House batteries

I have four six volt agm batteries- two sets of two to make 12 volts
I am going to replace them and would like recommendations on what is good. I know very little about batteries so am assuming I need deep cycle - is this correct ?
Thanks
Jim

Yes, you will definitely be using Deep Cycle for a house bank. I like AGM's. they can be discharged deeper, charge faster, have negligable amp loss over time (such as storing over the winter) and don't require a battery tender in storage, no maintenance.

However, if you are on anchor for long periods and primarily use a generator to top them off, their life span can be adversely impacted by not getting completely topped off regularly. This makes them better for someone who is regularly returning to a dock with shorepower.

I like that 6v gold cart batteries give you flexibility for installation location, are lighter (per battery) and have high amp hours, as compared to 8D's.

I too like golf cart batteries. But these are usually the old style flooded cell lead acid types but are one of the few with real deep cycle capability. They are about $100 each at Costco and other places.

AGMs or absorbed glass mat batteries are also good for the reasons Shrew noted above and as best I can tell there is no difference between starting and deep cycle batteries.

Amazon has Renogy and Windy Nation 100 Ah AGMs for about $200 each. These are the cheapest I have found. You can pay as much as $300-400 each for Lifeline or other high quality brand AGMs.

David

Jim,
As shrew stated, batteries are killed by their owners a long time before they "wear out" by use. What I mean by that is the owners treat them poorly by discharging below the 50% level (usually because they just don't know their batteries state of charge), or they regularly operate the batteries in what is termed a partial state of charge (PSOC), not fully recharging them before using them again.

If you recharge your batteries using a generator or by moving the boat (with the engine) you are for sure not getting to 100% charge. It would take at least 6 hours of generator run time to achieve the necessary 100% charge (that is an approximation, but is in the right ballpark).
AGM's have some advantages, but you pay for them. If you are not going to "treat them" well, then you are spending a lot of money for little gain. If you operate at PSOC regularly, then buy flooded lead acid deep cycle golf cart batteries. They take abuse as well or better than most other types and are the least cost per amp hour.
Installing solar panels will help with the PSOC, but if you just go dock to dock, then maybe AGM would be good??

If you have a Sams Club nearby check out their Duracell AGM's. They are roughly $180 ea and made in USA by East Penn. I have had good luck w/ East Penn AGM's and just installed 8 of the AGM GC2's in our motorhome.
Currently working on a plan to replace the 3 - 8D AGMs with either Gp31's or GC2's AGM's .
I have confirmation from East Penn that there is no difference between GC2s and Gp31's re Deep cycle advantages - they will both handle deep cycling as well as providing start capability per their respective specs. Either way you get roughly 200 AH for $400... hard to beat if looking for AGM's IMO

Before purchasing new batts , install a State Of Charge (SOC) meter to asses what you now have.

The meter is a good investment before new house batts so you can use them, but not harm them.