Good morning. A couple of comments. The layout of the C30 S is virtually the same as the R29CB. You should have four house batteries, not two. And you are likely to have the UB121100 AGM batteries in there as well, at least for the house and thruster. Given the size of your engine, I hope someone put a higher CCA battery start engine.
So bottom line, you should replace all four house batteries at the same time. One bad cell in one battery can draw all the rest down. Also, I can tell you that the Universal Power batteries that the factory puts in are mediocre in quality. You got four years out of yours, which appears about right when you go on the web. Some get more, some get less. The Lifeline and other more expensive batteries have a better reputation and are likely to last longer.
Your quote on price of batteries makes more sense for FOUR rather than cost for each. I replaced all four UB121100 AGM batteries through Northern Arizona Wind&Sun for a total cost, including shipping, of less than $700. So something is fishy in the math here and you should get a lot of clarity from the marina.
Finally, getting these batteries out of their impossibly cramped storage area is a pain in the transom and will take an electrician a while. Mostly because they weigh 65 lbs and you have to find a way to wire them up while upside down in the lazarette. I hope there is an access hole underneath your cockpit fridge. It makes things much easier. But, having said that, 8 hours is about twice what it should take in my view. As I say, something fishy here. I recommend you get another estimate.
Finally, as Mike and others will tell you a much cheaper option is wet cell batteries. Even though you have to service them (top up the water) they are less cost and last just about as long as the cheap Universal Power AGM batteries.
Hope that helps.
jeff
