I have an old Latitude E5540 that won’t boot. No POST, no lights, nothing. It was fine a week ago when I shut it down and disconnected it from the power whilst I was on holiday. It’s 9 years old and I’ve never replaced the CMOS battery so I tried that first. Nope. Tried various combinations of removing CMOS battery, removing main battery, swapping main battery, swapping charger etc with no luck. I found a Facebook post where the same issue is resolved but I think it’s in Hindi so I can only watch what’s going on. Looks like he just removes the motherboard and powers it up? Video is here in case anyone can follow it

https://fb.watch/te3NVaSDDs/

I’ve seen a few mentions of low voltage on the CMOS battery causing similar problems but no real solution. Does anyone have any suggestions please?