phone used while plugged in, battery drains

[nyolc8]

I just noticed if I use my phone long time while it's plugged in, the battery drains from full to about 80-90% without any smaller steps. It's strange that it drops such a huge amount of %. When the phone is not on a charger, the battery drains in much smaller steps. Any idea? I use the stock charger which was in the box. It's output is 750mAh. Is that too small for using the phone plugged in? Or something is wrong?

 

[Laura Knotek]

Are you using it connected to an AC outlet or a USB port on your PC?

 

[nyolc8]

AC, with stock charger