I've had an Optima Red Top (Starter) battery installed in my 2003 A4 3.0L Quattro 6-speed for about 18 months (see attached photo of battery installation); the Optima gel battery always starts the car, even in absurdly cold weather. My opinion of the Optima gel battery application in an A4 parallels the old American Express motto, "Don't leave home without it."
The battery cables attach to the SAE posts on top (the GM-style posts on the rear side of the battery are un-used; they're sheathed in non-conductive plastic, so that they won't electrically short to the chassis). You'll need a fairly long shaft-extension for your nut driver in order to secure the battery-retention bolts at the bottom of the battery case. It's a snug fit, but the Optima can be securely mounted in the A4 (the strap in the photo is just the carrying-strap that's part of the battery; it's not used to secure the battery).
The Optima "Red Top" batteries are designed for starter activities, so this should be the battery series that you consider for installation into your Audi. The Optima "Yellow Top" batteries are a dual-purpose design, primarily designed for deep-cycle applications (trucks with winches, cars with high-current-demand audio systems, commercial equipment, etc.), but the Yellow Top batteries can also serve as starter batteries. I believe the Red Top battery installed in my car is the Optima Red Top Model BC1, Group 34/78.
Nice information. Is anyone having any problems with a gel battery over the long term? It should be the same sort of electricity that is produced by the electro-chemical reaction of a lead acid battery, but I have stopped making assumptions like this since I have started driving German cars.
People that are using Optima batteries, are you happy with that?
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