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House battery capacity
There are a number of factors that influence the size of the battery bank. The usual consensus is a battery capacity around 3-4x of daily needs (although Surrette has a guide based on boat length). Calder advocates 4x daily need, or 4x max temporary load, whichever is greater. AGMs or Gel-cells may allow higher temp loads, so the maximum temporary load factor can be reduced to 3x with them.
A battery is considered "dead" when it only retains 80% of its rated charge. Thus, unless you use cheap Trojans or other bargain batteries, it''ll be a while before you wear down the battery to this point. To go to one extreme: the Surrette/Rolls CS series can sustain 3,200 cycles at 50% Depth of Discharge before it''s "dead". That''s 9+ years of daily use, assuming one discharge to 50% per day. The AGM batteries I use are rated for 1,000 such cycles...
Ideally you won''t discharge the house bank deeper than 50% on a daily basis, cycling it between 80% and 55% (if you used the 4x rule) since this allows bulk charging (much higher replenishment rates than at higher states of charge) which minimizes engine runtime, fuel consumption, and wear. A 4x battery bank also gives you three days of charge if the generator/alternator fails.
Have one starter bank and one house bank. Keep the two circuits completely separate except during charging. The two banks should have similar battery types in them (Gel, AGM, Flooded) although using an isolator/eliminator by Ample Power gets around this.
Have a look at my battery page for more info: http://www.vonwentzel.net