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Findings on Galaxy S4 charging current

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By spark001uk, Senior Member on 11th May 2013, 06:55 PM
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With the advent of the S4, and its higher capacity battery, one could understand a higher charging current. However, I seem to have uncovered some findings that the charging system on the S4 is a bit more complex than first thought!
This is a bit techy, and assumes you have a basic amount of electrical knowledge, ie voltages, currents, resistance etc., but I'll try and keep it as simple as possible for anyone who may not!

To explain: The mains charger supplied with the S4 is a model number ETA-U90UWE, rated 5V @ 2A. However, the phone will ONLY charge at full current (which as I have measured so far, depending on what the phone regulates it to, typically sits in the region of 1.2 - 1.5A), when using the supplied charger (or possibly one of equal or higher current rating, depending on how it's configured internally), AND the supplied usb cable, OR any other usb cable, provided its shielding (the metal outer surface of the connectors) is connected at BOTH ends of the cable. Use a cable that doesn't have this shielding, and the charge current drops, regardless of whether there is plenty of current available or not. Use a different charger with an unshielded cable and the current drops even more, again regardless of whether it can supply plenty more current.
My assumption on this, is possibly an effort by Samsung to avoid the scenario of sticking 1.5 amps down a flimsy cheapo cable, the wires of which will likely be too thin to carry it.

After doing some probing around with a meter, I have managed to find a slight difference with the charger itself, compared to a generic one. In a generic one, the two data pins are usually just shorted together, which tells most phones that it's a mains charger rather than a USB port. On the Samsung one on the other hand, the pins appear to be shorted together, and also connected via resistors across the supply line (known as a potential divider), which holds these shorted data pins at a certain voltage. This is what tells the phone what sort of charger it's connected to.

Attached are a couple of diagrams to show the difference between the two chargers. There are in fact various setups of resistors that different manufacturers use to set the charging current, so it's quite easy to run into compatibility issues!

To make this a little less confusing I have done some preliminary experimenting, and I set out my results here.

For the test, I used combinations of 4 different usb style mains chargers: an apple iPhone one rated at 1A, an iPad one rated 2.4A, an HTC 1A one, and the genuine S4 one. With these I used two cables - the supplied Samsung S4 one (which is shielded), and a cheap generic one (which isn't). I started by measured the charging current directly with a meter, by using a very short usb breakout lead I've made, enabling me to interrupt the 5V line. However, I soon noticed that the use of any extension cables, even shielded, can lessen the chance of maintaining a good shielding connection, so I continued the exercise relying on the "galaxy charging current" app to get a reading.

Charger.................... Cable................ Current (A)
HTC 1A.................Generic..................... 0.5
HTC 1A.................Samsung S4..............1.0

Apple 1A...............Generic......................0.5
Apple 1A...............Samsung S4..............1.0

Apple 2.4A............Generic......................0.6
Apple 2.4A............Samsung S4..............1.3

Samsung S4.........Generic...................... 0.8
Samsung S4.........Samsung S4...............1.3

So as you can see from these results, the original charger makes a difference, and the supplied cable (or a good quality shielded one) makes a further difference. If you have any further findings please feel free to add them here.

I can see that this is going to confuse some people, as it has me, as I'm sure some will inevitably try charging up their phone on generic chargers/leads at some point, with potentially long charging times resulting!

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11th May 2013, 08:02 PM |#2  
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Very nice findings! Thanks for sharing them.
Not only what you say about longer charging time, but also discharging may occur (it has with me) while connected to the USB. I left my phone with USB tethering and it shut itself down after a couple of hours (it was low on battery already). I wanted to keep all the accessories in "new condition" in case I sell the phone in a few months, but I guess this justifies using the official charger and usb lead.
I hope someone can shed some light on the detection mechanism or the particular characteristics of the official cable so it can be replicated in generic ones.
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11th May 2013, 10:17 PM |#3  
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From your findings, also Appe 2,4A charger has some control on the cable used, thanks.
11th May 2013, 10:49 PM |#4  
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Yes, but you won't be able to get more than about 0.75A out of it even with the samsung s4 cable.
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11th May 2013, 10:57 PM |#5  
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You could try to add an extension cable to check if the charging system needs exclusively an original samsung cable from the charger to the phone.

Original samsung charger => extension cable male/female => original samsung cable => phone

PS: is there an app to check the charging current?
11th May 2013, 11:13 PM |#6  
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My experiments used an extension cable, that's the one I made into a breakout cable. So no it doesn't change by adding a cable.

There is an app, called galaxy charging current, which I tried also. In the 0.75A results above it showed a max permissible current of 1000mA, with the fully samsung setup it showed 1900mA. In the lower scenarios is showed up as 460mA. Other than that it doesn't tell you anything, it literally just shows you a max possible current, not the actual current it's drawing.
12th May 2013, 11:58 AM |#7  
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I've found out what the issue was, my generic cable was obviously a cheap one and wasn't shielded! I have edited my original post to re-explain.

Also I did a further test on a PC usb port - the Samsung cable allowed for 500mA, but the unshielded cheap one only allowed about 350mA, so (as pintycar found out), the phone actually carries on discharging in this case!
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12th May 2013, 03:54 PM |#8  
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Interesting post.

I've been trying to mod a car charger so that the s4 draws more than 300mA so far unsucessful. I'm going to add in the 82K resistance and see what happens.
12th May 2013, 08:34 PM |#9  
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Originally Posted by demusss

Interesting post.

I've been trying to mod a car charger so that the s4 draws more than 300mA so far unsucessful. I'm going to add in the 82K resistance and see what happens.


Using a 5a digital bench power supply, I took a normal micro USB cable, cut it in two, added in the 82k resistors and tested.

Before: 450ma, after 1.73a

Then using a clamp meter I tested it with a 2amp Asus tablet charger - it varied between 1.48 and 1.61 amp.

I have since built a few more cables - one to use in my car (again 2 amp charger).

All in all, it works very effectively. Thanks for the initial research (saved me having to open my charger.)

BTW I suspect it is the charger where the resistors are, not the cable, otherwise the phone would try to pull 1.5a + from a laptop if you connected it.

Left my samsung cable at work so will test it tomorrow (if I remember)!

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12th May 2013, 09:23 PM |#10  
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Interesting. So basically for us laymen, you are telling us to use the original charger and cables to the extent possible ... right?

Can you clarify to me what an unshielded cable is? Maybe with a picture? I've got a blackberry's cable at work connected to my PC.

At home, I also have a cable and charger that I got with my Nexus S (I think this says Output 5.0V ~ 0.7A).

Would this be a problem or can I get by with these?

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12th May 2013, 10:12 PM |#11  
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I've been using the Samsung cable+charger+USB extension cable and charging the S4 usually takes about 4 hours, I'm going to try with just the Samsung cable next time and see if it makes any difference since people are saying that theirs can be fully charged within less than 3 hours.
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