USB chargers are tricky. Or at least they are when you don't follow the standards.

I have a device that uses 5V to charge a battery. The device has a USB socket, with the idea of connecting it to any USB power supply (maybe USB power supply is not the best way to call them, but I mean a 5V DC wall wart that has a USB port, whether that wall wart adheres to USB specs or not regarding power delivery, I don't care as long it outputs 5V), but doesn't do any handshaking to ask for power. I was wondering what will happen if you use a charger from brand X or Z, will it output 100mA? will it output 500mA? will it provide as much as it is rated (e.g. 2A on an iPad charger)?

My question is, given that different manufacturers seem to use different ways to get more than 500mA from their chargers (at least to my knowledge) how can I get the maximum current a power supply can provide without needing to negotiate the power delivery? I was thinking in using an active load, but I don't know if that's the best way to test it. Is there any considerations I need to keep in mind to do the test?

I would need to get the active load, as I don't have any at the moment to test it myself and I wanted to ask first before purchasing it.

EDIT: The question seems to be too open to interpretations, but I don't know how to word it while keeping it short, sorry. I guess my question is: when using a charger that provides more than the usual 500mA, and that uses a proprietary system to allow the drawing of more current (e.g. a tablet charger from a reputed brand), what should my load by in order to extract the maximum current from it, without having to use ICs to do any kind of negotiation? Can I just connect a variable resistor and check when it starts to drop voltage? Can I get 2A from tablet charger that is obviously capable of doing it, without requiring whatever resistors it needs in its data lines?

I appreciate the answers given so far, as they are useful, but not exactly what my (terribly worded) question was looking after. I don't want to know how to measure that current, I know I can use one of the many flavours of ampmeter available, and I don't want to know how to do the negotiation to get more power. I want to know how to set up my test so if I get X amps, that's the maximum I can get from the charger without trickery.

  • \$\begingroup\$ Are you familiar with the USB dedicated charging port standard? That's good for up to 2A, and many (though by no means all) chargers are mostly compliant. And it's much better than the 100mA you get from a PC port without negotiation. \$\endgroup\$
    – Jack B
    Apr 24, 2017 at 11:28
  • \$\begingroup\$ Most modern Wall-wart type USB chargers will provide at least 1.5A, none should provide more than 5A as that is defined as a safety limit in the USB specification. If a charger is sold as a general charger then they typically do not require you to enumerate to request the current you want. If a charger for a specific device it would be within its rights to insist on enumerating before providing any power but I suspect few actually do. The only charger I have seen that did was for an old Motorola phone (now no-longer manufactured) \$\endgroup\$ Apr 24, 2017 at 12:20
  • \$\begingroup\$ In the case of USB power supplies, there are charge detection IC's that figure it out for you. Unfortunately there are several different ways that USB chargers signal to devices what they are capable of supplying more than 500 mA. So it is probably best to use the IC rather than try to figure it out on your own. One example: BQ24392. \$\endgroup\$
    – user57037
    Apr 24, 2017 at 15:43
  • \$\begingroup\$ If you are testing in a lab, you can try the active load. If you are designing a device which needs to know how much current is available, see my previous comment. But you can also probably figure it out by inspecting the voltage on D+ and D-, and also testing to see if D+ and D- are shorted together. These are the ways that the power supplies inform the device how much current is available. \$\endgroup\$
    – user57037
    Apr 24, 2017 at 15:46
  • 1
    \$\begingroup\$ I checked the BQ24392, a very interesting IC, indeed! \$\endgroup\$ Apr 25, 2017 at 16:37

2 Answers 2


The way to check the actual output is with an inline volt/ammeter.

There are several kinds, so I will pick one at random from eBay as an example: USB Tester

enter image description here

This one simply measures the instant V/A, but there are others that also take time into account to tell you the total amount of charge in mAh.

Some of the vendors can even spell in English correctly. ;-)

  • 1
    \$\begingroup\$ Yeah, it's not exactly what I was looking for, but the thing looks useful nevertheless to measure currents. I might get one of those! Thanks! \$\endgroup\$ Apr 25, 2017 at 15:30
  • \$\begingroup\$ The best part about using one of these is that they show you the actual draw (the load) that the device(s) are putting on the charger. This kind of unit has 2 USB ports on it for devices so you can get a good idea of your peak loads. And it answers your title question precisely. \$\endgroup\$
    – SDsolar
    Apr 25, 2017 at 18:02
  • \$\begingroup\$ Sorry, I changed my question. I didn't wanted to ask "how do you measure current", but "how do you set your test up in order to make sure that the current that you are getting is the right result, that your power supply is not capable of giving you more current to a dumb load without having to do enumeration or any other complex stuff" \$\endgroup\$ Apr 27, 2017 at 10:31
  • \$\begingroup\$ This same setup will definitely show what is being provided. But in general, there is nothing like a dumb load. The only exception to that, that I can think of, is a failure in a Lithium battery charger. And you will see excessive current on this meter. But if you don't have a meter then the first thing you look for is excessive heat coming from the battery. \$\endgroup\$
    – SDsolar
    Apr 27, 2017 at 14:21

You ask, "how can I measure the maximum current a power supply provides"? This is an ill-posed question. A power supply can provide the maximum current that it is designed to, which is usually written on the power supply case. So you don't need to measure anything in the first place. This is maximum guaranteed, without overheating or dropping designated voltage level. However, you indeed can research if the particular PSU meets it's advertised specifications and measure all parameters using known loads and just measure output voltage.

Another misconception is expressed in your comment as "or whatever the charger thinks it's best". Chargers do not "think", they just "supply" whatever they can within the limit of their "advertised" capability, which they advertise via various "signatures" on D+/D- lines, many variants of explanations can be found here. It is a function of device (or your charging circuit) recognize the signature and to take the current that does not exceed "provider" capabilities. If your device can't recognize the charger's signature, it should limit its consumption to bare minimum of 500 mA.


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