# Why are some USB chargers slower than others?

It seems with USB chargers for my android phone, not all are created equal.

The first pic is the official charger which came with my phone (galaxy nexus). It charges quickly, from 0 to 100% in a couple of hours max.

The second pic is a cheapy I got on ebay to use at the office. It works but it's slow as a wet week, can take 6+ hours to charge and if I use the phone while it's charging, it seems to drain faster than it's being charged.

The two chargers below seem to be the same spec (5V / 1A) and from my limited understanding that P = V*I they should provide the same power, right?

What else do I need to look for when buying a charger, and is there any way to tell in advance if it is going to be slow or fast (apart from testing them out)?

(click images for larger variant)

ps: I'm aware that charging from a computer USB port can be much slower than using the wallwart, because the computer USB ports can sometimes be current-limited to 500 mA.

• A related question to anyone who knows more about this than me: Is it true that the USB cable can have more or less "contacts"? I've heard that the cable needs an extra one or two contacts to be able to charge at full speed (i.e. sending extra power as well as data). Could be the cable that is the bottleneck rather than the actual power plug. May 28, 2013 at 8:27
• @bigbadonk420 - Yes, there are more or less contacts, but they don't have anything to do with charging speed. Micro USB connectors have a 5th pin called ID used in USB-on-the-go to determine which device is supposed to act as the host. USB 3.0 adds 5/7 new pins (2 differential pairs for SuperSpeed, a signal ground for those, and on the Powered-B connector, a reverse power pin (allowing the host to draw power from the device), and a ground for that new power pin). May 28, 2013 at 15:27
• Do not forget that P = V*I (1) can also be rewritten as P = R*i^2 (2) and P = V^2/R (3). Let's say that C is the charge of battery, C = P*t (4), so with (3) and (4) we have C = t*V^2/R (5) => t = R*C/V^2 (6). R is the resistance of cable (thin wires are more resistant, so time to charge will be longer); V is the voltage given - more voltage, less time; if you are using those 'xing-ling'-generic-power-adapters, they would rectifies AC network voltage improperly, so effective voltage given would be less than original power adapter voltage and time would be larger... Feb 10, 2016 at 19:46
• E.g. I had a LG phone that had a good cable I use since now. Newer phones I bought (Samsung) comes with thinner cables, so I saw that charging with other cables than the LG one result in more time to obtain 100% of battery charge. One time, I bought another cable (from those Chinese ones) and using it phone discharged instead of charged. That Chinese cable was longer (2m), so resistance R = r*L/A that depends on length of cable L was bigger than other cables... Feb 10, 2016 at 19:53

Lies, damned lies, and equipment specs. I can print up a lovely sticker that says it'll output 1000A. Doesn't mean it'll output that.

It's significantly cheaper to make a low-power supply than it is to make a high-power power supply. Making the latter means you need a bigger transformer (with heavier windings), larger capacitor(s), larger inductor(s) and higher current rectifier(s), all of which add up to more money. If you're trying to make stuff on the cheap and aren't worried about pesky things like ethics, you'll go with the lower end components and massage the specs (by testing at low temperatures, etc. This is especially common with computer power supplies which specify output at 25C, even though they would actually operate at ~40C unless you're using them inside a refrigerator), use theoretical values rather than actually testing the product (saving money on QA), or just outright lie about the specs, the latter of which is probably the case with this charger, for reasons below.

One big red warning light that your charger is substandard is that the CE mark (which is used to indicate conformance to EU standards) is fake. The shape of the letters and their spacing is wrong. A real CE mark should have the letters each forming a half-and-a-bit circle and should link up if you continue the arc, as shown below. A cursory glance at the mark on your charger shows it looks nothing like this.

The CCC mark (indicating compliance to Chinese standards) is also fake.

Other warnings signs include having no manufacturer listed and improper capitalization (should be mA, not MA, unless they're claiming is can supply a billion amps.).

In essence, to determine if a charger is likely to be substandard, apply the same thoughts as you would for determining whether a product is counterfeit. The only real difference between a substandard product and most counterfeits is whether a manufacturer's label is forged or not.

• It's definitely cheaper to construct adapters from cheap parts that may not actually meet the stated specifications. It's also cheaper to construct an adapter and ship it without testing to be sure whether it meets the specs. May 29, 2013 at 16:52
• Nice details about the CE mark. Jun 16, 2013 at 12:05

Aside from the actual power provided, there's another variable: Whether the device RECOGNIZES that the charger is capable of putting out the rated amps.

Devices have some different strategies for recognizing what rating a charger has. Now when connected to a computer, it's easy - the numbers come across during USB enumeration. But when connecting to a dumb charger, it's harder, and you don't want to try to draw too much, as that will cause the charger's over-current protection to kick in and cut it off.

I know that Apple uses a specific set of resistors in their charges to produce specific voltages on the USB data lines. Their devices then look for those voltages and take them to mean that they can draw some larger amount of current.

The problem with this is that there don't seem to be a lot of standards on this detection - different devices seem to do different things.

I'd guess that your cheapo-charger doesn't do whatever the galaxy nexus requires to recognize that the charger is capable of 1 amp, so it doesn't try to charge at the higher rate.

• Actually, now there is a standard — USB Battery Charging spec available here. In particular, a Dedicated Charging Port must have D+ and D- lines shorted for detection, and the device is allowed to draw up to 1.5 A from such port, while the charger output voltage during operation must not fall lower than 2.0 V. May 28, 2013 at 15:51
• Same as @SergeyVlasov. The USB charging standard has been around for quite a while, every charger you get either follows it, or the standard that iphone uses to ensure you buy their chargers as often as possible. They are different and a charger can not be complaint with both. If the device is "iPhone compatible"(or whatever their term for it is) then it wont be USB compatible and your phone will barely trickle charge from it. Jun 3, 2013 at 19:12
• @Kortuk - The Apple thing isn't in the charger. It's the cable that has the resistors in it. If you've got the right cable, all quality chargers are interchangeable. Jun 3, 2013 at 20:07
• @Compro01 this does not match my knowledge of how it is implemented, but are you possibly talking about the new design cable? Jun 3, 2013 at 22:28
• It's not just "USB charging spec vs Apple spec". There are several other configurations as well. See the datasheet for the ti.com/lit/gpn/tps2511, which tries to interface with all of them, and righto.com/2012/10/a-dozen-usb-chargers-in-lab-apple-is.html, which reverse-engineers several. Feb 10, 2014 at 15:18

Just like resistors, chargers have error margins, it's just not written on them, simply because it should be negligible in the case of known brands. Your phone will not even charge if the charger is deviating a lot from the designated voltage. It's not even supposed to harm your phone because modern phones have fail safe measures in them. Now if the charger is having the right voltage, the speed of charging is dependent on the amount of current it passes. And here is where the problem lies with cheap chargers. Cheap components affect the amount of current passing through them.

If smartphone is taking longer with cheapy charger for full charge, it means that the cheapy is giving less power. There's no exception or loophole with that. Grab a multimeter and measure the output by your own.

In this case, the printed specs (5V/1A) may simply be wrong or there may be other things like cable, connector etc which are screwing the output.

• There can be one or two components to a charger; either you have a detachable USB cable or you don't. If you do, this might be caused by the cable, which means the printed specs could be 100 % correct. Both chargers look like they have detachable USB cables. May 28, 2013 at 8:29
• In addition to @bigbadonk420, if the device does not have data lines shorted your phone should only draw 100mA. Jun 3, 2013 at 19:14
• @Kortuk Not applicable here. Mind it, device is charging faster with one charger..
– iOS
Jun 3, 2013 at 23:30
• @SachinShekhar it is if they use a 1.8A usb spec charger and try to charge an iphone or vice versa. Jun 4, 2013 at 18:58

Even 10A charger will not fast that fast because Android recognized your charger as 500mah.

Workaround https://dangerousthings.com/shop/android-fast-charge-usb-cable/ Or by shorting DATA+ and DATA-

• Why should Android do that? 500 mA is the specification for USB 2.0. USB 3.0 can deliver 900 mA. And using a wall plug, 2.000 mA are not an exception. So your "because" stands very questionable. Can you provide any source for that statement? Moreover: Shorting DATA+ and DATA- sounds like a very dangerous idea to me, and might harm your device. What your linked cable does is simply "faking" A/C mode by letting the device think it's connected to a wall plug. Read the description there. The question is just comparing different wall plugs, so this shouldn't apply.
– Izzy
Jun 20, 2013 at 10:44