It was 5 am in the morning stuck at McD and waiting for the train with my friends. All out of nowhere I asked them this question that I have been thinking about for some time.

How this battery-charger system in mobile phones and laptops work?

What I don't really understand is the ratings thing.

Lets assume that, the charger is 5V, 2 Amp And the phone battery is 1 Amp and (5V)

I know that means the phone is going to be able to take 1 Amp for charging from this 2 Amp rated charger. But

First is the phone battery Voltage 5V? Second how will this battery ONLY takes this 1 Amp, as from ohm's law the current is I=(V-charger - V-battery ) /R Here is an assumption, the phone battery is changing( from some min voltage value when the phone is 0% charged to max voltage value when its fully charged). Consequently the voltage value (V-charger - V-battery ) will chane as well So how will this drawn current be constant while the voltage is changing? Is there any current limiting diode? Any current mirror?

Is this the mechanism of this system works?


1 Answer 1


There's a charger controller IC inside the phone/laptop/whatever that regulates the charge current or voltage to ensure safe charging according to the battery's specs.

There are typically voltage and current control loops in the charger, and the mode is selected based on the state of charge of the battery, the temperature, etc. They can be linear or switch mode power control schemes.

For example: bq24195 Charger IC

Here's a link to some information on how it all works: Charging and gauging


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