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?

5 Answers 5


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).

  • 2
    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? Commented Jul 14, 2015 at 15:30
  • 3
    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. Commented Sep 26, 2015 at 16:29
  • 2
    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
    Commented Oct 19, 2015 at 0:41
  • 3
    @niico correct - the iPhone will only draw the amount of current it needs (1 amp in the case of an iPhone).
    – nc4pk
    Commented Oct 19, 2015 at 1:18
  • 3
    @nc4pk I was googling this question and I saw that someone stole your answer for advertising a product on Amazon : quora.com/…
    – Meds
    Commented Sep 23, 2018 at 11:28

To choose the right charger for the right device increases the rate of charging, increases the lifespan of the device and decreases the risk of burn. There is always a risk of burn, but it can be minimized by many methods.

  • Ohm's law V = R I where V voltage, R resistance, and I current. Choose a cable which has more/same charge capacity (I) than the phone.

General Summary

Exercises where try to answer the question marks

V (V) /   V (V) /
I (A)     I (A)
Cable     Phone     Decision
-------   --------- --------
5V 1A     5V 2A     No, burn risk
5V 2A     5V 2A     Yes 
5V 2.1A   5V 2A     Yes
5V 2.4A   5V 2A     Yes
5V 0.7A   5V 2.1A   No, burn risk  
5V 0.7A   5V 0.85A  No, burn risk

5.2V 2A   5V 2A     ?   
5.1V 2A   5V 2A     ?   
4.9V 2A   5V 2A     ?
4.8V 2A   5V 2A     ? 

5V 1A     3.8V 1.3A No 
5V 3A     3.8V 1.3A Yes
9V 3A     3.8V 1.3A ?
9V 3A     5V   2A   ?
5V 3A     5V   2.1A Yes (Edza case)
  • burn risk - drawing more current from the cable which it supports is always a burn risk
    • feel the charger with your hand; if it is very hot, reject; overheating will happen; since most chargers do not have protections besides the covering, the seller cannot recommend combinations with burn risk

[new table here where device has Snapdragon 810 or 820, since the situation will be little different in dynamic configurations]

Practical table where try to answer the question marks

For most USB chargeable smartphones the battery voltage is irrelevant. The USB charging circuit is often set to 5 V 0.5 A

V (V) /          V (V) /
I (A)            I (A)
Cable            Phone      Decision
---------------  ---------  ----------
USB1 (5V, 0.5A)  5V 2A      ?
USB2 (5V, 0.5A)  5V 2A      ?
USB3 (0.9A)      5V 2A      ?
USB-C            5V 2A      ?
Apple's iPad
5V 2.1A          5V 2A      Yes
Kindle fire 
5V 1.8A          5V 2A      No, burn risk
Car charger
5V 1A            5V 2A      No, burn risk 
Car charger
5V 2.1A          5V 2A      Yes


Phones in examples: Oneplus 2, ...
Tools to study volt and ampere which your phone is charging: Ampere

  • So a 3 Amp 5V charger would be fine for a phone that charges fine with 2.1 Amps, because of Ohms Law?
    – Edza
    Commented Jun 27, 2017 at 7:58
  • @Edza Please, see the body. Can you confirm the specs concerning your case. Is your phone 5V? If yes, then fine to charge with the charger the phone. Commented Jun 27, 2017 at 10:33
  • Regular phone. My cable is rated at 15W, and am about to buy a 3 A power adaptor. Only problem I have to order from china, nobody sells them near by or on the internet.
    – Edza
    Commented Jun 28, 2017 at 11:54
  • @Edza Yes, the availability of electronics is a common problem in Europe. Order from Amazon, Ebay and/or China. Let's hope EU makes the things easier for us later by providing equal competition for EU and China. Commented Jun 28, 2017 at 17:15
  • 1
    5V 3A 3.8V 1.3A Yes 9V 3A 3.8V 1.3A ? 9V 3A 5V 2A ? Shouldn't these cases be no? The voltage difference creates a risk of damaging the components.
    – jhanschoo
    Commented Jan 2, 2018 at 17:04

This ohm law is wrong application for a battery under charged, the battery is not a resistance device, but a capacitance device instead, so if the charger supplies 2 Amp the phone battery will accept 2 Amp charging current as this ohm law: P = IxV , V = 5V constance so current I will change if the charger power is higher than the device require. The statement " Thus, the device will only draw as much current as it needs and no more. " is not correct in this case even though the statement " 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)." is OK from his experience, nothing happen to his device because only more heat generate when charging with more power charger and the time also cut off, but heat will degrade your battery in the long run.


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...


In case mobile charger of higher Ampere is used, than recommended, the mobile will get charged but after some days, the mobile battery gets swollen.

  • 6
    Sorry, wrong answer. The mobile device determines how many Amperes it draws from the charger. If the device is happy with 1 A, then it draws only 1 A even if the charger is able to supply more. Commented May 25, 2017 at 8:01

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