I'm looking at two Napa batteries, but I can't quite figure out the actual construction difference aside from the obvious warranty difference. Napa Legend 7559 (75 month warranty, group 59 size) CCA: 590 CA: 700 Weight: 37 lbs. http://www.napaonline.com/Catalog/CatalogItemDetail.aspx?R=NBP7559_0398897352 Napa Legend Premium 8458 (84 month warranty, group 58 size) CCA: 580 CA: 725 Weight: 28lbs. http://www.napaonline.com/Catalog/CatalogItemDetail.aspx?R=NBP8458_0398897353 They are different group sizes, but the dimensions are very close. I can't see where the nearly 10 lb. weight difference comes from. Both are normal lead acid batteries. I believe both are JCI batteries, as Napa carries Optima too. I'm kind of leaning towards the 8458, but the 7559 has better availability. Any thoughts on the weight difference?