A b2b (or dc-dc) charger is a type of isolator that boosts charging voltage to the house battery. These typically cost more and deliver less current than normal isolator types

This kind of controller boosts the chassis voltage to voltage appropriate for proper Absorption.

The most common example is the CTEK D250SA, a 20A solar/alt controller. The SmartPass accessory bypasses the D250SA when the house battery wants > 20A of current. This will generally happen in the first half of Bulk mode. When the house battery demand drops below 20A control is handed back to the D250SA for boosting to Absorption voltage.¹⁾
Note: the CTEK D250SA²⁾ panel voltage (Vpanel) maximum is 23v.³⁾