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12volt right and 24 volt battery?

Discussion in 'On the Road' started by KB7KJY, Oct 10, 2019.

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  1. KB7KJY

    KB7KJY Ham Member QRZ Page

    Hey Everyone,

    Chugging along on my RV project and have run into another QRM related question.

    I was planning on using a 24 volt to 12 volt converter to feed my radio gear from the 24 volt house battery. I was looking at this one:

    I have not purchased it yet and thought I would check and see if anyone has a better suggestion?

    Back in the '80s I used small 24-12 converters in LMR installations in heavy equipment. They were analog devices and didn't produce any QRM. The one that I am considering has a digital portion and no FCC cert. I am concerned that it may be one more source of QRM that I have to deal with.

  2. KB7KJY

    KB7KJY Ham Member QRZ Page

    Rather than starting another thread with another RV question I am going to throw it into the mix here.

    I read the following on another site:

    "There is another potential problem exacerbated by power inverters, and that is a Ground Loop. With one exception, RV manufacturers don't make the chassis of their coaches—bus, truck, and van manufacturers do that for them. Little if any thought goes into how things are grounded throughout most RVs, and if you're not into amateur radio, you'd probably never notice. Correcting built-in ground loop problems can be a very exasperating undertaking, especially when the coach and/or chassis manufacturer won't cooperate."

    My RV is not typical. It is all steel. Floor, walls and roof.

    I am grounding the starting batteries, house batteries, Solar charge controllerer, inverter, generator and 120v load center to the frame/body. I am hoping that this will avoid the issues described above.

    Any thoughts? Am I missing anything?

  3. KF5LJW

    KF5LJW Ham Member QRZ Page

    You would be better off using the vehicle 12 volt battery system rather than a converter. The Victron model you linked too is a decent entry level device, and should be fairly quite.

    Back in the day 24/12 converters were analog series voltage regulators, also made good space and hand warmers. They were just under 50% efficient. If your gizmo inputs 100 watts, requires you to draw 200 watts from the battery. The lost 100 watts is burned off as waste heat to warm the cabin or hands.

    Today they are Switch Mode converters around 90% efficient but can be very noisy. Manufactures cut corners yo save money and do not put much filtering on output.
    K0UO likes this.
  4. KB7KJY

    KB7KJY Ham Member QRZ Page

    Auto-correct got me again..... I have a 12 volt rig. Not a 12volt right....
  5. KB7KJY

    KB7KJY Ham Member QRZ Page

  6. KB7KJY

    KB7KJY Ham Member QRZ Page

    I appreciate the suggestion.

    I don't know about about running it off of the starting batteries.

    If I used a 24v to 12v battery charger that might be OK. I would still worry about draining the starting batteries if there was any trouble with the charger.

    If I manage to drain the house batteries it would be inconvenient. If I drain the starting batteries I may have a REALLY long walk.

    Thanks again.
  7. N8AFT

    N8AFT Ham Member QRZ Page

    Get yourself a battery isolator and do it up....
    Cole Hersee makes fine ones.
    That way you won't affect the crank battery and you keep the coach charged too.
    A 12v coach battery. I run an FAA certified Sealed Deep Cycle AGM 12v 100ah.
    K3XR likes this.
  8. K3XR

    K3XR Ham Member QRZ Page

    It's been more than a few years but did this with a couple of the VW camper buses I owned. Best way to insure you don't run the vehicle battery down.
    If I am fixed at a campsite with power just bring along a smaller switching power supply and run off of AC.
    N8AFT likes this.
  9. M6AGL

    M6AGL Ham Member QRZ Page

    You should still be able to use a small switch mode converter but the devil is in the details. It needs a tuned filter on the input and output, and also put in a sealed earthed aluminium case. Plus you need to pay attention to the design notes of the PWM IC. That is short high current paths - and star ground or ground plane design.
    I have been working on PSU design of the PRC320 military radio and it has an initial stage step down from the 24v battery to 12v operating at 20KHz, and from there to other stages for other supply voltages. So this proves it is possible with good design and proper filtering, as this is on a UK military radio costing £12k.

    I would have a go with one and design an input and output filter for it, then put it in a diecast chassis box, and earth that to the bodywork. I redesigned a modern replacement, and got the ripple down to 20mV in the same space as the original, and using through-hole components for easier future repair...EXCEPT the Buck and boost stages. SMT is just closer to the board and the inductors are fully enclosed type. Make sure you choose a type with block type enclosed inductors, and check for heating under full load.

    Worth a try.
  10. K0UO

    K0UO Platinum Subscriber Platinum Subscriber QRZ Page

    20191010_184619.jpg Most motorhomes and RVs may have dual batteries setups but they're all 12 volt. Two 12 volt batteries in parallel.
    The only thing I have that uses 24 volts nowadays is my airplane and heavy caterpillar equipment.
    All modern converters will have terrible noise I've tried many, by far the best one you can buy and it is reasonably priced is
    Astron N2412-24 DC to DC

    Last edited: Oct 11, 2019

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