It is called a Battery Isolator. It uses diodes to separate the alternator output to the battery sets, and prevent/isolate the batteries from discharging into one another.
They do work BUT and in battery charging system this is important that they cause a voltage drop of approx. 0.6V which to our systems is a lot.
THere are lots of work arounds.
But even better are the newer combiners/smart combiners, or ACR.
THese create virtually no Vdrop as they are simply a large relay OR use Mosfets for VERY Low Vdrop to the batteries.
Some electronic controls are used to sense voltage buildup for closing and joining the battery sets
OR
voltage dropping to open thus isolating the battery sets to prevent an unintended discharge.
THis one has two inputs from two alternators and three outputs to three batteries.