PC doesn't boot with new CMOS battery

Help!

my PC doesn't boot after i replaced the CMOS battery with a new one.

When i push the start button nothing happens. I have to remove the new battery to power it on and run the system without battery.

Motherboard is H81M-DGS. I've been trying to clear the CMOS through jumpers following the motherboard's manual but no success...

Old battery was dead, measuring 0.2V. I measured the new battery (CR2023) to make sure i didn't got a faulty one and it reads 3.3V. I double checked the polarity (+ facing up).

When powering without battery and push the power button it powers on for a second then shuts off, then it restarts for 3 seconds, shuts off again and it restarts booting succesfully (don't know if it's a strange behaviour or it's supposed to work like this when battery is removed).

The only annoying thing is that i have to set date and time all the time i power the PC (i disconnect it from mains everyday). For the rest everything seems to work ok.

Any idea?

The power button does nothing actually. Fan doesn't start, power button LED stays off. The computer has been sitting unplugged for 4 months. It's almost 10 years old and everything was ok before. I will try to power on and wait.