Charging into USB jack...

Thread Starter


Joined Nov 29, 2005
A plain smart phone as millions. Its plain wall adapter rated 5VDC 1.5A as millions of them. -Say it charges the phone in 4 hours-
If a 5VDC supply capable of 10 Amperes* is used instead into the USB charging jack, will the phone charge in less time or the charging current is limited in the phone and not by the adapter capability ?

*(Yes, I know its wiring cannot handle such)

In other words, smart phones charging time is dependent on what ?


Joined Jun 22, 2012
The phone will charge at it's maximum designed level, not by the charger output, so if it's designed to charge a 2A, as long as the charger can maintain 2A it will charge, if the charger can only handle 1A, the phone takes longer to charge.


Joined Jan 27, 2019
Phone charging is potentially complicated. There are several ”standards” for fast charging. If you are using a purpose-built charger, it can have various supported standards. Apple, Samsung, and others have specific fast hanging protocols.

This can range from shorting the data lines to tell the phone it can do its best, there’s no specified limit because it is a dedicated charging port; 200Ω across the data lines means the charger can do “battery charging mode” and supply 1.5A.

Apple uses voltages on the D+ and D- lines:

D+ and D− at 2.0 V, 900 mA.
D+ at 2.0 V and D− at 2.8 V, 1 A of current.
D+ at 2.8 V and D− at 2.0 V, 2 A of current.

Other manufacturers actually interrogate the charger to see what is can do. Then negotiate a particular current and voltage. In order to provide more power over small wires, the voltage is turned up as high as 20V by negotiation. Type-C PD (Power Delivery) is similar to this.

In the absence of signalling or negotiation, devices are supposed to limit charging current to 500mA, the standard maximum power for a USB port.


Joined Apr 2, 2020
A real battery has an internal resistance. The recommended charge rate is based on the heat generated by this internal resistance. Most device manufacturers limit charging rates on this internal resistance (and ohms law).

For example, a LiPo battery is typically charged to 4.2v, if the internal resistance is 0.2 ohms and the charge rate is 1.5 A, then the ohms law voltage drop will be 0.3v and the current limiting circuitry may require 0.5 v drop (minimum), then, the 5v charger can't really charge to 4.2v any faster than 1.5A (even if the charger can supply much more).

USB c chargers can communicate with usb c enabled devices and charge at higher voltages. Thus allows much faster charging and higher current.


Joined Jun 7, 2009
My IPhone gave me a message the other day that it was in a scheduled charge mode that would complete in 5 hours time to maximize battery efficiency (or something along those lines). Further googling indicates user usage profiles can affect charge profiles.