Latitude 5420 white led continuously illuminated, cannot power on until it turns off

Hello,

I have a Latitude 5420. It doesn't have it's battery (it was long time stored without any charge) and it works with AC supply.

When I plug the cable, the LED lights up and stays white until it finally shuts off. It takes about two or three minutes to do so.

And all during that time, it is impossible to power on the laptop. I can only do this when the LED is off.

When the laptop is on, everything works fine.

The LED doesn't blink nor become amber, it remains white all those minutes.

I tried to power off, keep power button pressed during about thirty seconds, and the LED blinks white two times and then shuts off.

I ran the BIOS diagnosis and it only complains about the lack of battery. Nothing else.

Any idea?

Thank you in advance!