Originally Posted by MarkSF
Obviously when overnighting, you'll want to drain the deep cycle battery (2?), keeping the starting battery (1) for reserve. In that case the next morning select 1, start the engine, run for 10 mins, then switch to 2.
If you need to select #1 to start the motor then you're drawing the house bank far to low and ideally need a bigger bank. You should never need to switch off the house bank. If you do need to then there is a bank size issue or the battery is getting long in the tooth..
Even 50% SOC a single group 24, 27 or 31 should easily start most small Yanmar, Universal, Beta, Nani or Westerbeke's...
A single group 24 bank leaves you with just 37.5 Ah's to use from a 100% full charge. When working off the alternator and only ever getting charged back to about 80% of full. Then you have only 22.5 Ah's to use before hitting the 50% DOD longevity threshold.
22.5 Ah's is far to little capacity for a weekend on most 25-36 +/- footers if you use the devices and lights most use...