Why mobile devices use same pins for charging and for pwr transferring

[amit016]

Mobile battery has generally 3 pins, 3rd for safety and 2 pins for supply. How can mobile devices get electricity during charging from same pins from where it is transferring power to mobile circuit?

Please help

 

[meBigGuy]

I guess there are two ways that can work.

1. When charging, systems can get power from the charger. The charger both runs the system and the battery charging circuits. Then, when the charger power goes away, the system switches to the battery and run off it. It is fairly easy for the power circuits to switch without interrupting system operation. So, think of the mobile device as getting power from the charger if it is plugged in, or from the battery if the charger is not plugged in.

2. I've also worked on systems that charge the battery and run off it at the same time, and it causes no end of problems because the system draws a variable current, and the battery requires a precision current to charge correctly. But it is possible to work that way (I expect there are charger chips that do it all). You just need to compensate for the current being drawn by the system.

Not all devices will work the first way, but that is the easiest since you can start a system with a dead battery without waiting for it to get some charge.

All systems go through power conditioning circuits after the battery, generally a combination of switching and linear regulators to supply the various subsystems. Nothing generally runs directly off the battery.

Don't get confused about the two pins. When charging, power flows into the battery, and when supplying, power flows out. If a system and a charger were both connected to the same two pins (like #2 above), the system would essentially steal power from the charger while it was charging the battery. That is, the charger would power the system and the battery.