When charging, I know that USB socket that you plug your microUSB cable into will charge your device at SOME rate. But that rate varies wildly, depending on the original source of the power (high quality mains converter, low quality mains converter, laptop, desktop, foreign mains via a travel converter, etc.) and the rate can vary from "will quickly charge even if device is being actively used with wifi", to "will charge quite slowly, and only if the device is airplane mode", to "will charge over the course of many, many hours if don't use the device at all".

Trouble is, it's all but impossible to tell which of these cases a source is, without leaving it for a bit to find out.

Are there any devices out there (or apps, I suppose?) that I can plug the microUSB end of the cable into, which will give me a numeric value (presumably just the current?) for the charging power that the source is offering. (Sorry my physics is enormously rusty so I've probably used the wrong term in there somewhere.)

I've asked the exact same question for apple's chargers, in the apple.stackexchange site, here.


closed as off-topic by Samuel, Andy aka, Leon Heller, Ricardo, PeterJ Jun 20 '15 at 1:22

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  • 2
    \$\begingroup\$ Please don't cross post questions. This is a question either for shopping or the existence of an app, I'm voting to close it here. Your assumptions are not right. A wall adapter will output the current specified on the adapter. The output from a computer USB port will depend on the version of the USB and the ability for the device to properly request a higher current. If you want to test the current, use an inline multimeter. \$\endgroup\$ – Samuel Jun 19 '15 at 18:20
  • \$\begingroup\$ You can buy these: dx.com/p/… \$\endgroup\$ – pjc50 Jun 19 '15 at 18:50

Unfortunately you've more-or-less asked a shopping question, which is off-topic here.

However I'm going to try to give an on-topic answer as if you had asked an electronic design question because its something I had to figure out for myself recently and I think it may be useful to others.

As you have recognized, different USB charging devices are capable of supplying different charging currents to whatever USB device is plugged into them. Some far more or less than others.
For example, something that's plugged into a PC's USB port should not expect to be able to draw more then 100mA initially, and later up to 500mA if it asks nicely during enumeration and the PC allows it.
Dedicated chargers however can routinely supply in the region of 2A without struggling.

Some manufacturers have designed their devices to only charge properly when connected to their own chargers, and to only trickle-charge when connected to anything else. My old iPad2 is a good example of this. There is likely a good reason for this and I would guess that at the time the iPad2 was designed, higher-current charging over USB was an unusual idea, so Apple came up with their own method of detecting when it was plugged into a capable charger and it was therefore safe to draw more current.

So if you're designing a new USB device which needs to charge an on-board battery, how does your device know how much current it can draw from the charging device (PC/charger/whatever) its plugged into?
These days there are some standards and methods which can help us here - some more standard than others and some specific to particular manufacturers.

The most common way for a charging device to identify itself is by using particular combinations of resistors on the USB D+ & D- data pins.
But detecting all of these possible combinations without interfering with the normal operation of the USB data lines manually could be tricky. The drawing below is taken from the datasheet of a MAX14578E which is an IC designed specifically to detect & interpret these configurations and shows some of the many possibilities. Maxim Integrated MAX14578E USB resistors example This particular IC does all of the detection for you and then either reports the results over I2C or sets the state of a few of it pins which you can use to control the current limit of your battery charger circuitry. It then passes the USB signal lines through to your devices USB transceiver for normal USB comms to continue.

Here is another snip from the same datasheet: Maxim Integrated MAX14578E Detection Results

It shows the list of possible charger devices which this particular IC is able to detect and how much current you should expect to be able to draw for the special or dedicated chargers.
Once you have this information its up to you to design your charging circuit to draw only as much current as its allowed to.


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