# Questions about Charging Speeds and Wattage

1. Feb 20, 2015

### musiliu

Hi, I'm not sure if this is a correct place to ask this, but i have a question about the charging speed for the usb chargers for phones and tablets.

from what I have gathered, the device being charged is the one that pulls the current from the charger, so if a 1 amp input device is charged with a 2 amp output charger, only 1 amp max is being supplied right?

But how come I read on lots of other websites that the wattage (voltage * amps) matters for how fast something charges? For example, it says a charger with 5v and 2 amp output = 10 watts and they say that will charge something faster than a charger with 5v and 1 amp output = 5 watts..
But I don't understand because I thought the device only pulls 1 amp if that is the maximum input current?
Are these websites wrong?

2. Feb 20, 2015

### phinds

You have in each case specifically stated what the output current IS, which make the supply capability irrelevant (assuming it can supply the stated current). Wattage is based on what is actually supplied, not what a source is capable of so if a device can supply a million amps but is only supplying 1 amp at 5 volts, then the power used is 5 watts.

3. Feb 20, 2015

### musiliu

Ok, so that means that those websites are wrong then? It doesn't matter how high the output current is if the maximum current input stays the same?
But how come people say their devices do charge faster if they use a charger with a higher output current?
For example, I read people say their iPhone that has 1 amp maximum current input charges faster with an iPad charger that's capable of outputting 2 amps?

4. Feb 20, 2015

### phinds

I can't even parse this. It makes no sense to me. You don't seem to get that a capability has nothing to do with what is actually supplied (given that the capability is greater than what it is required to supply). I don't think you understood my post at all.

Try to state, very briefly, exactly what it is that you think is going on, or what you think the problem is.

5. Feb 20, 2015

### musiliu

sorry, I am not sure how to explain in correct physics terms

I just want to understand how it is possible that a usb charger with an output capability of 2 amps can charge a device with an input capability of 1 amp significantly faster than a charger with an output capability of 1 amp? This is stated on multiple web sites talking about charging an iPhone with an iPad charger and they say it charges a lot faster...

This is why I don't understand, because the device can only draw a maximum of 1 amp from the 2 amp capability charger, why does the 2 amp capability charger charge so much faster than the 1 amp capability charger?

6. Feb 20, 2015

### phinds

OK, thank you. I now understand your question and I see your puzzlement and I share it.

I see absolutely no reason why it should work as you describe unless the charger with the 1 amp capability is slightly under capacity and the device being charged is requiring slightly more than one amp. Under those conditions, it's likely that a higher capacity charger would work noticeably better.

But if the charger can supply, say 1.1 amps and the device is only drawing 1 amp then I see no reason at all why substituting a charger with a 2 amp, or a million amp, capability would make any difference at all.

7. Feb 21, 2015

### davenn

Hi musiliu

OK the situation is that there are different rate chargers .... for example if you wanted to charge your phone from a USB port on your computer, it can only supply current at a maximum of ~ 500mA ( 0.5A) (At 5V ) There is circuitry in the computer that limits the current out of the USB port
But chargers that plug into a wall outlet often have a much higher output capability, anything from ~ 1A to 2A ( At 5V)
The "intelligent" charging circuit inside your phone is able to use this higher current capable charger and allow the phone battery to charge quicker ( at a higher rate)

Also keep in mind that the charging circuitry to charge the phone battery is different to the PSU circuitry that takes the battery supply and regulates it for use by the different parts of the phone

cheers
Dave

8. Feb 21, 2015

### phinds

Dave, I agree w/ what you are saying, but basically it boils down to your agreeing w/ me that the way he states the problem is not what's really happening.

9. Feb 21, 2015

### davenn

yeah, he's just not understanding the actual process

I was hoping my comments may have clarified it a bit ?

10. Feb 21, 2015

### phinds

Certainly clarified it for me. I've never thought about what amperage you can get out of a USB port and I have no idea about the intricacies of phone chargers