We have a large (L-14) 340 ah house bank and very small engine cranking battery, for the 18hp diesel
sources are the engine alternator
, dockside AC charger, 4 panel solar
array...our primary source, and we used to have a wind
unit as well.
All of these charge sources go to the large house bank alone, and none to the small engine battery.
The engine cranking battery is parallel connected to the house bank, using a "blue Sea" battery combiner. This device connects the two battery banks only when the charging source is on, and line v above 13v, then when the house bank is being discharged, the "combiner" disconnects the line so that the banks are NOT connected, and the engine battery can not be discharged.
Since our standard engine alternator is internally regulated, and set at a higher level than the other "smart" sources, it could potentially over charge the combined engine battery, because the engine battery is normally 99% charged from the get go, and the house bank might only be 80% charged. So, if we crank the engine in the morning for say... an all day motor
down the ICW
, after 30 minutes or so, I will flip a switch connected to the combiner, and disable it, keeping the batteries disconnected. This is the only function that is not automatic. It is optional, and if forgotten, it doesn't really matter.
We have used this system for the last 15 years, 12 were full time liveaboard
. It has worked flawlessly. It takes perfect care of the $500 batteries, and although the $59 battery is slightly at risk of gradual overcharge, it has never been a problem. I seem to get about 3 years out of the engine cranking battery regardless.
The beauty of this system is it it the simplest way to go about it, and not having the charging sources split up to go to both batteries, avoids the "diode" dilemma, which creates a v drop in the line and confuses the charging sources v regulators. This takes excellent care of your most expensive batteries, and tops off the engine battery as well. It is also a way to deal with the fact that the two batteries are of such different size.