Apple 30-pin usb CABLE (not the charger) charge rate mystery...Any ideas? - Page 1
Forum section: electronics / projects

author: salil
author_rank: Regular Contributor
timestamp: June 28, 2014, 06:02:21 pm
content: Does anyone know how Apple devices that use the 30-pin connector determine the exact rate at which they charge based on the usb cable used? I'm aware of the voltages Apple has on the data lines of a charger to signal how much current a device can draw from that charger but what about cables? I did a test using my iPhone 4S and 2 (incase one meter was off, but they gave the same results) of these USB volt/current meters:http://www.ebay.com/itm/USB-Charger-Doctor-Mobile-Battery-Tester-Power-Detector-Voltage-Current-Meter-/251570967031?pt=US_Computer_Power_Supply_Testers&hash=item3a92cc51f7...and found I was getting the following charge rates0.25A - Generic 1.5m 30-pin USB cable1.00A - Generic very short 30-pin USB cable0.92A - Genuine Apple 30-pin 1m USB cableAll three of these tests were done within seconds of each other - so the device (iPhone 4S) being charged had essentially the same charge level and I used the same genuine Apple 5w (1A) USB wall charger for all tests.Does anyone know what's going on here? How do you deal with this?There's also the issue with iOS devices refusing to charge off MacBook USB ports I won't get into that here, but I think it may be related to this issue.EDIT:Another interesting thing I've noticed is that when trying to charge an iPhone 4S via a Macbook Air (2013, 11", it's supposed to have high current USB ports) using some of the 30-pin USB cables that charge at a slower rate when plugged into an Apple USB wall charger, the device refuses to charge. By refuse, what I mean is that the iPhone will franticly alternate between charging and not charging. On the other hand, when I plug this same cable and device into an Apple wall charger, it charges but never at full speed. I've heard a lot of reports of MacBooks with high power USB ports refusing to charge Apple mobile devices from other people as well, so I'm confident it isn't simply a problem with my Macbook or iPhone. The only solution people seem to have is to use Apple's genuine cables. Unfortunately, Apple doesn't sell cables in all lengths and form factors (not to mention a little overpriced and not very durable) that I use regularly and I'd really like to understand what's going on with these cables.

author: wraper
author_rank: Unknown
timestamp: June 28, 2014, 06:52:54 pm
content: Cheap "slow charging" cables lacks one wire. Not sure but it might be that old iphones had the same wiring and additional wire was added later for newer models.

author: Len
author_rank: Frequent Contributor
timestamp: June 28, 2014, 07:51:26 pm
content: Quote from: salil on June 28, 2014, 06:02:21 pmThe length & cheapness of the cable could be the problem. If the cable has significant resistance (due to length and skimping on copper), it could mess up the magic voltage values set by the charger.

author: salil
author_rank: Regular Contributor
timestamp: June 29, 2014, 09:54:31 pm
content: By "cheap" what exactly does this translate to in terms of construction that would significantly affect performance?My $1 cable charges faster than the official Apple cable that came with this iPhone (purchased from Apple directly, I'm certain it's genuine)...I'm pretty sure the $1 cable has lower resistance since it's very short, but I've seen (I don't have one) Apple certified 10ft 30-pin cables that are supposed to charge at full rate? Just ultra heavy gauge wires? I'm now tempted to make a 12/3 AWG USB cable, :p but if the connectors aren't making better contact with the phone and charger, I guess that wouldn't be of any significant help...?If in fact charging rate is determined based on the resistance of the cable, does anyone have any solutions to this dilemma? One thing I thought of was using a very short 30 pin usb cable that is plugged into a tiny fast charger which gets charged rapidly by the USB charger (probably a 10w 2A iPad charger just so it stays well ahead) and then the phone draws current from it over the very short 30 pin cable. Seems awfully clunky, but it's better than charging at 50% or less speed.All these 30-pin USB charging cables have the same 4 wires going to the same pins on both ends and a stranded wire wrapping around them for shielding.

author: Len
author_rank: Frequent Contributor
timestamp: June 30, 2014, 12:04:11 am
content: Quote from: salil on June 29, 2014, 09:54:31 pmI'm confused. From your first post, the "Generic 1.5m" cable is the one that was making it charge slowly. Is that correct? Yes, I would expect a genuine Apple cable + charger to charge the phone at the rated 1A.

author: aargee
author_rank: Frequent Contributor
timestamp: June 30, 2014, 05:20:26 am
content: As you know Apple works out the charge rate by the resistances on the D+ and D- of the USB connector. Depending on the Apple charger and its resistor combinations will let the device charge at different rates.How this would apply to cables is anyone's guess unless the wires themselves are resistive. You could try measuring between the D+/- and V+ and GND on the cable alone to see if there are resistors built in?

author: salil
author_rank: Regular Contributor
timestamp: June 30, 2014, 04:42:28 pm
content: Quote from: Len on June 30, 2014, 12:04:11 amYes, the 1.5m generic cable is the slowest, but 2nd slowest (not really that "slow" but still not the fastest) is the Apple 1m one and the fastest is the very short $1 generic cable.Quote from: aargee on June 30, 2014, 05:20:26 amI guess the data lines would have some added resistance as does the introduction of any wires/connectors. I'll measure it and post what I find. Thanks.My guess right now is that the iPhone is somehow measuring the resistance of the USB cable. If that's the case, does anyone know where I can purchase especially low resistance USB-A connectors so a low resistance USB extension cable can be constructed? I've bought a couple different types of USB connectors on eBay from China but I'm not sure how to select which will make the best connection.Another interesting thing I've noticed is that when trying to charge an iPhone 4S via a Macbook Air (2013, 11", it's supposed to have high current USB ports) using some of the 30-pin USB cables that charge at a slower rate when plugged into an Apple USB wall charger, the device refuses to charge. By refuse, what I mean is that the iPhone will franticly alternate between charging and not charging. On the other hand, when I plug this same cable and device into an Apple wall charger, it charges but never at full speed. I've heard a lot of reports of MacBooks with high power USB ports refusing to charge Apple mobile devices from other people as well, so I'm confident it isn't simply a problem with my Macbook or iPhone. The only solution people seem to have is to use Apple's genuine cables. Unfortunately, Apple doesn't sell cables in all lengths and form factors (not to mention a little overpriced and not very durable) that I use regularly and I'd really like to understand what's going on with these cables.

author: Jeroen3
author_rank: Super Contributor
timestamp: June 30, 2014, 05:18:35 pm
content: I've tested a bit with my iphone. Set lab supply to 5.1 Volt, as is the Apple Genuine charger open-loop, and inserted this into the iPhone. Instead of 5V from charger.PC USB = 500 mA flat, doesn't change with voltage.1A apple charger = 570-620 mA, changes with voltage.2A apple charger = still 570-620 mA, and also changes a bit with the voltage.I've also noticed my genuine 1A apple charger drop to 4.8 Volts with usb mini extender cable with iPhone charging. And the 2A one drops to 4.9V, yet being 5.2V open-loop.Tested with iPhone 4S @ 20% battery.So I would throw this mystery to the extra Ohms in the measurement cables(extra usb, little usb board, banana leads), and the mystery charging currents inside the iPhone.

author: Len
author_rank: Frequent Contributor
timestamp: July 01, 2014, 07:40:33 pm
content: Quote from: salil on June 30, 2014, 04:42:28 pmI wouldn't worry about the difference between 0.92A and 1.0A. That's, what, 10 minutes on a 2 hour charge? How much time & money are you willing to spend for that? If it bothers you, just use the shorter cable.I don't think there's anything funny going on there. My guess is that the iPhone is charging at its nominal 1A rate, but it's being affected by the voltage drop in the USB cable. It's typical for a 1m-long USB cable to have a resistance of a few tenths of an ohm - that could drop the charging voltage from 5V to around 4.5V, which can affect the charging performance.On the other hand, the difference between 0.25A and 1A looks like the phone's charge-rate-sensing isn't working right, probably because of excessive resistance on the data lines of that particular cable. (And again, I'm just guessing.)

author: retrolefty
author_rank: Super Contributor
timestamp: July 01, 2014, 08:06:01 pm
content: Quote from: aargee on June 30, 2014, 05:20:26 amThe adafruit folk had to reverse engineer that when they were researching their charger kit product. It's not series resistors on the D+ and D- lines, but rather two resistor voltage dividers wired on each D+ and D- lines. The Apple product 'measures' the applied voltage on those lines to allow higher charge rates or not.

author: salil
author_rank: Regular Contributor
timestamp: July 01, 2014, 08:23:08 pm
content: Quote from: retrolefty on July 01, 2014, 08:06:01 pmYeah, that's correct. Still not sure why the resistance of the cable seems to be impacting charge rate so much. I used the same Apple wall charger for all tests.Quote from: Len on July 01, 2014, 07:40:33 pmHopefully not more than a couple hundred dollars. 8% difference isn't huge, but still disappointing when using an official Apple cable that literally costs 20 times the price. 20% or more is pretty bad though and would be something worth investigating.I've been trying to figure out how to get the maximum, consistent charge rate for a while so I've got some time to figure out a solution. Using the short cable isn't really an option in a lot of scenarios like in a car such or anywhere really that the device needs to be used while charging except when using an external battery pack attached to the phone. My iPhone's battery goes through at least 3 charge cycles per day and getting a 2nd iPhone seems like a hassle to keep them both synced up all the time, especially when there is minimal data service. I tried switching to Android twice just for the ability to have removable batteries, but couldn't find enough 3rd party software to replace what I use on a regular basis.A work around I've been considering is a custom belt clip attachment which uses a strong magnetic connector that allows for quick docking of the phone when not in use (snaps into place kind of like magsafe in physical design). This would ensure the phone is always charging when not in use (without making the phone thicker or tethered to a cable) and a decent number of battery cells could be fit on a belt. I also was considering wiring a Scottevest with additional battery cells if need be, but that isn't ideal for the weather here right now.

author: Rerouter
author_rank: Super Contributor
timestamp: July 01, 2014, 08:52:41 pm
content: the other option you have is to break the data lines on your lowest resistance cable and tell the apple device that its a 2 / 3A charger with the right resistor pair (the resistances on the data lines are what it uses when there is no chip on the other end negotiating power constraints,)