mains electricity and the low-voltage side you touch; cheap fakes skip them to save pennies, can't hold their voltage steady, and run hot. A few simple rules:
- Buy chargers and cables from a shop or brand you trust, not "too cheap to be true" bargains. A real charger feels solid, has the safety marks (UKCA or CE in the UK) and the maker's name spelt correctly.
- Stop using any charger that gets too hot to hold, buzzes, crackles, smells or has a loose plug.
- Never charge under a pillow, duvet or blanket. Batteries and chargers make heat while charging and need air.
- If a battery ever looks swollen (a phone screen lifting, a laptop that rocks on the table), stop using and charging it and get it looked at.
So, does a big laptop need a big charger? Yes, to charge quickly, especially while you're using it. Does a small phone mind a big charger? Not at all. It orders 25 W and the battery chip stops it at full.
Related lessons: why phones and computers get hot · why a full battery dies in the cold · why copying to a USB stick is slow.
Grown-up twin: Tooltern's help pages on a laptop that won't charge over USB-C or says "slow charger", which USB-C cable charges a laptop and a phone getting hot while charging, plus the power bank calculator.