Laptop turns off after 2 seconds, why?

I have a problem with a laptop that immediately pops off with a click (as if a circuit breaker is tripping) when turned on. This was exactly for this reason that it was in the cellar for 2 years. Now I wanted to get the thing to work, after turning it on after such a long time, it suddenly worked for 1.5 hours. That's why I first ruled out a defective motherboard. Until I switched it back on the next morning, it went off again after 2 seconds. I have already disassembled it and renewed the conductive paste on the CPU and on the AGP chip - it doesn't help.

Laptop: HP Pavilion

The peripheral components such as hard disk, keyboard etc. Also had it expanded and only tries to start with a minimal hardware range, the result remains the same (2 seconds off)

Where can it be? Anyone have a guess?

Battery and power supply OK? A battery actually has a thermal contact to protect it from overheating as far as I know

Exactly without a battery it went for 1.5 hours but now it switches off after 2 seconds even without a battery. How can I check a power supply unit if it is defective?

See if it is charging, but apparently it doesn't, for me the first approach would be to get a new power supply, but you better think again, if it is not that you put 30 to 50 euro in the sand, yes you can so do not say for sure that it is definitely because of it

Sorry for the late reply did not see it