|The specs call for 19V and you are feeding it 15.3v it is little wonder there is a problem.|
When you put a flat battery in with the computer on the powers supply has to supply both the battery and the computer. It obviously cant do both a the same time. Try leaving the battery in for a few hours with the computer turned off so the power supply only has to supply the battery.
Besides the voltage, what is the amperage of the power supply compared to what is required. That is likely to be more critical than the voltage.