Jim, Chris and all,
Batteries are my specialty. I give seminars each year in February in Georgetown, Exumas and have been helping cruisers with battery decisions for 30 years now. I agree with a lot of what Chris says but the idea of breaking your house bank into 2 is not one of them. The larger the bank the more efficient the charging system can be if you are deeply (up to 50% of capacity) discharging them on a regular basis.
But lets be real here. I am talking about liveaboard cruisers, not people who weekend or two week cruise and then are back tied up to a dock and have shorepower. That is a totally different useage with different answers. Also cost comes into play.
I talk for an hour on this subject so I am just going to give you a taste of that.
If cost is a factor you can not beat 6 volt golf cart size batteries. Period. I just took 6 Trojan T-105's that are 7 years old, tired but still working, and installed them in my Allmand to see how long they will last. I replaced them on my Saga with Lifeline AGM golf cart batteries. Not a cost effective change but Lifeline wanted me to try their batteries and to be honest, I was curious enough in my advanced age to want to as well. At over double the price of the Trojan wet cell T-105's obviously price alone won't support the change since they would have to last 15 to 20 years to equal the overall cost of good wet cell golf cart batteries.
I don't recommend group 27 or 31 size 12 volt batteries because their cell construction, by definition, must be weaker than the much larger cells found in 6 volt golf cart or 12 volt 8-D batteries which are physically much bigger. On the other hand if you don't have the space or don't want to make the custom battery boxes that are required for most golf cart installation's then the group 27 or 31's are the next best thing but you will have to accept a shorter lifespan (4 to 5 years vs 7 plus for the larger cell types)
If you want the best price for Trojan T-105's I highly recommend Dale at Tee Time Golf Cars at Ormond Beach, FL, phone
386-615-1980. His current price is $96 per battery assuming you have a core to exchange. Obviously if he is located too far away to drive there, transportation costs factors in. I don't believe shipping heavy wet cells is practical unless you can get them before the acid has been installed and ship the acid separately.
A plus for AGM's or Gels is they can be laid on their sides to fit tight spaces although I try to avoid this. Also no worries keeping up with a water addition schedule because they are sealed and don't require it. Another plus is for the weekend warrior who spends more time with the boat not in use, these batteries have a much lower self discharge rate than wet cells that often translates into a longer lifespan when used in this manner.
As Chris pointed out, Gels are very fussy with their charging rate. Anything over 14.1 volts will greatly shorten or terminate their life span. AGM's can use virtually the same charging rate as wet cells though an Equalization charge should be used either as a last resort or at least very sparingly to avoid shortening their life.
Finally, most batteries die from systematic undercharging. Every battery, no matter what type, does not respond well to being partially discharged for any lengthy ( more than a matter of days) period of time. This last fact I have found to be most startling to even the most experienced of cruisers. It is one of the reasons I am such a big proponent of solar and /or wind as supplemental charging system that can efficiently charge that last 10 to 15 percent of charge that occurs at such low amperage. To determine if a battery is fully charged, assuming it is relatively new or in good shape, it should take no more than 1.5 % of its 20 hour rating to hold it at 14.5 volts, assuming the battery is at 77 degrees F. That target voltage goes down by .15 volts for every 9 degrees of battery temperature above that target and up by the same amount for every 9 degrees below 77 degrees F. And obviously Gel batteries have a lower target of less than 1 % at 14.1 volts at 77 degrees F.
I apologize for the length of this response but I felt shortening it was too likely to result in miss-understandings.
If there is interest and travel time is reasonable I would be happy to do one of my talks for an Allmand Owners get together.
Keith on Camelot