Welcome to the Learning Curve
AlamedaJohn,
Be patient: it' a whole education unto itself. It took me a while to aborb (no pun intended) the basics and relationships, and I'm still a beginner.
If I can offer a simple summary:
1) The two factory batteries are likely very small. They will not store many AMP HOURS of power (amp hours is a term you should become familiar with: it describes your storage capacity & consumption rates). You may even have Group 24 batteries installed, which are very small indeed. Installing larger replacements is your first good bang-for-buck. Group 27s, Group 31s if you have the space to just drop them in, is easy to do. When installing, combine both new batteries into one large house bank by connecting them together and using just one of the two circuits you have on your panel (instead of one battery connected to 1 on your master switch and the other to 2, you will use just circuit 1). One large house bank is more efficient for several reasons, than two small banks of equal total capacity.
(Other posters: I'm leaving out discussions of 6v batteries for simplicity's sake and because I sense from his post that John seems to want to be able to step up from day sailing, not cross an ocean).
2) Buy DEEP CYCLE batteries: the lead plates are thicker, they are designed to discharge deeply (50% is customary) and still provide many "cycles" of discharge/recharging, which is the opposite of a starting battery, which is designed to provide a lot of current for very short periods, only discharging a little bit. You want deep-cycle batteries for your HOUSE BANK. A third type, the "multi-purpose" battery, is a compromise design between these two performance profiles. It does nothing well: avoid it.
Deep-cycle batteries will start your relatively small diesel engine without any problems. You can add a dedicated starting battery later if you want, and use a BATTERY COMBINER to let it charge from the house bank, and connect it to circuit B on your master switch, to easily combine the power of both banks for that emergency start that will be required someday.
3) Learn the difference between combining batteries in SERIES, or in PARALLEL. Connecting the two house bank batteries in series (positive post to negative post) will double the voltage and fry a lot of your 12v equipment, start a fire, etc. You want to connect them in parallel (positive to positive, negative to negative), which will retain 12v and double your amp-hour capacity safely. Read up on this in detail: my description is just sketchy overview.
4) Charging the house bank efficiently is next. The alternator you have, if it is the factory installation, has one-stage, internal REGULATOR, designed for automotive application, where long running times are assumed. You will shorten your charging times greatly if you replace it (the regulator) with a multi-stage SMART REGULATOR, which lets the alternator charge at higher voltages the more deeply your house bank is depleted (the more deeply the bank is depleted. With three charging levels, instead of trickle-trickle-trickle, the smart regulator gives you FLOOD, then STREAM, then TRICKLE.
5) A BATTERY MONITOR, which has been mentioned below, will allow you to precisely monitor the state of charge/discharge of your system and protect your investment by using it more efficiently. It replaces a lot of guesswork with data.
6) If your shore power battery charger is very old, replacing it with something up-to-date, which will take better care of your new batteries, is a good idea.
7) A bigger alternator has been suggested, and is valuable, but I put it last in my bang-for-buck list. I think money is better spent on the regulator to maximize the performance of the alternator you have. A bigger alternator is still a good thing, though.
Consider this simply a list of topics to become educated about. Other very knowledgeable posters here will disagree with my application, but these components are the major players.
Fair Winds,
Jeff
