Why have a 1-2 switch at all? In fact Ample Power says that all battery 1-2-both switches are prone to failure because of the high loads put on them by water makers and the like. The only switch that they recommend is a emergency bypass switch should the dedicated starter battery fail.
Please take Ample Powers stuff with a grain of salt as there is some questionable advice in there.. Since reading that many years ago I have been looking everywhere for these so called melted switches that are going to happen. Perhaps thousands of switches and not yet found a melted one.. Just yesterday alone I saw the backs of 6 battery switches.
Battery switches are required under ABYC standards if the cranking amp capacity greater than 800 or the Ah capacity is 100Ah or greater. Cranking amps is the default. This 800 CA exemption however was intended to allow for small boats such as skiffs and open boats to avoid a switch in a corrosive environment. Surveyors follow ABYC and a boat, with a cabin, without a battery switch will very likely get flagged. If you need to pass an insurance survey you will need a battery switch. Beyond that they are just a wise investment.
What happens when you smell smoke, as happened to myself and another member here on this forum during a race? You dive for the battery switch and cut it immediately. No battery switch............?
What happens when you have a battery that develops a catastrophic internal fault? Do you get in there and physically un-wire it risking battery acid in the face if it goes thermal? With a battery switch you simply isolate the bank/battery.
I work on boats for a living, as a marine electrician, and have never once seen a physically melted US type battery switch, (European Beneteau/Jenneau, Hanse type switches I do see melted and failed) that was properly rated for the use. Even then I see piles and piles of switches that were undersized for the use (usually on power boats) and they still survive..
This one came off a commercial lobsterboat. It started a very large 3208 Caterpillar diesel and ran massive pumps, and equipment, for nearly 25 years. It was only removed during a re-wire and the proper HD switch was installed. I though for sure this one would be my golden ticket to Ample's "melted switch phenomenon" but it was not. Despite this boat ideally needing a "heavy duty" battery switch this standard grade switch was in fine health even after 25 years......
I have opened up a number of switches to autopsy them and found even 35 year old switches in fine health. For sailboats finding a melted one has proven to be the proverbial needle in the haystack..
The loads imposed by sailboats barely even constitutes a load to a battery switch. I work on boats with 1600HP+ engines that start with Guest, Perko or Blue Sea battery switches. I have bow thrusters that pull 300A continuous (600A in-rush) that run through battery switches and again not a single failure/melt down. A water maker does not even begin to constitute a load to a battery switch....
Even Blue Seas smallest battery switches, the "mini" series (m-Series) can handle
300A continuously, 500A for 5 minutes, 775A for 1 minute and 1500A for 10 seconds. There is NOTHING on our sailboats that can exceed these load ratings and this is the absolute smallest battery switch Blue Sea makes...
The Blue Sea e-Series is:
350A continuously, 600A for 5 minutes, 1000A for 1 minute and 2000A for 10 seconds
The Blue Sea HD Series is:
600A continuously, 700A for 5 minutes, 1150A for 1 minute and 2750A for 10 seconds.
Keep in mind that there is not even battery cable big enough to handle the ampacity that a Blue Sea HD switch can, and they are reasonably priced....
Even 4/0 105C battery cable can only handle a max ampacity of 445A outside an engine space and 378A inside an engine space. A Blue Sea HD switch can handle 600A without issue. Even the mini series continuous rating exceeds the max ampacity of 1/0 105C wire and the standard series continuous exceeds 2/0 105C wire.....
Again, some of what Ample has written has not held true in the real world. Take for instance their article on breaking in lead acid batteries. Trojan Battery vehemently disagrees with his suggested process yet it is posted as fact by Ample power.... I think I will trust the actual battery manufacturer on how best to break in service batteries....