Fast Charging Explained: PD, QC, and Why Your 40W Charger Feels Slow

You bought a 40W charger and your phone still takes ninety minutes to fill. Nothing is broken. Fast charging is a negotiation between three separate pieces of hardware, and the slowest one sets the speed.

The three-part chain

Every charge involves a charger, a cable and a device. Each has a maximum it can handle, and the actual charging speed is the lowest of the three. A 65W charger with a 60W cable and a phone that accepts 18W will charge at 18W. The extra capability is simply unused.

This is by design and it is a safety feature. Devices negotiate before power flows, which is why plugging a laptop charger into a phone does not destroy the phone.

USB Power Delivery (PD)

PD is the open standard that runs over USB-C, and it is the one worth caring about. Rather than a fixed voltage, the device and charger negotiate a voltage and current the device asks for — 5V, 9V, 12V, 15V, 20V and beyond.

Because it is an industry standard rather than one company's system, a PD charger works properly with any PD device: iPhones, most modern Android phones, Nintendo Switch, iPads, and many laptops. If you buy one charger, buy a PD one.

PPS (Programmable Power Supply) is an extension of PD that lets the voltage be tuned in very fine steps rather than fixed jumps. It runs cooler and is gentler on the battery. Samsung's fastest charging modes need PPS specifically, so a plain PD charger will charge a Galaxy but not at its top speed.

Quick Charge (QC)

QC is Qualcomm's standard, found on phones using Snapdragon chips. Older versions ran over USB-A. QC 4 and above are cross-compatible with PD, so a modern charger supporting both covers nearly everything.

Many manufacturers also run proprietary systems under their own branding. These typically hit their headline speeds only with that brand's own charger and cable. With a third-party charger they fall back to standard PD speeds — still fast, just not the number on the box.

The cable is not a detail

This is where most people lose speed without realising. USB-C cables are not interchangeable.

• A basic USB-C cable carries 60W (20V at 3A). Fine for phones.
• Above 60W the cable needs an e-marker chip — a small controller that tells the charger the cable can handle more current. Without it, the charger refuses to exceed 60W, correctly.
• 100W cables (20V/5A) and 140W+ cables under the newer USB PD 3.1 spec exist for laptops and the fastest phones.

A cable rated for 120W or higher will not make a phone charge faster than the phone allows, but it removes the cable as a bottleneck. If you are buying one cable to cover everything, buying a high-wattage one is sensible.

Also worth knowing: charging speed and data speed are separate specifications. A cable can carry 100W and still transfer data at slow USB 2.0 rates. That is fine for charging, less fine if you move large files.

Why charging slows down near the top

Charging is not linear. Lithium-ion batteries take a high current comfortably when fairly empty, then must taper as they fill to avoid stress and heat. This is why "0 to 50% in 30 minutes" is a common claim while a full charge takes much longer — the last 20% is deliberately slow.

Heat also throttles charging. Charging in direct sun, under a pillow, or inside a thick case while gaming will slow things down as the phone protects itself. If fast charging seems broken, check the temperature first.

Does fast charging damage the battery?

Modern phones manage this actively, and normal use is fine. Two things genuinely help long-term battery health:

• Heat is the real enemy, more than charging speed itself. Keep the phone cool and uncovered while charging hard.
• Staying at 100% constantly ages a battery faster than cycling in the middle range. Most phones now offer optimised or adaptive charging that holds at 80% overnight and finishes before you wake. Leave it on.

What to actually buy

For most people: a 20W–33W PD charger and a decent USB-C to USB-C cable covers a phone and earbuds properly. Step up to 65W or more only if you are charging a laptop or want one charger to run several devices at once. Paying for 100W when everything you own tops out at 25W buys you nothing.