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Battery to battery chargers

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)

images-na.ssl-images-amazon.com_images_i_3131jbgz9ll._ac_us218_.jpg 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.

Bigger is not always better, since battery types accept current at different rates and buying more charging capacity than the battery wants3) is a waste. 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 for battery health 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 for battery health 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, for the following reasons an appropriate charging capacity for lithium would be as small as possible while still meeting your power needs:
    • 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

common examples

CTEK D250 series

The most common b2b charger with 'dwellers is the D250 series, all of which are 20A smart chargers with integrated MPPT:

  • D250 (pre-2018, discontinued) - the original, Vabs == 14.4v, Vfloat == 13.6v.
  • CTEK D250SA, a 20A solar/alt controller. Vabs selectable 14.4v and 14.7v. Vfloat 13.6v. Added support for smart alternators.
  • D250SE which has 14.2 Vabs and 13.3 Vfloat for lithium settings.

House and starter batteries are combined when the starter battery holds >13.1v for 5 seconds.4) 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 charging5) 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.6)

  • Smartpass 80 - original model. 80A + 20A = 100A.
  • Smartpass 120. 120A + 20A = 140A.
  • SmartPass 120S as above, but with support for smart alternators.

Note: the CTEK D250SA7) panel voltage (Vpanel) maximum is 23v.8) There are user reports that the unit is repairable after overvoltage or polarity damage.9),10)

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.11)


images-na.ssl-images-amazon.com_images_i_41ic_8sivjl._ac_us218_.jpg 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 images-na.ssl-images-amazon.com_images_i_51hu4-7nxol._ac_ul160_sr160_160_.jpg


images-na.ssl-images-amazon.com_images_i_417f858gcdl._ac_us218_.jpg Redarc makes a 25A b2b charge with MPPT similar to the CTEK.

Ring Automotive

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.

An extensive write-up.


The FlexCharge NC25a-12 is a 25A charger that claims exceptional efficiency15), 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:

  • Orion-Tr 12|12-20 20A
  • Orion-Tr 12|12-30 30A
electrical/12v/b2b.txt · Last modified: 2020/10/11 19:48 (external edit)