What decides usb amperage?

[orangpelupa]

So, what decides usb amperage?

I'm confused because

- official LG charger and Xiaomi power bank can charge at different amperage depending on the cable it use. Cheap local brands onky charge at max 500ma, while official Samsung usb cable at 800-1000mA.

- no brands usb charger can charge at max amperage at 1.3A, using any usb cables.

- charge only usb cable (no data connection) can draw 1.3A through official LG charger and xiaomi power bank.

Now frankly, I'm confused. What decides the amperage? The cables? The charger? The phone? All of them?

What exactly is used to decide the amperage? I don't think usb cable have a chip inside them...

Thanks

 

[Deleted member 13524]

An ideal charger will provide as much current as the electrical charge demands.
The electrical charge, in this case, are the battery management ICs, which determine how much current you can feed into a battery without causing overheating and/or a reduction of healthy charge cycles (i.e. the "lifetime" of a battery).

This is obviously limited to how high is the current that the charger may provide. Since there always has to be a rectifier circuit in wall chargers (to go from alternate current to direct current), the maximum current output at 5V depends on the quality of the components that are used in it.That's why chinese knock-offs can only output 500mA and overheat/blow-up while Samsung's can output a lot more without getting even warm. It the same principle as seen between bad quality and good quality power supplies.


That said, AFAIK some "charge-only" usb cables without data connection have one of the data pins short-circuited to the 5V pin. This "tells" the smartphone's battery IC that the cable can pass a higher current because there's no "power" being spent on sending/receiving data.

 

[orangpelupa]

@ToTTenTranz

Yeah you are correct I think. Git similar info here

https://www.maximintegrated.com/en/app-notes/index.mvp/id/5801

It's actually part of usb standard to short the data line.

Doesn't this means USB charger ideally already have their data line shorted on their end?

I think that's why my Chinese no brand usb charger can charge at higher amperage than official LG.

Official LG charger is set in the factory to only charge St max amperage when married with official LG usb cable with proper resistor resistance.

 

[Blazkowicz]

afaik USB starts at only (up to) 100mA, which is well enough for keyboards, mice, gamepads etc. then it is negociated to (up to) 500mA or whatever is supported.

I think there's a really tiny CPU in every USB charger/PSU just for that.
A buddy has a charger that went really slow, I have a theory that it's stuck at 100mA.
I might be terribly wrong thinking of it : if only the power pins are connected then no negociation can take place? So the stuff will default to 500mA and then there's a lousy de facto standard to allow 1A depending on "impedance" measured on data pins. But some vendors don't agree or some provide for 2A hence special "charging ports" or chargers.

I have some cheap mobile stuff : apparently the vendors decided to support 500mA only - this include e-cig batteries.
Note that there are dumbphones with USB. That works fine with 500mA : charging for only 10 minutes gives you some useful battery life if you're in an emergency.

Cheap smartphones with USB C will be a better way forward I think : 3A ought to be the new minimum. That would put higher constraints on cheap ass chargers and cables though. USB 2.0 with no USB 3.0 or 3.1 support is also perfectly "legal" with USB type C.