# USB Chargers **URL:** \ **Category:** Factual Questions\ **Created:** [September 9, 2012, 10:33am UTC](https://boards.straightdope.com/t/usb-chargers/634176 "2012-09-09T10:33:29Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [September 9, 2012, 10:33am UTC](https://boards.straightdope.com/t/usb-chargers/634176/1 "2012-09-09T10:33:29Z")
Short version: are all USB power bricks intercompatible? Long version: my wife and I have cell phones whose chargers are USB cables. One end plugs into the phone, and the other end - a regular USB - plugs into a brick, which then plugs into the wall outlet. My wife also has a Kindle that only came with a USB cord, so to charge it, she has to plug it into her computer. I assume that a USB port is a USB port is a USB port, so if we were to, say, plug her Kindle charger into the brick that came with the cell phones, we could then charge the Kindle from the wall. Is that correct? Is there any kind of device-specific stuff going on in those bricks, so that the Kindle would get the wrong voltage or wattage or flavor of reticulated splines or whatnot? I’m starting to see house outlets with USB ports, so I’m guessing it would be fine, but I wanted to double check before I explode the Kindle. Thanks. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [September 9, 2012, 10:41am UTC](https://boards.straightdope.com/t/usb-chargers/634176/2 "2012-09-09T10:41:49Z")
Sort of. You won’t damage anything. You will find that some bricks have a maximum current output that is too low for your device, and your device will refuse to charge off them. Won’t damage anything though. ---
**Author:** ![percypercy](https://avatars.discourse-cdn.com/v4/letter/p/8baadc/32.png) [@percypercy](https://boards.straightdope.com/u/percypercy)\ **Post date:** [September 9, 2012, 2:29pm UTC](https://boards.straightdope.com/t/usb-chargers/634176/3 "2012-09-09T14:29:54Z")
My Kindle Fire will charge off my cell phone’s charger, but it is much slower to recharge using the cell phone charger. ---
**Author:** ![DrCube](https://avatars.discourse-cdn.com/v4/letter/d/a3d4f5/32.png) [@DrCube](https://boards.straightdope.com/u/DrCube)\ **Post date:** [September 9, 2012, 3:05pm UTC](https://boards.straightdope.com/t/usb-chargers/634176/4 "2012-09-09T15:05:59Z")
Yes, they are all the same voltage. Maximum currents vary, but that just means it might take longer to charge. The only time I had to worry about it was with my Raspberry Pi mini-computer, which is powered from a USB cord. So if you actually need it to operate a device, like a computer, you might need more current. But if you just need it to charge something, it shouldn’t matter. ---
**Author:** ![Dog80](https://avatars.discourse-cdn.com/v4/letter/d/6bbea6/32.png) [@Dog80](https://boards.straightdope.com/u/Dog80)\ **Post date:** [September 9, 2012, 3:09pm UTC](https://boards.straightdope.com/t/usb-chargers/634176/5 "2012-09-09T15:09:45Z")
You can take a typical microUSB cable and short the DATA+ and DATA- wires on the phone’s side. This forces the phone to enter into fast charge mode. ---
**Author:** ![AaronX](https://avatars.discourse-cdn.com/v4/letter/a/7bcc69/32.png) [@AaronX](https://boards.straightdope.com/u/AaronX)\ **Post date:** [September 9, 2012, 3:24pm UTC](https://boards.straightdope.com/t/usb-chargers/634176/6 "2012-09-09T15:24:01Z")
Some devices are more picky. My kindle will charge off anything I’ve tried. Smartphones are more choosy. Tablets are the worst, eg the iPad won’t even charge off some computers. I think the Samsung Tab is similar. ---
**Author:** ![Digital\_is\_the\_new\_Analog](https://avatars.discourse-cdn.com/v4/letter/d/a88e4f/32.png) [@Digital\_is\_the\_new\_Analog](https://boards.straightdope.com/u/Digital_is_the_new_Analog)\ **Post date:** [September 9, 2012, 7:48pm UTC](https://boards.straightdope.com/t/usb-chargers/634176/7 "2012-09-09T19:48:19Z")
> [@Dog80](#): > > You can take a typical microUSB cable and short the DATA+ and DATA- wires on the phone’s side. This forces the phone to enter into fast charge mode. I’m not sure this is true in the general sense. I know of some old devices that don’t function this way, back when the uUSB spec was fairly new. Also, what are you suggesting to plug it into? A laptop may actually enforce USB spec, which would require it only source 100mA before enumeration. Attempting to draw more can cause the laptop to shut the port down - I’ve seen it happy plenty of times. Most desktops seem to connect the USB power directly to a 5V output on the power supply with no limit in place, so you may get away with it there. -D/a ---
**Author:** ![Michael63129](https://avatars.discourse-cdn.com/v4/letter/m/3da27b/32.png) [@Michael63129](https://boards.straightdope.com/u/Michael63129)\ **Post date:** [September 9, 2012, 10:18pm UTC](https://boards.straightdope.com/t/usb-chargers/634176/8 "2012-09-09T22:18:45Z")
> [@DrCube](#): > > Yes, they are all the same voltage. Maximum currents vary, but that just means it might take longer to charge. In at least some cases, nonstandard voltages may be used, as reported in a [recent thread](http://boards.straightdope.com/sdmb/showthread.php?t=663845) about a 12 volt USB adapter, although it isn’t clear if it was a standard USB adapter or a 12 to 5 volt adapter for a car (in any case, in post #4 it is mentioned that 12 volt USB adapters do exist). But most of them output 5 volts with current the only concern (note, some AC to USB adapters have no internal control over output current, being just a basic 5 volt SMPS; overloading these will cause them to enter a shutdown-restart cycle, if it isn’t so cheap that there is no overload protection). ---
**Author:** ![Dog80](https://avatars.discourse-cdn.com/v4/letter/d/6bbea6/32.png) [@Dog80](https://boards.straightdope.com/u/Dog80)\ **Post date:** [September 9, 2012, 10:20pm UTC](https://boards.straightdope.com/t/usb-chargers/634176/9 "2012-09-09T22:20:24Z")
> [@Digital\_is\_the\_new\_Analog](#): > > I’m not sure this is true in the general sense. > > I know of some old devices that don’t function this way, back when the uUSB spec was fairly new. > > Also, what are you suggesting to plug it into? A laptop may actually enforce USB spec, which would require it only source 100mA before enumeration. Attempting to draw more can cause the laptop to shut the port down - I’ve seen it happy plenty of times. > > Most desktops seem to connect the USB power directly to a 5V output on the power supply with no limit in place, so you may get away with it there. > -D/a From [here:](http://en.wikipedia.org/wiki/Universal_Serial_Bus#Charging_ports_and_accessory_charging_adapters) > [@](#): > > A portable device can recognize the type of USB port from the way the D+ and D- pins are connected. For example, on a dedicated charging port, the D+ and D- pins are shorted. With charging downstream ports, current passing through the thin ground wire may interfere with high-speed data signals. Therefore, current draw may not exceed 900 mA during high-speed data transfer. I have a Galaxy Nexus and a heavy duty 2 port car charger that can provide 2 Amps max. However my phone was charging too slowly. If I had the satnav application turned not only it wouldn’t charge, but it would actually discarge, albeit at a slower rate. I did the shorting trick with a spare USB cable and boom, the phone charges as fast as with the AC charger! I later found that with the [Franco Kernel](https://play.google.com/store/apps/details?id=com.franco.kernel) there’s a fast charge option that doesn’t require a special cable. If it is turned on, USB data transfer is disabled. ---
**Author:** ![Digital\_is\_the\_new\_Analog](https://avatars.discourse-cdn.com/v4/letter/d/a88e4f/32.png) [@Digital\_is\_the\_new\_Analog](https://boards.straightdope.com/u/Digital_is_the_new_Analog)\ **Post date:** [September 9, 2012, 10:51pm UTC](https://boards.straightdope.com/t/usb-chargers/634176/10 "2012-09-09T22:51:16Z")
That works great if the device chooses to differentiate between a USB connection and a charger only by checking the status of D+/D- lines. I worked on at least one phone at my last company that used the USB ID pin of the enhanced uUSB connector as a resistive load to determine the rate of the charger. If you had D+ and D- shorted together, but no appropriate load on USB ID, it was rejected as an invalid charger. Again…it’s device specific. It’s likely to work in many cases, depending on your device and what you are plugging it into. -D/a