I'm not quite sure if this is the right thread, but I found this old battery in my dad's old car that's been sitting for about a year. I'm not very educated on electronics or batteries so would this work on the XJ? The one I have now is getting pretty old.

 

It's an Optima which is good.

Chances are it will fit in the stock tray, it only has 680CCA which is less than the type-34 battery my XJ has - but will definitely still fit.
They are all 12v, so as long as you are able to secure it, and the battery doesn't contact the hood - I think you should be good to go

100% make sure it is secure unless you want it to short out on the hood, and burn your rig to the ground. Good condition battery tray and hold-downs are cheap insurance against that

 

I'll second this one. As long as it fits in the tray and the battery terminals are on the correct sides, it should be fine. Optima's last a long time and can be taken in and swapped for a new one without a core charge if it does go bad. As he mentioned above, make sure it doesn't hit the hood. Optima batteries are pretty tall and the XJ tray is fairly shallow so this could be an issue. I have a basic run of the mill battery in the front, and a secondary yellow top Optima in the back wired through my Amp to take some of the hit off of the alternator when the bass hits, but this isn't necessary. I just got tired of killing alternators every 2 months.