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A b2b (or dc-dc) charger is a type of isolator that converts alternator voltage to specific voltage setpoints instead of just passing it through as a relay might.
Because of their electronic nature they are limited to a rated amount of current1) which can protect your alternator from damage. This can be especially important with Lithium and AGM batteries that can demand huge loads from the alternator when discharged.2)
Lead-acid batteries require two things to get fully charged:
DC-DC chargers address the first requirement, but unless you are driving for hours the isolator still won't have enough time to fully charge the batteries. DC-DC chargers were first marketed for cruisers (powered boats) since they run the engines (“cruise”) for hours each day.
Charger throughput is generally limited to 20-30A since higher rates would dictate $$$ internal components. This isn't as big a limitation as it might seem, since lead-acid current acceptance drops substantially throughout Absorption. Untested theory: b2b chargers might work well beside normal isolators; this is essentially the $$$ CTEK SmartPass setup described below.
DC-DC chargers come in different ratings (10A, 40A, 60A, etc) which increase significantly in price as the rating increases.
Bigger is not always better, since battery types accept current at different rates and buying more charging capacity than the battery wants4) is a waste. Assuming a 100Ah battery:
Isolated DC-DC chargers have negative terminals output side (battery bank) “galvanically isolated” from the input side (alternator).
Galvanically isolated is a nice thing to have, especially if you are charging house batteries in a trailer from the tow vehicle engine. The issue relates to how the house battery is grounded to chassis ground when the tow vehicle is disconnected. All in one RVs and vans don't have this issue (probably) so I would not expect them to need galvanic isolation. – HaldorEE5)
The most common b2b charger with 'dwellers is the D250 series, all of which are 20A smart chargers with integrated MPPT:
House and starter batteries are combined when the starter battery holds >13.1v for 5 seconds.6) The batteries are isolated when the starter battery is “<12.8V, for 10 sec… or service battery voltage > starter battery voltage”.
The SmartPass accessory adds 80A-120A (see below) of alternator-voltage charging to the D250's output when Vbatt is low enough that the alternator can still charge it directly. This combined charging7) will happen in the first half of Bulk mode. As house battery voltage rises to normal alternator voltage the smartpass can no longer contribute. At that point the 20A D250SA continues voltage-boosted charging to Absorption voltage.8)
Note: the CTEK D250SA9) panel voltage (Vpanel) maximum is 23v.10) There are user reports that the unit is repairable after overvoltage or polarity damage.11),12)
The MPPT charger will charge the house battery at up to 20A. CTEK specifies 50-300W of panel, though the 23v Vmax limits the unit to [electrical:solar:panels#panel_voltage|nominal 12v panels]. Note: polycrystalline panels may be more useful here, as their Voc should be a bit lower than mono.
The D250-series trickle charges the starter battery from a solar panel at intervals of 3 seconds if the service battery is fully charged.13)
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.
Ring makes a 30A DC-DC charger with 50v max input MPPT. (manual)
Kisae makes a DMT1230 DC-DC charger 30A charger with 45v MPPT input. They also make a 50A model (DMT1250).
Customer support is generally regarded as excellent.
The FlexCharge NC25a-12 is a 25A charger that claims exceptional efficiency17), the ability to dump loads, and a proprietary charging algorithm.
Victron makes several DC-DC chargers, some of which are configurable by bluetooth. 12v examples:
Bogart's SC-2030 can work as a DC-DC charger as well as a PWM charge controller.
DC-DC charger (B2B) options and general discussion on DIY Solar Power Forum.