Most suited for very light use, a dual battery system charges both the house and starter battery from the alternator.

In order to keep from getting stranded it is set up as a one way system: the house battery can be charged from the alternator but the house cannot pull power from the starter battery.

“the simple continuous duty solenoid is the easiest simplest most effective way to have battery isolation”¹⁾ - Sternwake

The house battery is sequestered by one of the following methods:

solenoid - the generally preferred method is to use a continuous duty solenoid to connect the house circuit when the ignition is in the RUN position.²⁾ Solenoids are generally cylindrical. A solenoid “costs” about 0.8A to keep the electromagnet energized.³⁾

isolator - Battery isolators use diodes to keep power from flowing from the starter battery to the house battery. Isolators are generally brick shaped an have heat fins. An isolator “costs” about 1v in losses across the diodes.⁴⁾

manual switch - A manual battery switch normally has 4 positions: A, B, A+B, and Off. A would be for the starter battery and used during starting. B would be used for house use when one is not driving. A+B could be used to combine both sets for starting or for charging while driving. This kind of setup is prone to user error. A manual switch has no current or voltage losses.

House batteries require a lengthy absorption stage that takes multiple hours. It is unlikely to get the house battery completely charged unless one is driving significant distances. This can have an impact on battery longevity.