USB is supposed to supply 100mA and negotiate up to 500mA but pretty much all computers output 500mA even without negotiation. In theory what should happen is you connect something to your computer and it is powered at 5 volts at 100mA then it tells your computer that it will need a LOT more power like 500 and the computer checks and says "sure I have enough" or "sorry maxed out something else is already using 100mA"
But nothing does that.
Some USB ports have a little + symbol near the USB logo, those should output 500mA regardless and sometimes even more. Others max out at 100 or even something strange like 350mA. It all depends on the device and that explains why some things just don't work when you think they should. Ever plug in a USB hard drive only to have the light blink?
Now if you look up USB charging http://en.wikipedia.org/wiki/Universal_Serial_Bus#Mobile_device_charger_standards
You will see many different "standards" USB has a few common elements. You probably already know USB has four connectors: 5 volts, Data+, Data-, ground (negative).
China joins data+ and data- together.
Apple sends something like D+ = 2 volts, D- = 2.8 volts.
Some companies ignore it.
Some phone companies like Motorola use a resistor on the end of the cable that connects to the phone (the side with mini/micro USB has 5 pins) and typically only use a passive component like a resistor set at 68k to prove it is a genuine cable. Without it the phone will say incompatible charger.
Great. Knowing all this means I should be able to come up with a combination to charge my phone at the full 1000mA current rather than 500mA. Or does it?
Charger 0) Computer USB port - 5v 500mA
Charger A) Samsung USB charger - 5v 700mA
Charger B) Motorola USB charger - 5v 850mA
Charger C) HTC USB charger - 5v 1000mA
Charger D) Galaxy S4 charger - 5v 2A (2000mA)
Charger E) Apple iPad charger - 5v 2A (2000mA)
Logically they would charge from slowest to fastest in order of amperage. By the way amperage is like the amount of power something can provide. We have all heard of voltage; a car battery is 12 volts, your outlet is 120 volts, a AA battery is 1.5v, and a 9v battery is 9 volts. Why can't you use 2 AA batteries and a 9v battery to start your car? Because they aren't able to deliver enough amperage. I don't understand this well enough to explain it but I will say too much voltage will break things, while more amperage is usually safe because the device only uses as much as it needs. So when you connect an AC adapter to your laptop if it says 19vdc - ---(o--- + 2A and your original adapter says 19vdc - ---(o--- + 1.2A it will be safe to use.
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| Didn't need to chop the cable in half, just needed to sever the power line so I can put the meter in between |
So I chopped a USB cable in half and wired it so I could measure the charge speed.
Charger 0) 450mA
Charger A) 600mA
Charger B) 350mA
Charger C) 350mA
Charger D) 350mA
Charger E) Didn't have to test
What happened?
Well the first two made sense; the computer negotiated a reasonable charge current and began to charge and the Samsung charger could "communicate" to the phone that it can deliver that charge speed.
The Motorola and HTC chargers might have used a different "standard" to indicate the "high power" charger status as I would expect with the Apple charger but it's hard to imagine both of them would do that.
The one that completely threw me was the S4 charger which only charged the phone at 350mA the same slow speed it charged with my other chargers and ultimately slower than a desktop USB port. Why that happened is up for debate and research. It's possible my USB charging cable has a resistor set in it to indicate it is a "data" cable, the cable could be too long, my test set up could have been interfering with the results because I had to cut the 5 volt power line and connect my meter in between the severed cables. (See diagram)
In conclusion I do not know why it charges slowly.
Now I shall shop for a new battery because I think the used battery that came with my phone has seen better days.
