Lots of good advice on here suggesting, actually demanding, AGM.
For those that might not be aware - that is Absorbed Glass Matt and they don't leak like "sealed" lead acid batteries still do from time to time.
In the C5 - that is a PROBLEM and here is why.

 

55$ walmart cheapo. Lasts years. Disconnect for storage and fires right up. Just saying.

 

Got it from Forum member Bret. His Forum name is:
BLOWNBLUEZ06@RKT Performance
He's a great guy and will walk you through on how to install as it requires some programming so you don't get a charging fault.

 

I checked all the usual parts stores and have not found a single AGM battery to fit my 2004, other than Optima. And then, they only show the red model.

Ive checked Napa, O’Reilly, Advanced, Auto Zone…. Some have AGM batteries, but not for the Vette.

Is there another size I need to ask for?

This is the Napa Legend I have:

 

https://www.corvetteforum.com/forums/c5-tech/4688104-battery-group-78-a.html#post1605926331

If the factory sized Group 86 top post AGM battery is hard to find, look for the slightly larger Group 34 size top post AGM battery and yes it will fit in the factory battery tray.

 

Optima yellow top working great in my car

 

I recently put a Group 65 Duralast Platinum AGM battery in my 2004 Corvette. Its a tight fit and you need to just use the big washer on the hold-down, not the rubber part. It fits flush on the tray.
Its a very powerful battery, I believe 800 cca.
I have had one in my 69 Vette for almost 7 years now. (They were side post back then, too!)
I tried the redtop optima but never got more than 3 years out of them, right around warranty expiration. And when they die, they die!!!

VERYSOON

Oops! My 2004 is top post. Specs on battery are 750 cca/935 ca/120 reserve. The battery in the 69 is side post with approx same specs. (I have too many cars!)

 

The following is NOT going to be received well by the users of the Optima Red top battery, so I will be reaching for my flame suit as soon as I hit the post reply.

Please be aware that the Optima Red top batteries (Group 34, 78 and 34/78) are all under rated for use in the C5 Corvette. The battery requirements for the C5 Corvette include a 120 minute reserve capacity rating and you can get away with a 115 minute reserve capacity rating, but the Optima Red Top only has a 100 minute reserve capacity rating. (Please note that the Optima Yellow top has a 120 minute reserve capacity).

So what does that mean? I am quoting here;
"Commonly referred to as RC, reserve capacity is the amount of time, in minutes, that a fully charged 12V battery can run before dropping to 10.5V. It is measured in reserve minutes. For example, if a battery has a reserve capacity of 120, that means it can supply 25 amps for 120 minutes before the voltage drops to 10.5V." Basically this tells you how long you can run your; lights, stereo or other electrical items on the car and still have enough cranking amps left over to start the engine. It can also tell you things like say, how long your C5 can stay parked in the garage with the alarm activated without a battery tender attached and still have enough power to start the engine.

So the question is; Can you get away with using an Optima Red top in my C5? (which by the way some Torch Red C5 owners seem to love the under hood color coordinated effect) The answer is... well, sort of yes. But just understand that its not recommended and the limitations involved (like the need to keep the battery tender handy) nor is it the "optimal" choice (pun intended). I will get off my soap box now.

 

^ I too had bad experiences with the red tops many years ago. I'm hoping that this yellow will serve me better down the road 🤞

 

Yep, the tray, etc., was designed to hold a size 78 battery, which is what all the early cars had. And the 34 is the exact same physical size, but with top posts. you will also see 34/78 batteries for sale, which have both sets of posts.

 

Is this a good alternative to the Gold DieHard and Optima Yellow top?

Seems like the Optima reviews are 50/50 on lifespan



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That should work fine.
I've had absolutely no problems with the current two Red Tops I'm using, one in my red 99 and the other in my 1991 Eagle Talon TsiAWD. Is the Red Top my first choice, no, the Yellow Top is. The red 99 already had the Red Top in it when I purchased the car and it's preformed flawlessly. The Red Top in my Talon is because the car is red but it it too has preformed flawlessly for 8 years now and my demands for battery power exists in that car
As far as the 34/78 fitting in our C5s, it fits great.

 

After I watched @Toys4Life C5 YT video on the battery I've been looking to buy an AGM battery myself. Just joined up to the forum to see what brands folks are going for. Curious what you end up with.

 

My truck has a 10 year old Walmart Maxx group 78 battery. A non-AGM being OK since there is nothing expensive underneath it. When it gives up, it will be replaced with another one. At $120, they are a good buy

The C5 however has an Odyssey Extreme in it, which recently replaced a 2008 era Sears Platinum - which was a re-badged Odyssey sold by Sears at the time. The current Diehard Platinum is not related - except by name!

 

Bellied up to the the parts counter and picked up a DieHard AGM at Advance Auto Parts. Advance bought the brand so they carry them all over.

 

Walmart batteries are pretty good - I typically get about 6 years out of them in MN which is good considering our brutal winters.
Yep, I put the Wall Mart batteries in all my cars Except my C5. Can't risk it leaking on my PCM and main harness - Catastrophic problem if that happens - and it happens once in a while.

 

One more (somewhat academic) data point. The heavier the battery, the more lead there is, which means bigger plates, which translates to longer life.

 

Just pulled my Costco Interstate battery to inspect and clean the ground that is under the battery tray. That is when I noticed it is 6 years old, and still going strong. At about $100 and with an extra year of warranty with the Costco Citi card, it seems hard to beat for in service life and cost.

I understand the concerns about a battery leaking so I do have a battery mat between the battery and tray.

The Citi card extended warranty process does work. I recently had an Interstate motorcycle battery go bad while still under the Citicard extended warranty and they fully refunded the purchase price.

 

So true! My original Delco on the ‘98 - all it took was replacing the battery and normal removal untorquing the connectors - I put the battery on the garage floor and acid started leaking a bit all over the floor. 🙄

 

I used to run Optima Red-Tops in my 2001. Never had a problem with them but showed issues when using my Solar Tester.





Switched to Odessey in 2016 and swap out every 4 years (overkill "hopefully")...