'02 Ignition clicks, then car dies?

I've owned the vehicle for a month and a half and never had any electric related issues or issues with starting. Got to my car today and turned the ignition, everything lights up like it should, then when I turn the key to start the engine, there's a click and everything dies. Everything electronic is not working.

Any suggestions on where to begin? I would assume the battery is bad, but it happened so suddenly with no warning... has this ever happened to anyone here?

 

Definitely start with the battery and then go from there

 

The battery reads 4.8 volts, I suspected it’s bad since I found a spare one in the trunk (assuming previous owner was about to swap it). The spare one reads 11.85 but there’s a sticker on it that says 7/17 — I’m assuming it’s the expiration date?
Should I try to charge the spare battery or just go ahead and buy a new one?

 

Try to use the old one. No harm in it.

That sticker is the build date.

 

Thanks,
I went ahead and put in a brand new battery just so I know I won't have any issues in the future, and since I needed to move the car fast.
When I just put it in it read above 14 volts. Then I drove around for a bit and noticed it dropping lower and lower, before parking it at home it read 13.45 while engine is running. Isn't that way too low for a brand new battery? Should I assume the alternator is bad? Or could this happen with bad battery connections? The red cable looked pretty corroded...

 

Either could be a problem. What voltage does the battery say while running?

Your battery should have been 12v new. I would suspect a battery that reports much over 12v just sitting out. Are you testing with the car running or not running?

Battery can be like 11.8v with car off or a bit higher. With car running, you should see 13.5 v or more.

An alternator will put out from 13.5v to about 14v.

So your current battery voltage is as high as normal alternator output.