If you actually want to find a file on a long abandoned laptop, it may be easier to pull the drive and use an external enclosure to read it with a current system. If you have the hardware. No need for any of the other hardware, rtc, nvram, let alone the os to work.
If you are just checking one or two devices, charger is the way to go. Now if you need to check like a dozen for some reason, pulling the drives might be easier.
Depends how bad a state the rest of the machine is in, I've tried booting some really knackered machines and the time I spent waiting would probably have been enough to remove the drive!
Make one drive mount read only in a guaranteed working and current system instead of powering on hundreds of old components along with long dead software, batteries, authentication long forgotten... Not to mention where that supposed charger has gone and will it work or die after years of being abandoned.