But I think I found out how it changes from the red led to the charging screen.
When it is at the red led state the phone tries to boot on its own. It then freezes on the sony logo and turns off with no red led but with the charging screen enabled (on power button press). By the way the charging screen shows the same battery level (minimum). And some hour later it goes back to the red led.
I tried something else only moments ago. While the phone was at the "charging screen" state I pressed the power button. I got one vibrate and the phone attempted to boot. It went straight to the Xperia logo and then started the graphics. But after several repetitions of the graphics it turned off and went back to "red led" state.
Are we sure it is charging or that even the led will eventually change colours?
According to Toledo_JAB he waited 51 hours for his battery to dry out (while NOT plugged in) and then started to charge.