There are scenarios where one might want to prevent an isolator from combining the house and chassis 12v systems:

1. the house battery needs less voltage than lead-acid (lithium, for example)
2. the combined circuit can feed higher-than-optimal voltages back from solar charge controller to the chassis' electronics.

Isolator/relays/solenoids, etc, require a relatively small amount power to operate. If this power is disconnected the isolator will turn off and the batteries will not be combined.¹⁾ Using a High Voltage Disconnect to kill power to the isolator will effectively stop the alternator from charging the house battery at a given voltage setpoint.

Given that:

Alternator charging methods combine the batteries only at certain times:

by 12v trigger. Example: 12v source that's only active when the key is in the RUN position. These have 4 lugs: starting+, house+, trigger+, trigger-; or,

by voltage sensing. These usually have 3 lugs: starting+ in, house+ out, starting-. The starting- “ground” is only used to complete a small circuit so the relay can run itself. So it's usually very thin.

If you disrupt (or “ground”) the relay cannot turn on, cannot the batteries.

So-o-o-o-o-o…..

You can:

Use a high voltage disconnect on the house side set to something very safe (13.5V?) that would disrupt the trigger (or ground) circuit.

Example: you start out the morning with house batteries at 12.5v or something. Drive to McDonalds for coffee and free wifi.

On the way the alternator brings the house bank up to 13.5V. The HVD disrupts the trigger (or ground) and the relay/isolator turns off.

Caveat: I am currently doing this on my van at 13.8V even though I have FLA batteries. I do this to keep solar from pushing 14.7v into the chassis while driving if the batteries happen to be in Absorption.