Batteries are the heart of your electrical system.  We have met more people with sad stories about their batteries.  We offer two suggestions 1) buy the best quality you can afford and 2) have sufficient capacity.  The capacity should be 4 days usage in AH, and also 4 times the alternator when hot.  One hundred AH/Day needs 400 AH of battery capacity.  We would like to see, considering space limitations, two size 8D which will provide about 440 AH.  Two size 8D Lifelines are 510 AH while two 4D will provide 420 AH.  These should be your ship’s batteries, plus other smaller batteries like Optima for engine starting, windless, generators, ect. 

Our recommended battery configuration consists of one large house bank and a smaller starting battery with auxiliary batteries as needed.  All auxiliary batteries are charged from the house while it is being charged through a 60 amp diode and a thermal circuit breaker.  Connecting batteries in parallel to form a large bank is okay if the batteries are 1) the same size, 2) the same manufacturer, 3) approximately the same age, 4)all capable of full charges and in good condition.  How ever after 2 or 3 years periodically separate them electrically and test each battery and each cell.  Amp hour monitors make checking the capacity easy.  Batteries will eventually develop a bad cell.  When connected in parallel, the bad cell causes circulating currents reducing the life of the good batteries.  When a bad or low cell is detected, do not put that battery back in parallel with other batteries.  Use it up and replace it.