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Watch this video parts 1 and 2 on C5/C6 electrical systems and ECL. Click Here
You want a battery with a least 120 RC (Reserve Capacity).
Personally, the AC-Delco Professional series with an excellent replacement warranty meets the requirements and will run you under $120. 78-6YR/7YR Different for '04 86-6YR
I love this forum. I spend most of my time on the C1-C2 pages, but came to the C5 page today to see about a battery replacement for my 2004 AE.
I found this post from Jet-Jock, and it's like a miracle.
My C5 has been sitting for 9 months, that is a very long story, but the battery, won't take a charge. After reading this post I checked the battery and it is the AC Delco Professional with 7 year warranty, and it is only 49 months old!! If I am reading the Delco warranty correctly it is free replacement within the first 50 Months. I'll be headed to the dealer in the morning!
Thanks to Jet-Jock, and all other posters on this Forum,
George