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By DesperateDucky
OK so, Ive been scavenging the web all night and i cannot find the answer to my question and it is getting very frustrating. So here it is.

How would i go about creating a circuit that charges the battery while plugged in but the device the battery was powering can still be used while plugged into the wall.. cellphones do this, battery powered headphones do this, a lot of things use this feature yet i cant find it on the web. I was thinking that maybe a transistor would play a roll in this? While plugged in the transistor switches the discharge from the battery off while the wall power takes over. i dont know. please help.. :(
By skimask
Try this...

Get a car battery.
Hook it up to a head light.
Light is lit up.
Battery runs down.
Light goes out.
Disconnect head light.
Connect charger.
Charge battery.
Disconnect charger.
Connect head light.
Light is lit.
While the head light is still lit, connect charger.
Light is still lit right?
Disconnect charger.
Light is still lit right?

Replace the word "head light" with anything.

Do you see it?
By skimask
Assuming the battery isn't totally dead and sucking away all of the current supplied by the charger, and/or the charger is able to handle the current the 'dead' battery wants to draw as well as the circuit being powered....yes....