How do cell phones know their charge rates?

I got a new phone and realised that the charger had a rating of over 2A. So I was thinking, how does the cell phone know not to draw more than the rated supply? Some phones charge at more than 500Ma, but if I connected it to a computer (rated at 500Ma) what stops it from drawing more and damaging the computer? How does it know?

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When a connection is made over USB the master and slave (PC > phone) NEGOTIATE the power that the PC will allow to be supplied. If the negotiation doesn't happen, the USB port is supposed to only let the device have 100mA, although some cheaper USB ports don't do that and rely on thermal-resetting "polyfuses" on the motherboard. There is no way they can supply 2A.

The USB negotiation is in units of 100mA. "Can I have 2units" - if the PC can supply it, it allows 200mA to be drawn

IIRC, If the negotiation happens USB2 can supply up to 0.5A, USB3, 0.9A.

But how does that differ from wall chargers, I always assumed they were basic 5V 2A SMPS's, and that the phone would vary it's effective impedance to change the current drawn.

The phone checks if its been addressed by a USB host, if it has, it asks for power, if it can't, it pulls what it wants.

David97 (author)  steveastrouk2 years ago


Aaah thats the word "polyfuse" <=> a reset resistive fuse like component.


They're great, so long as the overcurrent transient doesn't blow something before they heat up.

iceng2 years ago

No fair asking three questions in one ;-)

First I assume you used a USB port that is limited to 1/2 amp.

I'm reasonably sure the phone has a reset resistive fuse like component and the better phones watch the battery temperature to limit current.

The charger should have the main battery protect current, voltage, temp, time and charge limit.

seandogue2 years ago

As far as I know, the phones' own charging subsystem (inbvoard to the phone) controls the actual charging process once the electrons leave the cable from the USB charging power supply (ie, the dongle for your car, the USB on board your laptop or desktop, etc. The cable only provides bulk power for the phone to tap.