Is the ESS Battery a deep cycle battery? I hope the JLU can be jump started if the ESS battery goes bad.Additional info that you might be interested in..
You can have a fully charged Main battery and if the ESS battery is either dead or disconnected, the JL will not start, even though the main battery is fully charged. On the other hand, if the main battery is missing or dead the ESS battery can start the JL/JLU.
If you wanted to isolate the positive side, disconnect N1 & N2 in the power distribution box.
N1-Main battery, N-2 ESS battery.