You're getting good advice. This looks corroded to me. As suggested take a battery lead off, then take each terminal off one-at-a-time (super important...take ONE off, clean, get a can of contact spray and reinstall carefully...tightened not stripped!
You want good clean connections everywhere.
I wouldn't buy a bunch of stuff like cables, etc. It's better to troubleshoot.
Electricity everything relates to ohms law.
There's nothing wrong with trying to start it with a charger on it.
I would get a can of electronic spray. Disconnect battery. Clean those terminals, one off, wire brush, spray a little, Vaseline is ok, reinstall...do the next one.
That spray is good for when you unplug a connector, spray, plug it back.
I bet it's something simple...a bad corroded connection.
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