Table of Contents

DC-DC charging of auxiliary batteries using your car's alternator

see also:

  • I don't sell any of these nor do I receive any remuneration if you buy them, and I have not personally reviewed all of them, they are listed here to give you perspective

Introduction

  • buy one with Bluetooth so you can program it to use lower currents if need be and more easily monitor what it is doing
  • if you want maximum currents from DC-DC chargers, you probably will need to add on active fan coolers
  • consider buying a 50A unit designed for Australian conditions
    • eg. Redarc BCDC Alpha 50
      • has several advantages over the Victron Orion XS 12-12-50A:
        • takes longer to heat up as it has better thermal dissipation, current throttling starts later at 55degC not 40degC and it keeps working until 80degC not just 60degC so it will deliver much more current
        • has built in 48V 1000W MPPT solar controller which the Victron doesn't have
        • can charge a 12V starter battery via auxiliary battery or solar - the Victron doesn't have this capability
        • IP67 not just IP65 so it can be placed under the hood if need be
        • allows larger diameter wires in case you have a long distance to the starter battery
        • external thermometer lead which goes to battery negative terminal - Victron has internal thermometer
        • output voltage is up to 24V not 17V
  • or consider buying a 98% efficient digital technology charger as this will be much more compact but deliver less current in warmer conditions
    • eg. Victron Orion XS 12-12-50A
      • has several advantages over the Redarc Alpha 50:
        • more compact and 1/3rd the weight
        • much better Bluetooth app and integrates with the much cheaper and better Victron server system
        • has a few more set-up options over Redarc - eg. no input trigger option in Redarc
        • has strain protection for connectors
        • has negative isolation, Redarc doesn't
        • may need to add a heat sink or a fan underneath it - the Redarc may need a fan on top of it
        • much cheaper on special at ~$450 vs $899 and has 5yr warranty not just 2yrs but you do need a good reseller for local support
  • see also https://www.youtube.com/watch?v=GaOc9Gx4CqA
  • do not use a AC-DC or solar charger directly onto your main car battery while it is connected to a DC-DC charger and a partly full auxiliary battery
    • the DC-DC charger will detect the voltage of the main battery charger and commence charging your auxiliary battery potentially at a rate higher than the charger to the main battery can provide, and this will result in your main battery going flat!
    • also if your DC-DC charger is connected to your vehicle ignition cable, don't leave your key in the ON position without vehicle running as this may drain your main battery
    • see https://www.youtube.com/watch?v=rDXixCzg6iw

Installation tips

  • ideally you need to connect the car's engine ignition wire to the DC-DC charger so the charger can detect when to start and stop
    • NB. if you do not have an engine ignition wire, then you can use a relay to detect the ripples the alternator generates such as the EasyAs relay 1)
  • the DC-DC charger needs to be kept cool - if it heats up, it will drop the current

Example 98% efficient DC-DC chargers

Older style DC-DC chargers with solar MPPT controllers

Dual battery wiring kits (charger not supplied)

1)
https://www.youtube.com/watch?v=HdXL6s1lXEM
2)
https://www.youtube.com/watch?v=nXvUIUsOSAU