That switch, 4 position, ON, OFF, 1, 2 , should be as follows:
OFF no connection to the batteries at all. No battery charging either. Also means the engine won’t
start.
ON both batteries are connected in parallel to operate ALL loads, engine included, OR to recharge both
battery sets. Also means the eng. start battery is used as part of the house load supply so it can
be discharged to the point the engine cannot be started if this is forgotten about.
1 means only one battery [bank] is connected to the loads. TO me it is usually to the engine start
battery only.
ALso will recharge only the engine start battery. Most alternators are wired to the STARTER so the
ALL should be used while running to charge the house battery [house].
2 means only one battery [bank] is connected to the loads [house], To me it is usually to the house battery only
not including the engine. May not recharge the house battery depending upon the wiring from the
alternator.
Iv’e seen the 1 & 2 switch positions used in reverse but strictly speaking it does not matter as long as YOU are clear about which battery is connected AND any one doing work on the electrical is made aware.
This arrangement is what often is stock from many boat builders.
Often engine starting is done in BOTH and left that way untill you get to your destination and shut the engine down.
This is fine as long as YOU DO NOT FORGET TO CHANGE THE SETTING so the eng. start battery is isolated and protected from the general loads. THe catch is many people do forget and flatten the entire system so the engine cannot be started. Thus all the various configurations to protect against that AND recharge the batteries independently.
Mine was set up this way. One of the first changes I made was to run the alternator output lead to the house set since that is usually, and it is on mine, the set that gets most seriously and often well discharged, heavy use.
I have also installed a COMBINER, to give the engine starts a charge while running. The combiner operates to join the batteries at a particular voltage ensuring the house is well on its way so then they both get what they need, USUALLY.
Just a caveat. Once the engine is running DO NOT turn the switch TO OR THROUGH the OFF or you will likely damage the alternator. You can switch between as long as you go through the ALL position.
Now to your other questions:
I do not know what your engine has for an alternator. Cummins used various units ranging between 60A to approx 100A, maybe more depending upon engine age. There should be a plate on it that will tell you along with the model number.. Look underneath as I have seen mounting positions that put the plate below.
I will guess though that it could be 100.
Alternators do NOT produce much current at idle or close to idle. Maybe 10% of the name plate rating so that 100A alt. may only produce about 10A. at idle. Untill the engine is revving close to half revs or about 1,800 revs the alter. will NOT put out even close to the nameplate rating. There are alternator output curves sheets which will tell you how much output can be expected at the alternator operating rpm , NOT the engine operating rpm.
Even the engine needs electrical to remain running, about 6-10A or so. The fuel control shutdown is energized to allow fuel flow to allow the engine to run.
So yes, it is possible to deplete the battery from a too heavy load if allowed to operate that way long enough. If you use all the batteries and the load is heavy enough to the point the engine quits you may not even be able to restart.
Just depends upon what you operate and for how long and that you MUST determine.
Get the output curves from the mfgr. website once you have the mfgr., model and A rating.
Find out what you are running NOW and do a good estimate of what you intend to install. Keep notes of all the individual pieces and their ratings.
Estimate how many hours all the gear is expected to operate.
A good read and a serious primer for a lot of this is:
—Nigel Calders Boat Owners Mechanical and Electrical Manual and study it. THere is a lot there. Get the fourth edition , the most recent as it will be up to date. You could go for an older edition which will cover the basics of batteries,, wiring and doing it properly, how to estimate loads and so on at much less cost. New about $50 or so , used what ever is available.
Ask questions here too but you must find out what you have now and expect to install before anyone can help a lot more.
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