Apple 2.4A Charging: How Apple's USB-A Fast-Charge Signal Works
Apple 2.4A is the USB-A signal that lets an iPhone or iPad pull 12W from a 5V charger. How the data-pin voltages work, and why a Mac drops the same brick to 7.5W.
Before USB Power Delivery, Apple’s fast-charge protocol for USB-A was remarkably simple. No negotiation and no higher voltage, just two fixed voltages on the data pins that tell an iPhone or iPad how much current the charger can handle.
The top tier is Apple 2.4A. It’s why Apple’s 12W USB Power Adapter reads “Output 5.2V 2.4A” on its label, and why so many car chargers and power strips print “2.4A” next to their USB ports.
It still matters in 2026, because Apple’s own devices disagree about it. A current iPhone honors it. A current Mac doesn’t.
How the signal works
A USB-A charging port uses four pins: 5 V, ground, and two data lines called D+ and D-. A basic charger leaves the data lines with nothing to say.
Apple’s chargers hold each data line at a fixed voltage using a small resistor divider. When an Apple device is plugged in, it reads the two voltages and picks a current limit to match. That’s the whole protocol. The charger’s output stays at 5 V the entire time; the signal only tells the device how hard it’s allowed to pull.
The voltage table
| Apple mode | D+ / D- | Apple adapter |
|---|---|---|
| Apple 0.5A (2.5 W) | 2.0 V / 2.0 V | None |
| Apple 1A (5 W) | 2.0 V / 2.7 V | 5W cube |
| Apple 2.1A (10 W) | 2.7 V / 2.0 V | 10W iPad adapter |
| Apple 2.4A (12 W) | 2.7 V / 2.7 V | 12W iPad adapter |
The 1A, 2.1A and 2.4A rows are from Texas Instruments’ TPS2514 datasheet, which calls them Divider 1, 2 and 3 and ties them to 5 W, 10 W and 12 W adapters. The 0.5A row comes from Adafruit’s reverse-engineering notes.
Two things worth knowing:
- D+ at 2.7 V is the iPad signal. It’s what separates the 10W and 12W iPad chargers from the 5W phone cube. D- then picks between 2.1 A and 2.4 A.
- Swapped pins are the most common mistake in write-ups of this table. Even TI’s datasheet had two of the rows wrong in one table until a later revision corrected them.
Apple’s labels print slightly more than 5 V, 5.1 V on the 10W and 5.2 V on the 12W, which helps offset the voltage drop along a cable at higher current.
Where you’ll see “2.4A” today
- Apple’s 12W USB Power Adapter, the brick that shipped with iPads for years and is still easy to find.
- Car chargers and power strips that print “2.4A max per port”, like the Anker PowerDrive 2 Alloy (5 V at 4.8 A total, 2.4 A per port).
- The USB-A port on multi-port chargers, like the Amazon Basics 68W, where the rating is only reachable by devices that speak an older fast-charge protocol such as Apple 2.4A or Qualcomm Quick Charge.
Third-party chargers rarely use a fixed divider. They use charge-port controller chips, like the TI part above, that watch the data lines and switch between signatures: Apple’s divider modes, the USB Battery Charging 1.2 mode with D+ and D- shorted together, and a 1.2 V mode used by some other phone makers. Anker markets its version as PowerIQ. That’s how one “2.4A” port can give an iPad 12 W and still charge a device that only understands the standard.
Why your iPhone still honors it
Apple moved the iPhone to USB-C, but a USB-A to USB-C cable carries no Power Delivery. Its USB-C end only tells the phone to expect default USB power. The data pins are the only other place a charger can say “you can take more”, so a USB-C iPhone still reads them.
In my testing, an iPhone 17 Pro Max pulls the full 12 W from Apple’s 12W brick over a USB-A to USB-C cable. I use exactly that setup as a slow overnight charger.
Slow is relative. 12 W is well under what an iPhone 17 takes from a 40W USB-C Power Delivery adapter, but a phone charging overnight doesn’t need more.
Why your Mac ignores it
Modern USB-C Macs don’t use Apple’s divider signal at all. With no Power Delivery on a USB-A charger, a Mac falls back to USB Battery Charging 1.2 (BC 1.2), which allows 1.5 A at 5 V. That’s a 7.5 W ceiling, no matter what the label says.
Every USB-A charger I’ve measured on a MacBook Air lands right under it:
| Charger | Label | MacBook Air draw |
|---|---|---|
| Apple 12W (Apple 2.4A) | 5.2 V at 2.4 A | 7.1 W |
| Apple 10W (Apple 2.1A) | 5.1 V at 2.1 A | 7.1 W |
| Anker PowerDrive 2 Alloy | 2.4 A per port | 7.1 W |
| Amazon Basics 68W, USB-A port | 18 W | 7.2 W |
| Desk power strip USB port | None printed | 7.4 W |
| Apple 5W cube | 5 V at 1 A | 4.6 W |
The 5W cube is the only one below the line, and that’s its own 1 A hardware limit, not the Mac’s.
Apple made that trade deliberately when it moved its Macs to USB-C and standard charging. The longer story is in The Wild West of USB Charging, and the Mac side is measured in detail in Can You Charge a MacBook With an Old Apple iPad Charger?
Why some Windows PCs needed an extra charging driver
Flip the setup around and plug an iPad into a computer instead of a wall charger. A computer’s USB port can’t use Apple’s divider trick, because its D+ and D- lines are busy carrying data. To the iPad it looks like an ordinary data port, good for 500 mA on USB 2.0 or 900 mA on USB 3.0. That’s why an iPad on a PC so often showed Not Charging next to its battery icon.
For computers, Apple’s answer lived in software. Once the device is connected, the computer can send it a vendor-specific USB request (request 0x40) that grants extra current on top of the standard allowance. Macs of that era sent it on their own, and System Information listed the grant as Extra Operating Current, often 1600 mA on top of the usual 500 mA.
Windows had no such request built in. So even when a motherboard’s USB port could physically supply more current, the iPad never took it and kept trickle charging. PC makers filled the gap with their own utilities:
- Gigabyte released its driver for 40 of its motherboards in April 2010, within weeks of the first iPad. Later boards paired its On/Off Charge feature with a dedicated charging controller that kept working with the PC in standby or switched off.
- ASUS shipped Ai Charger.
- MSI and ASRock had their own versions, Super Charger and App Charger.
Each utility did what a Mac did natively: it sent Apple’s request, and on boards built for it, the port hardware could actually deliver the extra current. Without the utility, the same port on the same PC stayed stuck at a trickle.
Linux got the same fix from open-source tools like ipad_charge, which sends the request with up to 1600 mA of extra current, and the Linux kernel now includes a driver, apple-mfi-fastcharge, that does the same.
On a USB-C port, none of this applies. The port advertises how much current it can supply directly on its CC pin, and Power Delivery negotiates anything beyond that, so there’s nothing left for a utility to unlock.
What it looks like in USB Connection Information
Plug an Apple 2.4A charger into a Mac with USB Connection Information and you’ll see a single entry of about 8 W (5 V up to 1.5 A), no Power Delivery profiles, and USB BC 1.2 as the protocol. The ~8 W is 7.5 W rounded up.
That’s not a broken charger. It’s the Mac’s half of the handshake, and an iPhone on the same port will still get its 12 W.
Should you still use one?
- iPhone or iPad, overnight: yes. 12 W is slow, but a phone that has all night doesn’t need more.
- iPhone, fast: use USB-C Power Delivery. The iPhone 17 family reaches 50% in around 20 minutes on a 40W adapter, and the iPhone 18 Pro needs an AVS charger for its fastest charge. The Apple 40W Dynamic Power Adapter with 60W Max covers both.
- Mac: skip USB-A entirely. Any USB-C charger that speaks Power Delivery will beat 7.5 W, and a MacBook Air charges at a sensible rate from 30 W and up.
If you want an Apple 2.4A charger of your own, the Apple 12W USB Power Adapter is still available on Amazon.
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