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Thread: DC/DC Charging Ques

  1. #1

    DC/DC Charging Ques

    Evening all, hoping for some advice on dc/dc setups. My plan is to run a dual battery system, a start and a house AGM (125 vicrtron super cycle) with the agm charged via DC/DC, start will also have the anchor winch on it. Now if the alternator output is 75 amps (at highish revs I presume) and i'm running a 40 amp dc/dc is it possible that I may be loading up the alternator to much in the initial stages after a start and winch up or at times when the start is not at optimum. It would be my understanding that the dc/dc wouldn't kick it until it sensed 13.? volts or so on the start side but I was doing some reading and saw a few stories about overloading alternators due to a big dc/dc unit taking amps at same time as start battery is taking charge. Anyone got any experience with this, I was planning on buying a bug ass dc/dc to charge the agm as fast as possible but just checking this is the right way to go. Motor is 225 Yamaha 4st, alternator output is spec'd at 75.

    Scott

  2. #2

    Re: DC/DC Charging Ques

    I know the REDARC 1240 dc/dc unit doesn’t kick “on” until the unloaded “start” ( alternator) voltage is 13.2 V, so if your start battery has taken a bit of a hiding the Dcdc won’t take any juice from the battery/alternator circuit until it’s been charged enough to bring the terminal voltage back up above the 13.2, so by that stage should not be maxing out the alternator anymore. Just don’t use the “LV” chargers as they will run off 12v.


    Sent from my iPad using Ausfish forums

  3. #3

    Re: DC/DC Charging Ques

    Thanks Swof, that's what I got from the manual of my unit as well, can't overload the alternator given it wont engage until the start is near enough to fully recovered.

  4. #4

    Re: DC/DC Charging Ques

    What about a Lithium battery? Just watching this by Mick Johnson. He is retro fitting them to all the guys in the Bass scene. They are saving 50-80kg in battery weight, hence getting extra speed but lots of positives from this talk, other than the cost of course. He is an auto eleccy at Chinchilla.

  5. #5

    Re: DC/DC Charging Ques

    If the dc charger is seeing the start battery voltage it is going to draw off it too. What will end up happening is the start battery will charge up, the dc will kick and drain the start battery until it reaches cutout voltage (or the alternator keeps up with both) and then it drops out again until cut in threshold is reached. I have that system in my car. Rather than a big DC charger you’d be better off with a smart pass/dc unit that allows full alternator charge until the battery accepts less than 20a then the dc cuts in. My 25a dc charger will draw nearly 40a. A 40a charger is going to draw nearly all your alternator output. They aren’t too efficient at bulk charging. They are very good at finish charging. That is why smart pass units exist. They are efficient so you’ll get the full 75a to the battery instead of 40a during bulk charge.
    Do you need that level of complexity to charge a house battery though? Are you running other dc sources? Panels?

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