I recently bought a 2004 xj8, it apparently still has the original battery in it when I got it from the dealer. Are there any good substitutes for it or is the size required. Group 49/H8 aren't terribly cheap.

 

just get the walmart h8 lol

 

Don't know where you're located, but I just went to AutoZone or Advanced Auto and told them I needed a battery. They had what I needed in stock and it's worked just fine for years.

 

I agree! Replaced the battery in mine right after getting the car. Got an AGM from Advanced Auto for 150.00. Lasted over six years and started getting weak. Car sits for weeks regularly.
Shopped around and picked up an AGM from Walmart for 150.00. Hope it goes another six or seven.
The recommended battery is a calcium ion battery. Think these were more common when the x350 were newer. Have read Home Depot carries them.
Batteries are very important. A weak battery can generate all kinds of CEL’s on startup. Plus load up the alternator.