Until a few years ago every manufacturer had its own charger and its own connector. Today USB-C has become the common standard, but behind the same socket hide different protocols, different power levels and very different cables. Understanding how charger, cable and phone talk to each other helps you avoid the wrong purchases and charging that's slower than expected.
Power: voltage times current
Charging power is measured in watts and is the product of voltage (volts) and current (amps). Old standard USB supplied 5 volts and little current: a few watts in total. To charge faster there are two routes: raise the current or raise the voltage. Pushing current too high heats up cables; that's why modern protocols mainly raise the voltage and bring it back down to low values inside the phone.
The key point is that the charger doesn't decide on its own. It always starts at 5 volts, which is safe for any device, and only goes higher after the phone has asked for and been granted a higher value.
USB Power Delivery: the negotiation
USB Power Delivery (USB PD) is the open standard that governs this negotiation over USB-C. It works like this:
- the cable is plugged in and the charger supplies 5 volts;
- charger and phone exchange messages over a dedicated line in the USB-C connector;
- the charger lists the profiles it can offer, for example 5, 9, 15 or 20 volts with their maximum current;
- the phone picks the most suitable profile and the charger switches to that voltage;
- during charging the phone can ask to change profile, for example when the battery warms up or nears full.
The same USB PD charger can therefore power earbuds, a phone and a laptop, giving each only what it asks for.
PPS: fine tuning
The version with PPS (Programmable Power Supply) lets the phone request voltages in small steps rather than only fixed values. This is useful because if the charger already supplies a voltage very close to the battery's ideal, the phone has less energy to convert and runs cooler. Some Android manufacturers require PPS to enable their fastest charging: with a USB PD charger without PPS the phone still charges, just more slowly.
Other protocols
Alongside USB PD there are proprietary protocols, such as Quick Charge and various brand-specific solutions that reach very high power with dedicated chargers and cables. With a charger from another brand, these phones usually fall back to USB PD at lower power. It isn't a fault: it's simply the common mode both sides understand.
The cable matters more than it seems
Not all USB-C cables are the same:
- ordinary cables are rated up to 3 amps, so up to 60 watts at 20 volts;
- for currents up to 5 amps you need a cable with an e-marker, a small chip that tells the charger what the cable can handle;
- many cheap cables only transfer data at USB 2.0 speeds, even if they charge well.
A damaged or poor-quality cable can lower the negotiated power or make charging cut in and out. When charging is inexplicably slow, the cable is the first suspect.
Why charging slows down near the end
Anyone watching the percentage notices that the first minutes are very fast and the last ones slow. That's by design: a lithium-ion battery accepts high current when it's empty, but beyond a certain threshold the internal charger switches to a constant-voltage phase in which the current gradually drops. That's why marketing claims often quote the time to reach 50%, not 100%.
Temperature also plays a part: if the phone is warm, because of the room or the way it's being used, the system reduces power to protect the battery.
How to choose a charger
| Need | Recommended choice |
|---|---|
| smartphone only | USB PD 20-30 W, with PPS if the phone uses it |
| phone and tablet | 30-45 W |
| laptop too | 65 W or more, multi-port |
| travel | compact multi-port gallium nitride (GaN) charger |
On multi-port chargers the stated power is often the total: plug in two devices and it gets shared. The labels on the back, almost always tiny, list the profiles available on each port and are the most reliable source.
Finally, safety: chargers without recognizable brands and with suspiciously low prices may have inadequate insulation. A charger is connected to the mains, often overnight and next to the bed: that's the wrong place to save a few euros.
In short
- Power is voltage times current: modern protocols raise the voltage after a negotiation.
- USB Power Delivery is the common standard; PPS allows fine adjustment and less heat.
- The cable can limit power: above 3 amps you need an e-marked cable.
- It's normal for charging to slow down near the end and when the phone is warm.
- A quality charger with clearly listed profiles beats a cheap anonymous one.
