Originally Posted by brokesailor
I understand all that but if so then you would never use the Engine Start Battery to start the engine, unless when the day comes the house is totally discharged, because if you do you cant switch back to the house. Doesn't that defeat the whole idea of a engine start battery?
On most sailboats with a 1/2/BOTH/OFF switch the house bank can be used for everything, and it makes it a lot easier. If you have a small bank then you may want to wait to fire up your instruments to avoid drop out during starting but the bank should easily start your small aux engine.
Once you have more than one battery in parallel or series/parallel as a "bank" you combine your cranking amps so most often your house bank will do a far better job at starting the motor than the "start" battery will.
I don't treat my start battery as a "starting battery" I consider it my reserve bank...
With MOST switches it is fine to move between 1, 2 & BOTH with the engine running. What you don't want to do is pass through the OFF position.
An easy test to see if your switch is make before break is to flip on the cabin lights and move between 1, 2 & BOTH slowly. If the lights dim or go off when passing between these positions you have a break before make or just a worn out switch..