When we bought our J/109 I installed two 125-amp hour 12-volt Victron AGM Super Cycle batteries for a total bank size of 250 amp hours.
This is a small bank, so I selected these batteries because they are advertised as being able to withstand more very deep cycles than most lead-acid batteries, which are severely degraded by cycling below 50%.
My strategy was that over say a weekend cruise, our limited solar (50 watts), and alternator charging only when entering or leaving harbour, would generally get us through without having to run the engine just to charge, or at least not for long, but result in returning to our wharf with deeply depleted batteries that we would then immediately charge on shore power.
This has worked well and is a good example of how electrical systems need to be designed for the usage profile. There is no best, just best for task1.
We are now coming to the close of our third season on these batteries so time to report and make recommendations.
Let’s cut to the chase: These batteries have exhibited a very strange and worrying charging behaviour.