Unfortunately there is no "right way". Everyone prefers something different. There are piles of switch and battery configurations out there and they all work and I've wired them all ways from Sunday.
*The vast majority of my customers like their 1/2/BOTH and have used it successfully for 20-30 years or more and in most instances doing everything off the house bank including starting. Many of them never test the "reserve battery" but a number of them have had to use it and I can't recall
any of them who did not get the engine to fire. Many have supplemented these switches and converted them from charge directing/use
to a use only switch
by adding a charge management device such as an Echo Charger or combining relay or, years ago, a battery isolator.
*Some of my customers like the 1/2/BOTH with an added On/Off for dedicated engine starting. This works well too.
*Some of my customers will only have a three On/Off system: On/House, On/Engine, On/OFF/Combine/Isolate
*Some love their Blue Sea Dual Circuit Switch
All these systems work none of them is any more right than the other except to the owner. Cases can be made for or against almost all of them but in the end it is what ever floats your boat and helps you sleep at night.
My case is usually: keep it simple, keep it redundant, keep the ability to 100% isolate a bad bank if necessary and run everything from the other. There are many ways to skin that Cat.
On my own boat I have a LiFePO4
lithium house bank and a AGM
"reserve" bank. The LiFePO4
does it all via a 1/2/BOTH used as an On/Off switch. Simple, easy & has redundancy. Even at 80% DOD the LiFePO4 has thousands more cranking amps than my 100% SOC AGM
. Why would I subject my starter to the voltage drop of an LA battery when I can feed it vicious starting current
and only ever touch one switch to turn it on and off. Works for me but maybe not others...