The common battery switch has positions 1-BOTH-2-OFF You can rotate the knob either direction, full rotations. If you go either battery through BOTH then you are safe, connection to the other battery is made before connection to the previous battery is lost. If you rotate the switch so that you pass through OFF then it is REALLY IMPORTANT that the engine be off! ALternators have the electrical equivalent of momentum, they can not stop the flow of current instantaneously. If your batteries were low, and the alternator was pumping out maximum current, and you switch to OFF you will create a very bad situation. It is very likely that you will damage the alternator and it is also possible that you can destroy all the electronics in your boat. Some battery switches have a "field disconnect" option that will shut down the alternator as you rotate towards "OFF". This requires additional wiring, and it also requires a voltage regulator and/or alternator setup that gives you access to the alternator field windings, which is not common. As long as you pass through BOTH, however, everything is fine. Back to the original subject: The "battery isolator" will at least provide protection for the alternator/electronics as it prevents any damage from my above described scenario because alternator current will always have a path through the isolator to the batteries. The down side, which I think is very significant for most of the people on this forum, is that the isolator will also dramatically reduce the effective output of the alternator unless you get a voltage regulator with "remote sense" (more wiring, harder to install, requires non standard alternator with remote voltage regulator). A battery disconnect switch (such as the brand name "Perko") is a very good idea in general, IF it is properly installed. Since all the battery current must route through the battery switch, including the starter current, you must use heavy gauge wire (#4 AWG minimum, #2 preferred). The switch wiring will certainly increase the wire run to the starter so you may need to increase the size of the wire over what it currently is to compensate for the extra run. The major problem with the battery switch is that you have to remember to operate it. It must be on "BOTH" while you are running so both batteries will charge. You must switch to either "1" or "2" when you stop otherwise you might run down both batteries and leave you stranded. If you add the battery combiner then you simplify the operation. Just set the battery switch on either "1" or "2" (I would pick the battery that had the shortest wire run) and leave it there. The combiner will connect the second battery when the engine is running and the alternator is working.
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