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I've just bought a portable battery, but the portable battery don't comes with an adapter for the house current; it only comes with a USB cable so I can charge it with my laptop. My mobile phone came with a charger, and I want to know if is safe to use that charger with the battery without problems, even though the output has a higher amperage. These are the specifications:

Battery input: 5V, 1A
Charger output: 5V, 2A

Will the output only give the quantity that the input is requesting, or will the input overheat because it cannot handle all the current it receives?

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up vote 27 down vote accepted

Yes, it is absolutely safe to charge a device with a charger that has more current capacity than needed.

Ohm's law tells us the relation between current, voltage, and resistance:

    I    =     V      /     R  
(current =  voltage  / resistance)

Since the voltage is held constant (5V), the only factor that determines current draw is the load (another term for resistance) the device places on the charger. Thus, the device will only draw as much current as it needs and no more.

Speaking from personal experience, I've had no problems charging my phone (which only draws 700 mA) with my Kindle charger (850 mA) or my iPad charger (2.1 A).

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Perfect answer. Thank you! :) – Jorge Fuentes González May 27 '13 at 13:42
Now, adding a bit of info to this, if my phone can 'load' up to 2A and my charger ouput is 1A, will only give 1A or will overheat? As I can see, if the resistance is reduced, it will carry more curent, so supposedly will carry more than 1A, right? – Jorge Fuentes González Jul 14 '15 at 15:30
A power supply (what you're calling the "charger") rated for 1A can only provide up to 1A and still operate within spec. If your phone tries to pull much more than that it will excessively load the power supply. At moderate levels of overload the result is likely only the voltage "sagging". But at more excessive overloads the power supply may overheat and may be damaged. – Jamie Hanrahan Sep 26 '15 at 16:29
So if I plug a 3.5A 5V USB charger into my iPhone (which comes with a 5V 1A charger - but Apple say can work with a 2A 5V iPad charger) - this will not damage the iPhone, right? – niico Oct 19 '15 at 0:41
@niico correct - the iPhone will only draw the amount of current it needs (1 amp in the case of an iPhone). – nc4pk Oct 19 '15 at 1:18

Expanding on tapped-out's answer. These chargers work the same way the internal power of a desktop computer would work.

When you buy, for example, a gaming computer you will notice PSU (power supply unit) options exist which run way above and beyond the requirements of anything you could ever likely get inside the case. Obviously if the PSU controlled how much power was taken in by a component you would fry most systems fairly quickly. Clearly it is done by the components which require power.

It is useful to use higher PSUs when, for example, you might decide to upgrade your components at a later date. If your new pieces require more power than their counterparts they will simply take more in.

I don't think anything I run these days has the right power supply. My television runs from a laptop charger...

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