Using a Battery Change over switch - Lesson please

[ditchcrawler]

As I see it the purpose of a 1/2/both switch is to keep one battery for starting & the other battery/batteries for domestics. I have a Red Flash AGM starter battery & two 110a/h domestic batteries.Therefore for me always start on 1 charge for 15/20 mins then switch to 2 to charge domestics.It is of couse possible to start & run on 2 for both operations but my battery set up is designed to make use of the abilities of specific batteries.It is unwise therefore to be dogmatic about the 1/2/both switch,but the system is designed to allow you to have a separate battery just for engine starting if you wish.Otherwise why have a 1/2/both switch.I have never yet forgotten to operate the switch correctly but give it time ( as I am more forgetful now past 60).This is a personal view of course.

 

[William_H]

One very good reason for not starting on both every time is that while this does give more power to the stater it can mask the failing of one battery. Batteries often first fail in not being able to start the engine while appearing normal otherwise. You could easily have one or even both batteries knackered form old age without knowing it because together they can still start the engine. So at least occasionally make sure the engine can start on either battery alone.

Cliffs comment about volt drop causing instruments (GPS) to drop out or reset may be very valid.

Starting batteries are indeed designed for starting with large current capability and not so good deep cycle capability. Especially for their small size and weight.
However a large capacity deep cycle will also generate plenty of CCA by virte of being a big battery. They are certainly OK for starting a small engine.

Yes there are plenty of people who say that an alternator running with no load can be damaged. I certainly can not see why that should happen from a technical point of view. So if I had one I woulldn't be afraid to go through the off position. But of course I may end up sorry too. I know the battery lead fell off my car once while driving with no harm done. Engine did stop however. olewill

 

[Anonymous]

[ QUOTE ]
Yes there are plenty of people who say that an alternator running with no load can be damaged. I certainly can not see why that should happen from a technical point of view.

[/ QUOTE ]I've seen it happen twice on the same boat for the same reason! I suspect that the cause is the energy stored in the inductance of the alternator - if you try to stop current flowing instantaneously in an inductor you get very high voltages, more than enough to blow diodes. If you use a make before break switch then there is no break in the current flow, so no voltage spike. I've never tested such a circuit but we know it happens and that seems like a likely mechanism.