Isolator or relay for dual battery system???
>A common starter solenoid has a 3.5~4 ohm coil, a "contiouous duty" solenoid, like in a forklift, has a 18 ohm coil. Any parts store can get them, just be specific. They look identical. Works great for dual battery isolation. <
You MUST use the continuous duty relay if you want to actually use the second battery to power the vehicle [bad alternator]. Otherwise the relay coil will overheat and fry. Either type of relay can be used to "jump" the main battery from the aux.
To keep both batteries charged properly you MUST use a battery isolator! If you do not, one battery will discharge the other over time, or one will fail to charge, due to differences that develop in their electrochemical makeup, even if they are exactly the same to start! If you are using a deep cycle battery for the aux the problem is magnified. Or if you start out with mismatched batteries [very bad idea] the system will fail rapidly. Ford used to use the continuous duty relay setup as an OEM solution because it was cheap and it would usually last thru a 1 year warranty period with new batteries. It never did work for long, and could not work due to battery chemistry problems. If you use an isolator you can use different types of batteries in the system, or even a new and old(er) one.
ALWAYS USE AN ISOLATOR! You can use the proper relay to bypass the isolator for those high current needs.