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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
The most common b2b charger with 'dwellers is the CTEK D250SA, a 20A solar/alt controller. It has connections for both alternator and solar panel input + MPPT function, although the MPPT algo may not be partcularly robust.
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) It requires >= 80w of solar for reliable solar charging.4)5) There are reports that the MPPT algorithm is not particularly robust and is better thought of as an add-on for the alternator charging circuit.6)
The Sterling Pro Batt Ultra series is alternator-charging only, but can be configured to exact setpoints. The BB1260-12 is a 30A example and costs about $50 more than the D250S above.
Redarc makes a 25A b2b charge with MPPT similar to the CTEK.