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Solenoid isolators
Solenoid isolators are like an automatic switch. When power is fed into the switching terminal of the solenoid, the solenoid is switched on, allowing the power to flow between batteries. This allows you to start from both batteries and winch from both batteries. The batteries are charged together in parallel when the solenoid is switched on. When the solenoid is switched off only the main battery will charge.
The power feed for the solenoid is usually connected to the ignition circuit, so the solenoid will switch on and off with the ignition. You can place a switch inside the cabin of the vehicle to manually switch the solenoid off and on (when the ignition is on) at your desire. If you run your auxiliary battery flat, (for example by using a fridge for a couple of days and not running the motor to charge the battery) there is a danger of flattening your main battery as soon as you turn the ignition on. This is due to the large equalising effect between the batteries. The manual switch is useful as you can stop the batteries paralleling when you turn the ignition on and flattening the main battery. You then switch the solenoid back on once you are travelling.
Problem: Can hide a faulty/flat main battery.
Note: Solenoids and manual switches create voltage spikes, therefore they are NOT recommended on EFI systems. They are also not suitable for charging dissimilar batteries.
