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DQOTD: USB Charging Do's & Don'ts?
Old 03-17-2014, 10:27 AM   #1
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DQOTD: USB Charging Do's & Don'ts?

Like everyone else, we have too many charging cables for mobile devices. Just got a camera this weekend that came with a USB charging cable but no 120V adapter. My sparky skills are sketchy but...if I want to charge the camera using a wall outlet, any reason I can't just use one of the chargers we already have instead of buying another just for the camera (an LG or Apple brick WITH the supplied camera cable)?

For example we have iPad USB charge bricks (5.1V 2.1A), LG USB phone chargers (5.1V 0.7A) and a cheapo 12V socket charger (5V 1A).

Obviously I don't want to damage a device/battery, so am I safe to use any of the chargers above? A range of voltage/amp ratings that I should not exceed (like a 20A rated for example)? I assume the amp rating will influence how long it takes to charge.

My primary question is about wall charging, but anything I should watch for in using any of the USB3 or USB2 ports we have on the desktop, laptop, monitor or car (haven't found output ratings for any of them - yet).

TIA!
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Old 03-17-2014, 11:01 AM   #2
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Any USB port should work, in general, though Apple requirements can be a little different. That's the idea of using USB. Make sure the USB port supports the number of amps the device would like. If the device indicates it is charging, you should be in good shape.
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Old 03-17-2014, 11:34 AM   #3
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Simple answer: You are very, very unlikely to hurt anything (* see caveat below), so just try it.

More complete answer: As Animorph alluded too, Apple set things up a bit different, and didn't go to the USB board to drive them as standards. This may or may not affect some specific devices.

Apple chargers place some resistors on the data lines of their chargers, and the combination of connections and values 'tell' Apple devices how much current the charger can supply. Non-Apple devices may not recognize this 'key'.

Non-Apple chargers place a short across the data lines to indicate that the charger is a 'wall charger' and can provide fast charging (I forget the spec, but at least > 500mA). IIRC, this is an actual USB standard now.

I have seen notes on the web to take chargers apart and short those lines so an Android device will go into fast charge mode. Our Samsung smart phones would not fast charge on our USB cigarette lighter adapters (the charger used the Apple technique).

Simple Solution: Instead of taking the charger apart, I took a generic cable, sliced it near the middle, cut the two lines that were not black or red (black/red are +5 and ground, the other two are the data lines), tied them back on the charger side, and shorted them together on the phone side. Now, with that cable (I marked it "CHARGE ONLY"), an Android phone sees it as a high rate charger and all is well. W/O that change, the Android phone fell back to slow charge mode. If I use a standard cable, I can use it with any Apple device, so that is more flexible than modifying the charger itself - and easier, IMO.


(* caveat) - it's possible that the device may try to draw more current than the charger is rated for, and that might damage the charger. But even that is unlikely, the charger will generally just deliver what it can, and the device will deal with it. In some cases, if the device really needs more current, it may shut down the charging operation, or the charger will shut down, so no damage will be done, it just won't charge. Or the device will just shift down to a slower charge rate. Hard to predict, easy to try.

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Old 03-17-2014, 11:44 AM   #4
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Here's the pertinent info from my notes:

USB - Wikipedia, the free encyclopedia

Quote:
Charging ports exist in two flavors:*charging downstream ports (CDP), supporting data transfers as well, and dedicated charging ports (DCP), without data support.

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. A dedicated charge port may have a rated current between 0.5 and 1.5*A. There is no upper limit for the rated current of a charging downstream port, as long as the connector can handle the current (standard USB 2.0 A-connectors are rated at 1.5*A).

Before the battery charging specification was defined, there was no standardized way for the portable device to inquire how much current was available. For example, Apple's*iPod*and*iPhone*chargers indicate the available current by voltages on the D- and D+ lines. When D+ = D- = 2V, the device may pull up to 500*mA. When D+ = 2.0*V and D- = 2.8*V, the device may pull up to 1000*mA of current.
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Old 03-17-2014, 12:53 PM   #5
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This is totally anecdotal, but my Fitbit started acting up after I took it on a trip where I didn't bring my PC along. I charged it via one of those USB wall chargers instead, and it was never the same after that. I don't know if it was a coincidence or not.

I've read that the PC USB ports charge at a lower current than wall chargers, so best to read your manual and see if the specs of your wall charger are compatible with the new device to be on the safe side.
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Old 03-17-2014, 01:06 PM   #6
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Just looked and the camera website says input 5V 1A, so I'd have to think the LG phone (0.7A) and 12V socket charger (1A) should be OK. Maybe the Apple brick too, but since it outputs 5V 2A, I'll just use the two that match more closely.

Thanks folks!
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Old 03-17-2014, 02:41 PM   #7
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Quote:
Originally Posted by Midpack View Post
Just looked and the camera website says input 5V 1A, so I'd have to think the LG phone (0.7A) and 12V socket charger (1A) should be OK. Maybe the Apple brick too, but since it outputs 5V 2A, I'll just use the two that match more closely.

Thanks folks!
I think you have that backwards.

If the camera says it wants/needs a 1A input, then you want a charger that is rated at 1A or higher.

For example, the Apple brick you say is rated at 2A - that means it is capable of providing up to 2 amps. But it will only provide as much as the device draws.

Just like your 110V AC outlet is rated/fused at 15A. It does not 'force' 15A into anything you plug in - devices draw what they will (a function of their design and the voltage supplied). You would not plug in a 20A draw device into a 15A plug 'to be safe'. It's just the opposite, you plug lower draw devices into sources with higher capability.


Quote:
Originally Posted by Spudd View Post
...

I've read that the PC USB ports charge at a lower current than wall chargers, so best to read your manual and see if the specs of your wall charger are compatible with the new device to be on the safe side.
Actually, the USB spec is a little fuzzy in this area, unless they've updated since the last time I checked, and I have not read up on the latest USB 3.0 power specs. There are specs on the minimum current that a port must be capable of in different modes, but nothing on the maximum (but as I stated above - a device will draw what it will draw, the max is a 'capability').

When a device is connected to a USB port, the spec says that it must not draw more than 0.1A (100mA) initially. Then it 'asks' if the port is a high or low power port. If the port identifies itself as a high power port, it means it can supply at least 0.5A (500mA). If it isn't a high power port, then that device either must stay on a 0.1A max current mode, or shut down if it requires more than 0.1A for normal operation.

Wall chargers don't have the 'smarts' to 'talk' to the device, so they just use resistors (Apple) or a Data line short (everyone else) to tell the device that it can deliver more than 0.5A. How much more is not spec'd as far as I know.

But once it sees it is a high power port, I don't think it cares. I know the Samsung will identify a USB charge versus a 'wall charger' in its status, but I don't think the currents are different.

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Old 03-17-2014, 02:54 PM   #8
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More info here:

https://en.wikipedia.org/wiki/USB#Power

And a little note on my comments on "high power" and "low power" ports in the previous posts. This will tie things together a bit:

So your computer will be a 'high power' port (at least 0.5A capability). Note that if you plug in a USB hub, you can find 4-port hubs that do not require any external power supply. Each of those 4 ports will identify themselves as low power ports. See, they get their power from the computer, so the 0.5A the computer supplies gets 'shared' across the 4 ports which can each supply 0.1A. So 4 ports at 0.1A each is 0.4A total and that allows 0.1A of 'overhead' to run the electronics in the hub.

For that 4-port hub to supply 'high power', it needs to have an external supply. And those supplies will be rated at least 2.0A to provide 4x0.5A to those ports.

Make sense? Or TMI? Or both?

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