Best way to use battery switch with this set up?

[ejbpesca]

My small cruiser has two batteries, a shore power charger, and a 1 - 2- ALL - OFF battery switch. Should I used one battery to crank engine and the other as house battery or just us the ALL setting for everything?

 

[boatman37]

i use one for startng and one for house. i wouldn't want to be running off both while anchored then not be able to start it to get back

 

[Silvertip]

How you use it depends on how it is wired. If you have all loads connected to the COM terminal, then there is no house and starting battery other than what's on the battery label. If you anchor for long periods without running the engine then whatever battery you want to keep isolated for starting is the one you DO NOT want selected when anchored. In other words, start on BAT 1 and anchor on BAT 2. Under way, select the battery that needs the most attention from the charging system. Remember, charging systems are not an unlimited source of power so to get maximum benefit for a battery, select only the one that is most deeply discharged. If your engine is properly tuned and you know how to start it reliably without repeated cranking, you can start an engine dozens of times without charging the battery.

If you have all house loads on BAT 2 for example, and only the starter on BAT 1, then ALL (or BOTH) would be used for charging at the dock or at home with the on-board charger. At anchor = BAT 2, Starting = BAT 1. You would not anchor with the switch on ALL as you could kill both batteries. So you see, you presented a very vague question that has multiple answers. ​You are the traffic cop for your charging system and power system. Only you know how its wired and how you boat.

 

[airshot]

On my boat, I have a standard marine starting battery for the engine only. This takes care of starting the main engine and nothing else. For battery #2 I use a deep cycle group 27 battery and this runs everything else on the boat including all s/s radios, depth finders, lights etc. This way I aways know I have power to get back. Because deep cycle batteries do not favor a fast or high charge rate I use a deep cycle battery plug in charger to keep the battery #2 at full charge for longer life. In case of a quick need I can switch to bat#2 and charge of the engine but have never needed to do that for the past 5 years. In fact bat#2 has gone on many fishing trips without being charged by the plug in charger and has never ran out. By using a deep cycle bat for the guest battery it will hold up much longer than a starting battery with constant drain.

 

[Silvertip]

What is often overlooked is the charging system output of the engine. An outboard with around a 15 amp output is almost worthless at keeping a deeply discharged house battery topped off, especially on short runs. That output is barely enough to keep the starting battery charged. Obviously the situation is less of an issue with engines having much higher charging system output.