Fast charging is not one thing. It is a negotiation, and both ends have to agree.
Plug a phone into a charger and the two devices have a short conversation before any real current flows. That conversation is USB Power Delivery, and understanding it explains most of the confusing behaviour people notice while charging.
The handshake
Over the Type-C connector, the charger advertises the voltage and current combinations it can supply. The phone picks one it can safely accept. Both sides agree, and only then does power move. If either end does not support PD, they fall back to slow, basic USB charging — which is why an old charger makes a new phone charge at a crawl.
This is also why the cable matters. A cable without the right internal wiring cannot carry the negotiation, so you get the fallback speed no matter how capable the charger is.
Why 20W and not more
Laptop chargers go to 65W, 100W, and beyond. Phones mostly do not, and the reason is chemistry rather than restraint. Pushing current into a lithium cell generates heat, heat accelerates permanent capacity loss, and a phone battery is small enough that the heat has nowhere convenient to go.
Around 20W is where the curve flattens for a typical phone battery: fast enough to add roughly 50% in half an hour, gentle enough that the cell is not being cooked to get there. Higher figures exist and work, usually with sophisticated cooling and split-cell designs, but the returns shrink quickly.
| Power level | Typical use | Rough phone speed |
|---|---|---|
| 5W | Legacy USB-A charger | Overnight territory |
| 15W | Magnetic wireless | Comfortable top-up rate |
| 20W | USB-C PD, most phones | About 50% in 30 minutes |
| 45W+ | Tablets and laptops | Diminishing returns on phones |
Why the last 20% takes forever
Charging happens in two phases. Constant current runs from empty to roughly 80%, at full advertised speed. Constant voltage handles the rest, tapering the current down as the cell approaches full to avoid stressing it.
So the 0 to 50% sprint and the 80 to 100% crawl are both working exactly as designed. If you are charging in a hurry, the first half hour is where nearly all the useful energy arrives — which is a good argument for short frequent top-ups over one long session.
Protection is not optional
Any bank worth carrying watches the cell continuously: over-current, over-voltage, over-temperature, short circuit, and over-discharge. Over-discharge protection matters more than most people realise — draining a lithium cell too far causes permanent damage, so the bank cuts off before that point rather than giving you the last few percent.
Common questions
Do I need a special cable for 20W charging?
You need a USB-C cable rated for the current and capable of carrying the PD negotiation. Very cheap or very old cables often silently fall back to slow charging.
Will fast charging damage my battery?
Modern phones manage voltage and temperature closely, and 20W is well within safe limits. Heat is the real enemy, so avoid charging in direct sun or under a pillow.
Can I use my phone while it fast charges?
Yes, though heavy use adds heat and the phone may throttle the charge rate to compensate.