australia:dcdccharging
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
- cars will not generate enough voltage to charge an auxiliary lithium battery so you will need to use a DC-DC charger wired directly to the cranking battery (these will have a built-in voltage sensitive relay (VSR) or similar mechanism to stop draining your car battery when the car stops) which will be powered by the alternator
- a 25A charger will suffice for batteries up to 200Ah, however a 40-50A charger will be best for 100-300Ah batteries
- these require professional level wiring from your car battery to the DC-DC charger - and this should be near your auxiliary battery so you don't lose charge voltage
- 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
Installation tips
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- crimp don't solder as solder gets brittle and breaks on corrugated roads
- cover wires with corrugated pipe to avoid wires rubbing away on sharp edges
- fix wires to chassis every 30cm
- charger should be near aux battery to avoid charge voltage drop over a longer distance and a fuse on the positive wire between them
- 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
- Victron Orion XS 1400 DC-DC Charger for 12V or 24V systems
- July 2025 model
- 50A: 700W (12V system) to 1400W (24V system) output at up to 40degC (ie 50A at 28V)
- max. wiring size is 4AWG 21.2mm2; 0.52kg; 138.1 x 124.4 x 53mm;
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- 50A 700W (12V system only) output at up to 40degC (ie 50A at 14V); max. wiring size is 4AWG 21.2mm2; 0.33kg; 137.3 x 123.1 x 40 mm
- this is highly recommended; new 2024 digital technology with 97-98% efficiency compared to older models which are 87% and need a heat sink, smaller and lighter for its 50A capacity than conventional models as it doesn't need a heat sink but if you need solar, you will need a separate solar controller.
- in ambient 9degC temperatures, in horizontal position on a table (which it shouldn't be) running it at 50A for 1hr resulted in it being 40degC at 550W (11.8V input in this case - you only get 700W if input is 14V) whereas the older model hit 67degC while only doing 320W due to over-heating, although the metal under-surface does get much hotter although it shouldn't be touching anything 2)
- you will need 16mm2 AWG5 100A rated cable with a 80A fuse near your batteries, and you will need to use pins on the ends of the cable to connect to the Orion XS - see https://www.youtube.com/watch?v=TJO47csZaFk - and as he suggests, add a second wire to the input pin in case you want to add voltage checking of alternator, or a component to trickle charge your main car battery
- you engine ignition wire if you have one goes into the L port - NB. the L-pin does not override the remote on/off function - if you use a remote on/off switch, it must also be wired as per the manual
- in the Bluetooth app you can:
- step down in input and output currents if your wiring, alternator (nearly all will cope with 50A) or charging battery cannot cope with 50A
- choose the battery charging algorithms depending upon battery chemistry
- decide whether to allow the Orion XS to “determine” when the car engine is running or not (if you have an engine ignition cable then turn this off!)
- if you have a smart alternator and no engine ignition cable, then this becomes difficult as the alternator may drop its voltage below 12.8V or even 12.4V even when the engine is running and you so not want to set the Orion XS cutoff voltage to below 12.4V otherwise it will cause issues with your car battery - you may have to settle with it being 12.8V, and you may wish to have the “start voltage” lowered to 13.0V, and set input lockout voltage for 12.5V and restart voltage 12.8V (you can drop these below 12V IF you have a large voltage drop from car battery to DC DC charger) - see https://www.youtube.com/watch?v=9KZM4ySD6LY
- add the DC-DC charger to the other VE devices via the VE network so they can share data with each other to optimise function especially if you have multiple MPPT solar controllers in use
- importantly, allow change of device PIN so rogues walking past can't change your settings or turn it off
Older style DC-DC chargers with solar MPPT controllers
- Redarc BCDC Alpha 50 - perhaps the best you can get for most purposes in Australia
Dual battery wiring kits (charger not supplied)
- https://www.australiandirect.com.au/Battery-Systems/Wiring-Connections/KADBWK8MMPP 6m with plugs and a KICKASS® Maxifuse Cable to connect to main battery with 80A Maxifuse pre-installed $55
australia/dcdccharging.txt · Last modified: 2025/07/13 16:29 by gary1