Reward for lost Kraken!
Join Date: Apr 2006
Thanked 138 Times in 135 Posts
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"was I lucky? or is my engine good?" Your engine is immaterial. The facts are that there are over 50 alternator designs in the general market, they are internally different with different electronics in the regulators.
A cheaper or older design can burn out in 15 seconds or less if the voltage SENSE lead is disconnected from the batteries, resulting in the alternator going to "battle speed" and staying there, i.e. 17+ volts output.
Note that I said "sense lead", not "primary output". You should be able to disconnect the primary output without any problem. Except, many boats tie the sense lead TO the output, in a variety of ways, because that's cheaper than tying the sense lead to each battery bank (which is proper engineering) and arranging to switch it as you switch the banks.
So, with all those alternator types complicated by all the kuldge wiring...there's no telling what is safe unless you can detail your installation AND your alternator construction. For the past 30-40 years, there have been alternators available with internal regulators that will safely shut down the output during a fault condition, instead of burning themselves out. The necessary "heroic" regulator chip is typically a one dollar part. That makes it totally unaffordable for many engine suppliers, who are bidding on contract for the cheapest parts.
Switching the battery banks while the engine/alternator is running? Perfectly safe IF everything was designed well, installed well, is working well. The only question is, does the prudent mariner rely on "IF" ? Using a battery combiner, or dual knife switches, instead of a rotary battery switch, takes out the "IF". As does getting the technical information on your alternator, directly from the maker.