AFAIK/AFAIN (as far as I notice) there're two distinct DC-stage charger behavior in our common market, one that is none of our concern as it's plain/linear characteristic, the other is this post is on:

How is the working mechanism or principal of the circuit on DC stage of a typical low DC voltage, ~ 5V, charger with the behavior:

• if a MM simply testing voltage on its two terminals it gives 0 volt
• if a battery is set in at its place on the charger and MM testing on the two terminals, reading it, it's giving the voltage of the battery plus a little bit more

Just wondering its circuit method for many days to no avail, please help shed a light sincerely

 

A smart charger will sense the voltage of the battery, make a decision about its state of charge, the initiate one of several charging regimens. This all depends on the battery chemistry.

What we call a phone charger, is not a charger at all, it is a 5V supply. The charger is inside the phone.