Remedial electronic engineering filter: how does USB charging work?
January 28, 2015 3:17 PM   Subscribe

I'm trying to set up several micro USB charging stations in my house and car, and I'm realizing that I retain very little from 9th-grade Intro To Electronics. Set me straight on how current works?

Suppose I have a 2.4 amp USB charging cable plugged into the wall.

- Am I right to understand that the practical effect of the amperage is that it determines the speed with which devices will charge? Will it charge a device twice as fast as a 1.2 amp charging cable would?

- If I attach a Y-shaped splitter to the charging cable so as to charge two devices at once, does that cut the current in half, causing each device to charge half as quickly? Or does it have a different effect?

- If Device A is plugged into one side of the Y-shaped splitter, which is plugged into the charging cable -- does the presence or absence of a Device B on the other side of the splitter affect the charge received by Device A?

- Why might it be the case that a device can charge when plugged directly into the charging cable, and can charge when plugged into the charging cable through a single Y-shaped splitter, but cannot charge when plugged into the charging cable through a succession of two Y-shaped splitters?

- Do all the same rules apply if I'm using a car charger rather than an outlet?

- Am I right to guess that the circuit created by the succession of Y-shaped splitters is in parallel rather than in series?

Thanks in advance!
posted by foursentences to Technology (7 answers total) 1 user marked this as a favorite
 
- It depends. Charging is largely managed by the device, partially because you have to be careful with LiIon batteries how fast you charge them based on where they are in their charge cycle. On early Apple devices, they figured out how much current they could draw based on a resistor set across the data lines. More modern devices (ie: my Samsong S4) seem to be able to sense how much is available before voltage from the supply starts to drop and charge based on that (ie: It'll charge a lot faster off of a 3rd party power supply than my iPad), but as it gets closer to battery full it's going to draw a lot less current.

- Charging will be split between how much each device thinks it can pull. Presumably if you draw all 2.4A you'll start to see some voltage drop, and one device may react to that differently or be at a different place in the charge cycle. What you know is that the charger will deliver max 2.4A to that Y.

- One device plugged in and that Y cable will be like a straight cable (with some weird capacitance on the dangling lead, but we're talking DC so that doesn't matter).

- See the above about how devices sometimes try to communicate with their chargers via the data cables. Multiple Y cables and shared data lines might get those devices confused.

- Yes.

- Yes.
posted by straw at 3:34 PM on January 28, 2015 [1 favorite]


It depends on the behaviour of the charging units. Some (most?) will try to charge at full current, and if the current can't be supplied, they'll cycle on and off, barely charging at all. Few will detect the reduced voltage and automatically reduce current proportionally.

So if you have two 1.2A devices (or anything adding up to 2.4A or less), you'll be fine. But one 1.4A and one 1.2A, and one or neither is going to charge much, just depends on the exact behaviour of each device and how they affect the other.

Most things charge at a constant current, though different resistors can be used to set that current.
posted by flimflam at 3:37 PM on January 28, 2015 [1 favorite]


Some devices will also attempt to negotiate with the charger to get the optimum current output. Some (including many Android phones) won't charge at their fastest speed if they are plugged into a standard charger with a standard USB cable. You can use charge-only cables (with no data wires) to bypass negotiation and force them into full-speed charging.
posted by mbrubeck at 3:49 PM on January 28, 2015 [1 favorite]


- If I attach a Y-shaped splitter to the charging cable so as to charge two devices at once, does that cut the current in half, causing each device to charge half as quickly? Or does it have a different effect?

If both devices pull the maximum current available to them, yes. In practice there's a maximum that most devices will use (typically 1.5A). So yes, it's a parallel circuit.

- Why might it be the case that a device can charge when plugged directly into the charging cable, and can charge when plugged into the charging cable through a single Y-shaped splitter, but cannot charge when plugged into the charging cable through a succession of two Y-shaped splitters?

Some devices are sensitive to the impedance of the charging cable (that is, the slight voltage drop between the charger and the device). iPhones in particular are notorious for this.

Generally the solution to that is "don't do that". Use a powered USB hub instead.

- Do all the same rules apply if I'm using a car charger rather than an outlet?

Yes, the power coming out of the USB port is the same (5V regulated), barring differences in amp rating.
posted by neckro23 at 4:00 PM on January 28, 2015 [1 favorite]


- Am I right to understand that the practical effect of the amperage is that it determines the speed with which devices will charge? Will it charge a device twice as fast as a 1.2 amp charging cable would?


The current rating is the maximum the charger CAN supply, and that means it CAN potentially charge a device twice as fast as a charger rated at 1.2A, however the current actually drawn is determined by the device being charged, so if a particular device draws a max of 1A when charging, connecting it to a 1.2A or 2.4A charger will make no difference.

Within the rating limits of the devices in question, you can think of the voltage as being determined by the power supply, and current being determined by the power consumer.
posted by HiroProtagonist at 6:57 PM on January 28, 2015 [1 favorite]


Do Y-cables really work with USB chargers? I'm an electrical engineer, and I can think of a couple ways that might blow stuff up, or at the very least simply not work.
posted by intermod at 7:10 PM on January 28, 2015 [1 favorite]


Stuff is very finiky when charging, and I wouldn't be surprised if you used a Y cable and nothing happens, even though it should. You'll be in much better position just having multiple USB ports to charge from.
See the wirecutter for a car charger, and something like this for the house (office). I haven't used that plugable product, so others may have a better recommendation.
posted by defcom1 at 12:12 PM on January 29, 2015 [1 favorite]


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