How your iPhone asks for power

USB Power Delivery, explained from the plug to the battery

USB Power Delivery (USB PD) is the language a USB-C charger and your iPhone use to agree on power. The charger lists what it can supply — a 20W charger offers 5 volts at up to 3 amps and 9 volts at up to 2.22 amps. Your iPhone picks one, the charger confirms, switches, and reports it is ready. The iPhone keeps control: it takes only the power it asks for, based on battery level, temperature and model. A higher-rated charger never pushes extra power into a phone that does not request it.

By ERONIX · Last substantive update: 20 Oct 2026

In 30 seconds

The charger offers, the iPhone chooses. A USB PD charger advertises a short list of power levels. The iPhone requests one. Nothing changes until the iPhone asks. ¹
A 20W charger offers two levels: 5V at up to 3A (15W) and 9V at up to 2.22A (20W). These are set by the USB PD power rules for chargers above 15W. ¹²
Your iPhone decides the actual watts moment to moment. Most iPhones from iPhone 12 to iPhone 17 draw about 18–19.5W from a 20W charger in the first part of a charge, then slow down on purpose as the battery fills. ³⁴⁵
Any compliant USB-C cable works at 20W. Special "e-marked" cables only matter above 3 amps. ⁶
Bigger chargers help only the iPhones that ask for more — for example iPhone 17 (40W or higher for Apple's 20-minute figure) and iPhone 18 Pro (60W with AVS). ⁷

In three minutes

1. Plugging in: the cable tells each side who it is talking to

A USB-C connector has two pins called CC (Configuration Channel). When you plug in, the charger presents a pull-up resistor or current source on CC — called Rp — and the iPhone presents a pull-down resistor called Rd. The charger sees Rd and knows a device that wants power is attached; only then does it switch on its 5-volt supply. A bare charger with nothing attached keeps its output off, which is one reason USB-C ports are safe to touch. ⁸

The value of Rp also carries a first, simple message: how much current is available at 5 volts before any negotiation — default USB power, 1.5A or 3A. ⁸⁹

2. The conversation: four messages and their receipts

Once 5 volts is present, the charger sends its menu over the CC wire. USB PD uses a short sequence; every message is acknowledged with a receipt called GoodCRC. ¹⁰

Step	Sent by	Message	What it means
1	Charger	Source_Capabilities	"Here is what I can supply" — a list of Power Data Objects (PDOs)
2	iPhone	Request	"I'd like option n, at up to this current"
3	Charger	Accept	"Agreed"
4	Charger	PS_RDY (power supply ready)	"The new voltage is now stable — go ahead"

In a typical session the whole exchange takes well under half a second. ¹⁰ If either side stops responding, the protocol resets to safe 5-volt power rather than guessing. ¹¹¹²

USB PD sets power rules so devices know what to expect. A charger rated above 15W must offer 5V and 9V; above 27W it must add 15V; above 45W, 20V. ¹² For a 20W charger, that works out to:

Fixed PDO	Voltage	Maximum current	Maximum power
1	5 V	3.0 A	15 W
2	9 V	2.22 A	20 W

Apple's own 20W adapter (model A2940) advertises exactly these two levels, as captured by ChargerLAB. ¹³ Some third-party 20W chargers add a 12V level or PPS (below) for Android phones; iPhones do not need either at 20W.

4. The iPhone stays in charge

Once agreed, the charger holds the voltage and makes up to the agreed current available. The iPhone's own charging circuit then draws what the battery can take. When the battery is low and cool, an iPhone on a 20W charger typically pulls about 18–19.5W. As it fills — roughly past 50–80% — it reduces current, then may drop back to 5V. ³⁵¹⁴¹⁵

ChargerLAB's session with an iPhone 14 Plus on Apple's 20W adapter shows this shape: about 19.5W for the first 37 minutes, then about 15W, then about 8.5W, with the bus dropping from 9V to 5V from about the one-hour mark; 50% at 32 minutes and 100% at 2 hours 11 minutes. ³

iPhone 14 Plus · Apple A2940

V09 iPhone 14 Plus on Apple's 20W adapter, 25°C room, ChargerLAB, 18 Sep 2023. Milestones only — the source does not publish a continuous trace, so none is drawn. · Data (CSV)

Show the numbers

Step chart showing iPhone 14 Plus reaching 50 percent at 32 minutes, 80 percent at 60 minutes and 100 percent at 131 minutes on a 20 watt USB-C charger.
Series	Battery (%)	Time (min)
iPhone 14 Plus · Apple A2940	50	32
iPhone 14 Plus · Apple A2940	80	60
iPhone 14 Plus · Apple A2940	100	131

5. USB-C iPhones and Lightning iPhones

iPhone 15 and later have a USB-C port. Use a USB-C to USB-C cable. Apple specifies an 18W or higher USB-C adapter for fast charging. ⁷
iPhone 8 to iPhone 14 (and iPhone SE 2nd/3rd generation) have Lightning. They fast charge from a USB-C PD charger through a USB-C to Lightning cable; the cable carries the PD conversation. ⁷ A poorly made Lightning cable is a common reason fast charging stops — choose a certified one.

6. When a bigger charger helps — and when it doesn't

iPhone	What Apple specifies for its fastest wired charge	What a 20W charger does
iPhone 18 Pro / 18 Pro Max	60W or higher, USB PD 3.2 with AVS, for ~50% in ~15 min ⁷	Charges, but well below the phone's potential
iPhone 17 / 17 Pro / 17 Pro Max	40W or higher for ~50% in ~20 min ⁷	Charges at ~19W; iPhone 17 reached 50% in 27 min in an independent test ⁵
iPhone Air / iPhone 17e	20W or higher for ~50% in ~30 min ⁷	The right fit
iPhone 15 and 16 families	18W or higher ⁷	Fast charges; Pro Max models can draw more on larger chargers
iPhone 8 to 14	18W or higher, USB-C to Lightning ⁷	Fast charges

See the full table at /compatibility/iphone-charger/.

attach
Rp/Rd
5V
Source_Capabilities
Request
Accept
PS_RDY
renegotiation

V01 How a USB-C charger and an iPhone agree on power. Schematic illustration — not product test data.

For engineers

E1. Type-C attach: Rp, Rd and Ra

Current advertisement by Rp, as reproduced in vendor documentation of the Type-C specification ⁸⁹¹⁶:

Advertisement	Rp to 4.75–5.5 V	Rp to 3.3 V ±5%	Current source (typ; range)
Default USB power	56 kΩ ±20%	36 kΩ ±20%	80 µA (64–96)
1.5 A @ 5 V	22 kΩ ±5%	12 kΩ ±5%	180 µA (166–194)
3.0 A @ 5 V	10 kΩ ±5%	4.7 kΩ ±5%	330 µA (304–356)

The sink presents Rd = 5.1 kΩ (TI TUSB320 implements 4.6–5.6 kΩ). A cable that needs VCONN presents Ra of 0.8–1.2 kΩ. Legacy USB-A to C cables must carry a 56 kΩ Rp so they never over-advertise. ⁸¹⁶¹⁷

Sourcing note: these are vendor reproductions of specification tables (Infineon, TI), graded manufacturer-declared. The vSafe5V window (commonly quoted as 4.75–5.5V) and vSafe0V threshold were not confirmed against primary specification text during this research; they are not stated as fact on this page. The 4.75–5.5V figure appears only as the pull-up reference range in the Rp table above. ⁸

E2. Physical layer and message flow

USB PD messages travel on CC using biphase mark coding at roughly 300 kbaud. ¹⁰ A fixed-supply contract follows: Source_Capabilities (repeated until GoodCRC) → Request (object position + operating/maximum current) → GoodCRC → Accept → GoodCRC → voltage transition → PS_RDY → GoodCRC. If the source receives no response after its capability retries, it may issue a Hard Reset (nHardResetCount = 2) and return VBUS to safe levels. ¹¹¹²

E3. Timing parameters

Values below come from open-source PD stacks and compliance-test documents that cite the specification, not from the specification text itself; treat them as implementation-verified, not primary. ¹²¹⁸¹⁹²⁰

Parameter	Value	Source
tSenderResponse	27–33 ms (PD 3); 24–30 ms (PD 2)	¹⁸
tTypeCSendSourceCap	100–200 ms	¹²
tPSTransition (SPR)	450–550 ms	¹²
tSrcTransition (source wait before raising voltage)	25–35 ms	¹⁹
Settling after transition	within 275 ms, within ±0.5 V of new voltage	²⁰²¹
Slew rate during transition	≤ 30 mV/µs	²⁰
tSafe0V	650 ms max	¹²²¹
tFirstSourceCap, tSrcReady	Not confirmed — omitted	—

E4. PDO types: Fixed, PPS, SPR AVS and EPR

Type	Introduced	What it does	Relevance at 20W
Fixed	USB PD 2.0	Discrete voltages (5, 9, 15, 20 V in SPR) with a maximum current	The only type Apple's 20W adapter offers ¹³
PPS — Programmable Power Supply (an Augmented PDO)	USB PD 3.0	Device sets voltage in 20 mV steps and current limit in 50 mA steps within a range	Used by many Android phones (e.g. Samsung Super Fast Charging). iPhones do not rely on it. ²²²¹
SPR AVS — Adjustable Voltage Supply, Standard Power Range	USB PD 3.2 (Oct 2024)	9–20 V in 100 mV steps; defined for chargers of 27–100 W	Not applicable to a 20W charger. Apple's 40W Dynamic adapter declares AVS 9–15 V and 15–20 V ranges and pairs it with the iPhone 17's charge pump. ²³²⁴
EPR — Extended Power Range (fixed 28/36/48 V, plus EPR AVS)	USB PD 3.1	Up to 240 W	Laptops and larger devices only ¹

The current specification listed by USB-IF is USB PD Revision 3.2, Version 1.2; Version 1.1 (October 2024) is the edition referenced by European interoperability rules. ²⁵²⁶

E5. Power rules and current ranges

SPR rails and their current ranges ¹²: 5 V 0.5–3 A; 9 V 1.67–3 A; 15 V 1.8–3 A; 20 V 2.25–5 A. A source's PDP (power data profile) fixes which rails it must offer. At 20W PDP: 5V/3A + 9V/(20/9 = 2.22)A — the derivation is inferred from the rule pattern and matches Apple's declared A2940 output. ¹¹³ A sink must operate whenever the source's power is at least the sink's requirement on a normative voltage combination. ²²

E6. Cables and e-markers

A cable must be electronically marked if it is full-featured or rated above 3 A; a USB 2.0 C-to-C cable "may" carry an e-marker but needs one only above 3 A. Without an e-marker, sources cap current at 3 A. At 20W the current never exceeds 3 A, so no e-marker is required. ⁶²² Cable resistance still matters: voltage drop over a thin or long cable reduces voltage at the phone; this is one reason measurements are taken at the cable's far end. ²⁷²⁸

E7. What a USB PD analyser shows

A protocol analyser or in-line tester (e.g. the POWER-Z KM003C used by ChargerLAB ⁵) decodes the CC traffic and logs:

the Source_Capabilities message (every PDO, voltage and current);
the Request (which PDO the iPhone chose and how much current it asked for);
Accept and PS_RDY timing;
VBUS voltage and current over time, from which watts and charge curves are derived;
any renegotiation (e.g. 9V → 5V near full charge) and protocol errors or resets.

ERONIX's interoperability protocol records this trace for every iPhone session (19_LAB_TEST_PROTOCOL.md §7).

E8. Inside the iPhone

Apple does not publish its charging circuit. What is documented: iPhone fast charges to about 80% and then slows, and software may limit charging above 80% when battery temperature is high. ¹⁵ iPhone 15 and later offer a Charge Limit of 80–100% in 5% steps. ²⁹ The iPhone 17 series pairs AVS with an internal charge pump. ²⁴ Between the USB-C port and the battery there is at least one regulating stage — a switch-mode charger or charge pump — that conditions incoming power; the exact topology is inferred, not published. ³⁰

Myths

"A 30W or 65W charger will overload my iPhone." No. The charger only offers; the iPhone requests what it can take. A compliant higher-rated charger is safe; it just won't be faster for a model that draws 20W.

"A 20W charger always delivers 20W." No. 20W is the maximum offered. The iPhone draws less as the battery fills, when it's warm, or when the screen and radios share the power. ³¹⁵

"Every USB-C cable is the same." For 20W, any compliant cable works. Differences appear above 3 A, with data speeds, and with quality and length. ⁶

"PPS makes an iPhone charge faster." iPhones fast charge on fixed PDOs. PPS mainly benefits phones that use it, such as Samsung's Super Fast Charging. ²²¹³

Questions people ask

Does iPhone support PPS? iPhones fast charge using fixed USB PD levels. The iPhone 17 series and later use AVS — a newer adjustable mode defined for chargers of 27W and above — for their fastest charge. PPS is used mostly by Android phones. ²³²⁴

What is AVS, and do I need it? AVS (Adjustable Voltage Supply) is the USB PD 3.2 mode iPhone uses for its fastest charge; it needs a charger of 40W or more on iPhone 17 and 60W on iPhone 18 Pro for Apple's quoted times. A 20W charger does not offer AVS. If you own one of those models and want the fastest charge, choose an AVS charger. ⁷²³

Why does my iPhone charge quickly to about half, then slow down? Lithium-ion batteries accept high current when they are low and need gentler, constant-voltage charging as they fill. iPhone reduces power deliberately past roughly 80%. ¹⁴¹⁵

Can a Samsung or other Android fast charger charge my iPhone? Yes, if it supports USB PD on its USB-C port. The iPhone will request a fixed PD level the charger offers. Proprietary-only fast-charge modes don't apply to iPhone. Check the charger is BIS-registered for its model (see how to check).

Do I need a special cable for a 20W charger? No. Any compliant USB-C to USB-C cable (iPhone 15 and later) or certified USB-C to Lightning cable (iPhone 8–14) carries 20W. ⁶

Comparative caveats

Wattage figures for iPhones come from third-party tests on specific units, chargers, temperatures and firmware; your results will differ. ³⁵
Apple's charge-time claims are Apple's own tests on drained preproduction units. ⁷
No claim on this page compares Fermi20 with any other charger.