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Thread: 12v vs. 13.8v?

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  1. #1

    Default 12v vs. 13.8v?

    I was wondering if there is any advantage or difference in running a radio off of 13.8 volts as opposed to 12 volts, because I have a friend who says that radios will only work properly with 13.8 volts and that 12 volts is not enough power. But I have ran the radio off of just a 12v gel battery and the radio seemed to work just fine with only 12.6volts from the battery(dropped to 11.9 on voice peaks running 100watts and lights on radio dimmed), when I hooked that same radio up to a 13.8volt power supply the only difference was that the lights didn't dim, and I seemed to be getting out exactly the same as I was when I running it off of 12volts. I also tried this with another radio and I did not notice any difference whatsoever between the 12volt battery and 13.8volt power supply (power output was the same and the lights did not dim). So my question is does the radio care if you run it off of 12volts as opposed to 13.8volts? Is it any better running off of 13.8volts rather than 12volts? Can it damage anything if you supply it with 1.8volts lower or higher than the rated voltage of the radio? If the extra 1.8 volts does not make a difference then why do they make power supplies that put out 13.8volts instead of 12volts(many 12volt power supplies actually put out 13.8volts NOT 12volts)? Is it so you can use them to charge the 12volt batteries (because you need more than 12volts to charge a 12volt battery and the battery charger I have actually puts out anywhere between 12.9 and 16volts depending on charge state of the battery)? It seems like using a 12volt 20A power supply would be more efficient than using a 13.8v 20A because it will use slightly less wattage due to a lower output voltage at the same current (assuming they have the same efficiency and are both putting out the same amount of current), which would result in a (slightly) cheaper electric bill when operating the radios. Have any of you noticed any difference between the two voltages?

  2. #2
    Join Date
    Jan 2005
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    ORANGE CNTY, CA
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    today's most radios newer than 1980, they have internal voltage regulator except final , or may be driver amplifier. those voltage regulators are usually regulated to 5 V or 8V or so.
    so, as long as input voltage stay above 1 to 2 V above 8V or so, most radio works fine, just have lower output power.
    I have 2 radios TEN-TEC OMNI-V designed around 1988, and Yaesu FT-2800M designed around 2004

    OMNI-V had circuit , it shut down the radio when input voltage become less than 10.5 V or so. when I connect to 12 V deep cycle battery, voltage often drop to 10 V under full load of 100 watts output, so I modified, so it does not shut down until 9V or so, it work fine now.

    FT-2800M never had trouble with battery voltage dropped to 10 V, it is just output power dropped to 50 watts or so.

  3. #3

    Default

    Voltage regulators can't work miracles so I wouldn't run too low. Low voltage can kill high power FET's.

  4. #4
    Join Date
    Jul 2001
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    21,474

    Default

    That "dimming of the lights" that you see is an indication ( however slight ) that you are not transmitting a linear signal, that is the variations in the RF output signal do not match the variations in the audio modulating signal.

    Although the distortion is slight, it IS distortion. Older radios did not handle this drop in power supply voltage as well as some of the newer ones appear to do, but in any case, operation should be adjusted so as to keep the light flickering to a minimum. Watching your ALC level is a good indication of how linear your output signal is. The ALC should just "flicker", or show a slight positive indication. More than that, and the distortion products will probably be noticable by the operator at the other end.

    JMHO 73, Jim

  5. #5
    Join Date
    Oct 2001
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    "Downeast" Maine
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    Having used battery power for a long time I concur with the previous post.. I have a FT-847 that runs fine down to about 10.8 to 10.9 at which point it just cuts out. (display blinks on & off and signal goes to crap) No noticeable damage to the rig. Pretty much the same results on a FT-1500 mobile rig.
    Look at the specifications page in the manual.. My 847 for example states 13.8 volts +/- 10%
    So 13.9 - 1.38= 12.42 Now I know it will go quite a bit below that but they have to be conservative with their figures.
    If you have a variable power supply find your critical cutoff voltage and plan to stay above it.
    In my experience with any radio.. the worst that has happened is I lost my programmed memories on a FT-2600.
    YMMV but in my case 11.5 to 15.5 seems to be a decent voltage range.
    73..JW
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  6. #6
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    Dec 2010
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    No difference from 12 to 13.8 volts. Your radio will pull the current it needs. I wouldnt worry about a volt difference. Now if the voltage drops to 10 or lower voltage or climbs above 14 volts I would be concerned.

  7. #7
    Join Date
    Aug 2008
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    Mpls. , MN.
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    Basically any product out there has limits , high & low .
    An good analogy would be a car , lets say the red line is 3,800 RPM , running it at 3,795 all the time and thinking that is below the red line is OK is a fools game .
    On the other end , the low end , running the car in high gear to keep the RPM down is very hard on the engine , something like going up a long hill in high gear with RPM @ 600 is almost worse than high RPM , its about momentum , not enough to go up hill , or too much and the parts want to fly away .
    13.8 volts is what the average car / trucks alternator running down the road puts out for running the electronics , high voltage for charging a low battery .
    A fully charged battery is 12.75 volts and car batteries are designed to be discharged a max of 20% on an occasional basis , that does not mean volts , but capacity which is more to do with amps @ volts over time .
    Running outside those limits and expecting the manufactures to back up what you destroy is figured into there costs [ that we pay !!! ] , it's too late , but I say quit running up the cost of everything .

  8. #8

    Default

    The original poster seems to have answered his own question. Yes it will function on just 12 volts. However if the lights are dimming that is an indication that as you transmit your supply voltage is dropping down below 12 volts. As the voltage drops the radio starts to draw current. The higher the current the more heat is produced in the circuit. HEAT KILLS!!!

  9. #9
    Join Date
    Dec 2010
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    Quote Originally Posted by W9OE View Post
    The original poster seems to have answered his own question. Yes it will function on just 12 volts. However if the lights are dimming that is an indication that as you transmit your supply voltage is dropping down below 12 volts. As the voltage drops the radio starts to draw current. The higher the current the more heat is produced in the circuit. HEAT KILLS!!!
    Your radio draws current no matter what the voltage is. If the lights are dimming your supply dosent have enough current, as long as the voltage is constant at 12-14 volts. Some people will use a 20 amp supply and the radio is rated at 20 amps... thats not good. You should use a 30 amp in that case.

  10. #10
    Join Date
    Feb 2002
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    Cleveland, Ohio
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    10.9 volts is a limitation of lead-acid batteries. If you discharge one below that level, you have permanently decreased its capacity. Having a radio that cuts off at that voltage is a good thing.

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