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electrical:12v:b2b

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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.

limitations

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.

CTEK D250 series

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)

Sterling

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.

Renogy

relays images-na.ssl-images-amazon.com_images_i_51hu4-7nxol._ac_ul160_sr160_160_.jpg

Redarc

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.

electrical/12v/b2b.1584982370.txt.gz · Last modified: 2020/10/11 19:48 (external edit)