Data-only USB-to-Lightning cables

Question

I don't want to degrade my battery by it charging while performing a lengthy iTunes backup, I would like to be able to backup my iPhone to my computer without it simultaneously charging. Does anyone manufacture/sell any data-only USB-to-Lightning cables?

Answer 1

It is unlikely for a Lightning cable to provide data sync without power because providing power is a very important part of the USB handshake process.

Consider the process of connecting a simple USB device like a mouse. How is the mouse to know it was connected to computer? Well, it "knows" this by having power applied. Now that it has power it will inform the computer of its capabilities, and as part of that information it will inform the computer of its power requirements. If, for some reason, there's not enough power for full capability (unlikely for a mouse but possible for something more complex like an iPhone) then the USB device will not be "enumerated" as an available device. Likewise a device would know the connection is lost by seeing loss of power. Loss of power for a mouse means not being able to operate further but for something with external power, such as a printer or an iPhone, the loss of power on the USB port means no USB host is connected so it looks for hosts on other ports or something.

An iPhone "knows" it is connected to a computer because power is available, just like any USB device would similarly be "informed" of a live connection. The iPhone would then inform the computer of its power requirements, which presumably includes enough power to recharge its batteries. If you don't want an iPhone to draw power from the host then it is on the iPhone to inform the host that it isn't going to draw power. Then presumably the iPhone won't draw more power than that required to maintain communications, which for USB is less than 100 mA.

USB cables can inform both the host and the guest devices on how much power it can carry. The levels on which a cable may carry does not include zero, there must be some power allowed on the cable but how much is actually drawn is up to the guest device.

I'd have to dig into the USB specs on how much the cable is allowed to limit power, which I'm not willing to do at the moment. Maybe the cable can advertise itself as capable of only US 1.1 power levels which might limit power draw to 2.5 watts to the iPhone. Again, how much power is actually drawn is up to the iPhone and not the cable.

The cable must allow the 5 volt Vbus power through as that is vital to informing any connected device that there is a live host on the port. The USB specification allows for 100 mA of current draw without negotiating for more power but, again, the amount of power that flows on the connection is up to the guest device on the bus. I'd have to dig into the USB specifications to be certain but if a cable cannot provide at least 900 mA of power to a connected device then it would fail getting certification. Without this certification it would only be sold by fly-by-night operations and that just means the cable becomes a fuse that blows if more power flows than some kind of real limiter on power flow while a data connection persists.

My best guess on this is that if you don't want your iPhone to charge while connected to a computer then you need to look to some kind of settings or software on the iPhone to prevent this. USB is not really set up for providing data without also providing power, with one very unique exception.

With the introduction of USB 3.0 there's a mode of operation in which data can flow without also having either device providing 5 volt Vbus power. I don't know if the Lightning port is equipped for operating in this mode. If Lightning was equipped for operating in a mode that allowed for USB 3.0 data transfer without Vbus power on the port then that would mean having software on the iPhone to initiate the connection.

As I see this it is dependent on the iPhone to initiate a connection with a host computer for USB data transfer without also drawing power that could charge the battery. I am not aware of any such software that prevents an iPhone from charging once connected to a valid USB host computer. If a cable exists to establish a connection by USB without also drawing power then it must come with software specifically designed to allow a data connection but not allow power to flow.

Answer 2

You can influence the power a Mac sends to connected usb devices.

Stopping (during the backup) and restarting (when you want fast charging) the usbd daemon will allow some control over power delivered.

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This is not exactly a cable answer you may have hoped to get, if you're worried about lightning cables frequently disconnecting/reconnecting to your Mac and can't get a solid connection to transfer data, then you might like to run the command sudo killall -STOP -c usbd in terminal which will reduce the power draw from 2100 mA down to 500 mA.

The USB 2.0 standard allows the host (i.e. your Mac) to provide 500 mA of power to your connecting device. However through some "Apple magic", iOS devices connected/charging from a Mac can draw an additional 1600 mA of power (up to 2100 mA of power), provided you use an Apple approved/official lightning cable. This is despite lightning cables being USB 2.0 cables that should only draw a maximum of 500 mA as per the USB 2.0 specification. This is why on Windows, connected iPhones will charge much slower (since they can only draw 500 mA) whilst on macOS, connect iPhones can charge faster at 2100 mA.

Unfortunately, because of this "Apple magic", I've found lightning cables can be pretty sensitive to the smallest of faults (i.e. a single pin is slightly bent, debris/dust on the pins, etc) that can make the 2100 mA power draw quite unstable when connected to a Mac. So as the Mac-iPhone connection is trying to negotiate a constant power draw of 2100 mA, any fluctuations causes the Mac/iPhone to think something has gone faulty and attempts to fix it by disconnecting/reconnecting in very quick succession (we're talking milli seconds). This is more prevalent when your iPhone is on a very low charge and more likely to affect Mac laptops (particularly on the left IO ports, but right IO ports are also susceptible). That's not to say it doesn't happen on desktop Macs, but it seems more prevalent for MacBooks strangely...

Anyway, I digress, by running that terminal command, you pause the usbd process, which is the "Apple magic" process that configures iOS devices to charge from an Apple Mac up to 2100 mA power draw. Once that process is stopped, the Mac will limit the power draw to your iPhone at 500 mA, which is much more stable and thus will allow you to connect and transfer data fine, (albeit it will charge as slow as on Windows).

If you want more details on the issue with usbd process, I've done some thorough troubleshooting and explained quite a bit about it in my own question here: iPhone-Mac connection issue and what does the usbd process do?.

Also note, whilst technically the lightning cable is faulty, I found that it's really the smallest of faults that causes this instability issue, even if the cable physically looks perfectly fine and still charges perfectly fast when charging directly from a power adaptor. I've had countless of official Apple lightning cables (bought directly from an Apple retail store) experience this issue within just 2 weeks of using the cable. This is despite being very careful with not bending the cable, keeping the pins clear of debris/dust, ensuring the pins do not get damaged, etc. There is really nothing you can do about that unfortunately. However, as I said, they will work perfectly fine charging from a wall charger, or connect perfectly fine to a Mac if it's drawing only 500 mA of power.

Again this might not solve your question, but if it's a lightning cable that's frequently disconnecting/reconnecting and all you want is a stable connection to transfer data, then I'd recommend running that terminal command to pause the usbd process, which will drop that power draw to a much more stable 500 mA (which is what the USB 2.0 standard specifies anyway).

Final note: If you want to restart the usbd process, you can run the command sudo killall -CONT -c usbd or restart your Mac.

This will lower the odds you will degrade your battery whilst performing an iTunes backup.

Answer 3

You can influence this greatly with normal hardware and iOS settings.

• For sure set the phone to not charge the battery past 80% - if your battery is at 80% at the start, no charging will happen and any charging will stop as soon as possible.
• Get an inexpensive self powered USB hub and connect that first to the Mac. It draws less current (500 mA) than fast charge direct connection (up to 2000 mA) to the Mac draws. This Anker hub is a high quality, reasonable price and says it doesn’t support charging: Anker 4-Port USB 3.0 Hub, Ultra-Slim Data USB Hub with 2 ft Extended Cable [Charging Not Supported]. Anker has a very solid reputation for cables and devices in my experience.
• Plug your phone via lightning cable into the USB hub and not the Mac.

The second two mean you will charge at the baseline USB rate and slow any charging that does happen, or you’ll find a hub that doesn’t charge at all.