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Thread: DC/DC chargers, How do they work and do you really need one.

  1. #1
    Join Date
    Jan 2017
    Geraldton WA
    Total Downloaded

    DC/DC chargers, How do they work and do you really need one.

    My understanding as a "Layman" abut these units is that with modern (Post 2006) 4WD's the alternator doesn't put out enough voltage to keep the Deep Cell batteries at 100% SOC and the DC/DC chargers can increase that voltage to get the secondary battery/batteries up to 100%SOC.
    How they actually do this I have No bloody idea But from all accounts they do the job nicely.
    As my D2a and my 79 series both have alternators that can keep secondary battery/batteries at 100% SOC I have simply installed a VSR (Voltage Sensitive Relay) in the engine bay that prevents the cranking battery from being depleted when I use the secondary battery when the engine isn't running.
    Both the VSR and the DC/DC units are designed so that the cranking battery is maintained at optimum voltage/SOC and it wont go flat when the secondary batteries are being used.
    However, I rarely use the alternator to charge my secondary batteries so I simply use an Anderson plug to tap into the trucks charging system if I need to.
    My main source of power for the secondary batteries is solar and with only one fridge running I very rarely need to top up with the alternator and it is pretty much a stand alone system.

    As far as I am aware most vehicles from 2006 onwards will benefit from fitting a DC/DC charger if they are running a secondary battery to maintain them at 100% SOC or if they are charging the batteries in their caravan/camper when travelling.

    This is my understanding of how these units work, If I have it wrong please let me know
    You only get one shot at life, Aim well

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  2. #2
    Join Date
    Apr 2019
    Total Downloaded

    DC/DC chargers, How do they work and do you realy need one.

    how they work? - dunno

    do I need on? - yes

  3. #3
    Join Date
    Jan 1970
    Avoca Beach
    Total Downloaded
    Yes most cheaper Lithium batteries have definite maximum amp charge specified and they would not like the big inrush at startup.

    I have a 40AH Lithium(supposedly) in my camper trailer and have fitted a 10amp DC/DC charger to supply it, 10 amps being the maximum specified by the battery maker.

    Lithium chargers also cut off when voltage is reached .

    So the reason for fitting a DC/Dc charger IMHO is different.

    I agree with Drivesafe when he says that a DC/Dc charger will not charge an AGM as quickly as a straight VSR , but Lithium are different animals that like to be cuddled.

    Regards PhilipA

  4. #4
    Join Date
    Apr 2008
    Brisbane,some of the time.
    Total Downloaded
    To stop confusion 2006 should be taken out of the equation.

    Some vehicles today donít have a variable voltage alternator,my 2012 D4 doesnít have one,although the 3.0l had one from 2010.

    I am pretty sure the sons 4 month old LC 76 doesnít have one either.

    But many,in fact most late model vehicles do have them.

    D2,D2,D2a,D4,'09 Defender 110(sons), all moved on.

    '56 S1,been in the family since...'56
    Comes out of hibernation every few months for a run

  5. #5
    DiscoMick Guest
    I think you're right.
    As I understand, if your alternator is constant output you probably don't need a DC-DC. If its variable you may.
    However, lithium batteries often do need a higher charging rate than many alternators provide. Check the charging specs for the particular battery.
    Sure someone will be along soon to give a lot more detail.

  6. #6
    Join Date
    Dec 2014
    Total Downloaded
    I don't know much about anything - but I do know that the DC/DC charger in my boat is more effective in keeping my house AGM battery charged than the VSR that it replaced.

  7. #7
    Join Date
    Aug 2006
    Total Downloaded
    really briefly.

    some lithium batteries need a higher voltage than the alternator will supply but at a lower current to fully charge

    some lithium batteries use the same voltage but need a specially shaped current during charging that most alternators wont provide (a computer controlled variable voltage+ current unit could be made to do it but unless you went all lithium you'd compromise the other batteries)

    IMHO, I would not use any generic DC/DC or lead acid charger for charging a lithium battery in a vehicle except for as a patch around to get some life back into someones electrics and even then I would only be doing it if I was absolutely certain that the rated current was not outside of the normal bulk charge current for the battery AND the max voltage was not beyond what the battery could handle.

    you can get specialist chargers for the various types of batteries and I would quite happily recommend fitting something along the lines of an early traxide or homebrew version there of and fitting it in front of the appropriate charger for the battery you want to use.

    if you're charging stock lead acid tech then if you're fitting a DC/DC unit you're wasting money that should be spent on fitting a traxide, better cabling and fusing.

    "In a Landrover the other vehicle is your crumple zone."

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    Fozzy, 2.25D SIII Ex DCA Ute
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  8. #8
    Join Date
    May 2006
    Total Downloaded
    The 2006 bit is rubbish, i ran Tims VSR in our 2013 amarok no dramas, currently Tim is giving it the once over before it goes into our 2018 Patrol.

  9. #9
    Join Date
    Apr 2012
    Whyalla, SA
    Total Downloaded
    An important thing to keep on mind:

    Smart Alternator doesnít necessarily mean it wonít charge in a more conventional manner. Just that it has outside control capability eg. ECU can control its outputs.

    In a similar vein, people losing their **** over the new Defender having 80 something ECUs. An ECU isnít always a fancy computer - the BT module is an ECU, the door sensing module is an ECU..

    A smart alternator isnít always operating at lower voltages - itís a matter of the control systems it uses.

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  10. #10
    Join Date
    Dec 2007
    Brisbane West
    Total Downloaded
    ​Mod hat on.

    The OP has asked for a simple explanation of DC DC.
    Please keep comments and answers civil and without bias toward posters or systems, stick to facts.
    This will prevent the thread being shut down.
    If you don't like trucks, stop buying stuff.

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