A b2b (or dc-dc) charger is a type of isolator that boosts charging voltage to the house battery. These typically cost more and are rated for less current than normal isolator types.¹⁾

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

Lead-acid batteries require two things to get fully charged:

1. a high enough Absorption voltage (Vabs)
2. enough time at that voltage for the battery to stop accepting much current.²⁾

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.

Different battery types accept current at different rates. Assuming a 100Ah battery:

* Flooded lead-acid accepts a maximum of about C/5, or 20A and requires a minimum of C/10 (10A). In this case, 10-20A of charging capacity would be appropriate. Less than 10A would be insufficient and more than 20A would be wasted expense. * AGM lead-acid accepts a max of about C/3, or 33A per 100A of battery capacity, and requires a minimum of C/5 (20A). In this case, 20-33A of charging capacity would be appropriate. Less than 20A would be insufficient and more than 33A would be wasted expense. * Lithium has very low internal resistance, and may be able to slurp up all the alternator has to give. However, there are reasons not to max out charging capacity:

• added expense
• lithium does not care about partial states of charge
• lithium tends to last longest with gentler charging/discharging
• the alternator will be happier with lighter loading

For these reasons, an appropriate charging capacity would be as small as possible while still meeting your power needs.

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 adds 80A of alternator-voltage charging when Vbatt is low enough that the alternator can still charge it directly. This combined charging³⁾ 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.⁴⁾

Note: the CTEK D250SA⁵⁾ panel voltage (Vpanel) maximum is 23v.⁶⁾ There are user reports that the unit is repairable after overvoltage or polarity damage. It requires >= 80w of solar for reliable solar charging.⁷⁾⁸⁾ There are anecdotes that the MPPT algorithm is not particularly robust and is better thought of as an add-on for the alternator charging circuit.⁹⁾

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.

relays

• DC1212 series DC-DC isolators in 20-40¹⁰⁾ output. Switched on by ignition circuit on the D+ port.¹¹⁾ DIP switches for battery type. manual (pdf)
• DCC-series of voltage sensing b2b chargers in 20A and 40A versions.
• DCC50S 50A model with MPPT charging. MPPT input limited to 25v.¹²⁾ - manual (pdf)

Redarc makes a 25A b2b charge with MPPT similar to the CTEK.

Ring makes a 30A DC-DC charger.