3. an intelligently controlled solenoid system (national luna or IBS for example)
- in this case a solenoid (high current relay) acts as the "boat switch", except in all systems i know of, you only have "A" and "BOTH" settings. the big plus is that you run a small cable to a remote controller to set the state of the solenoid instead of having to run giant cable in order to relocate the "boat" switch (and the solenoid stays near the batteries).

- the intelligence is in the form of a small microcontroller that can monitor either ignition state and/or time and/or each battery's voltage, and based on that information set the solenoid switch to "A" or "BOTH"

- - when i say monitor ignition-state/time/voltage, it depends on the system. some merely tie the batteries in parallel (i.e. the "BOTH" setting) when the car is running and for a few moments after the car has been shut off, then it sets everything back to the main battery (i.e. the "A" setting). other systems may be smarter and actually look at each batteries voltage and the incoming voltage and sense that it's ok to tie them together when there's a supply voltage (i.e. the car is on) and then sense that if for any reason one of them starts to loose voltage (bad battery, whatever), it shuts off feed to that battery.

- ussually these systems have an override so that when you want to, you can manually set the batteries in parallel (the "BOTH" setting) for winching power or for starting from the backup ("B") battery.

- the only downside compared to the "boat switch" + isolator method is that in most systems you only have the "A" and "BOTH" setting, you have no pure "B" setting... i.e. if your "A" battery actually shorts out, then you will have to physically remove it from the system... you wouldn't want to switch to "BOTH" in that scenario. now if your "A" battery is just low (more typical situation), then you can safely switch to "BOTH" in order to start the car still.

- these manufactures will tell you a solenoid (high current relay) is tried and true simple switch that is less prone to failure than fancy digital switches

- national luna and IBS both offer fancy remote controllers with voltage monitors for both batteries, audible alarms, etc.

- IBS's "BOTH" setting is timer based so you can't easily leave them connected for too long. national luna's "BOTH" setting has a warning light to show you it is engaged.

- IBS's system handles 500 amps (200 continuous) whereas national luna is rated at 400 amps (85 continuous). note the continous rating is very different, might matter for when you want to tie both batteries together for a large winch.