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12v vs. 13.8v?

Discussion in 'Discussions, Opinions & Editorials' started by K6ELV, Mar 23, 2011.

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  1. K6ELV

    K6ELV Ham Member QRZ Page

    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. AG6JU

    AG6JU Ham Member QRZ Page

    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. VK2FWAB

    VK2FWAB Ham Member QRZ Page

    Voltage regulators can't work miracles so I wouldn't run too low. Low voltage can kill high power FET's.
  4. AG3Y

    AG3Y Ham Member QRZ Page

    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. N1MLF

    N1MLF Ham Member QRZ Page

    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.
    Last edited: Mar 23, 2011
  6. AD4DQ

    AD4DQ Ham Member QRZ Page

    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. KD0CAC

    KD0CAC Ham Member QRZ Page

    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. W9OE

    W9OE Ham Member QRZ Page

    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. AD4DQ

    AD4DQ Ham Member QRZ Page

    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. KC8IUR

    KC8IUR Ham Member QRZ Page

    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.
  11. K8MHZ

    K8MHZ Ham Member QRZ Page


    I would like to see both measured and calculated data.
  12. K0RGR

    K0RGR Premium Subscriber QRZ Page

    At some given voltage, your radio will stop working the way it should. Increased distortion is common. Some radios begin to lose frequency stability as the voltage drops - they will start 'FMing' on SSB. My brother runs a Kenwood TS-120 from a car battery, and you can tell when it's running out of juice!

    Most radios can handle 14.5 volts with ease, since that's not an uncommon charging voltage in automotive environments. 15 volts is usually not fatal, either, but above that, I'd be very afraid. I had a Suzuki-built auto with a bad voltage regulator that was putting 15.5 volts on the battery. None of my mobile radio gear complained at all, but the car popped light bulbs fairly often until I caught on to the problem.
  13. WA9SVD

    WA9SVD Ham Member QRZ Page

    While most current (and recent) Amateur equipment is "happiest" at 13.8 Volts, (the typical automobile vontage, +/- several percent) many WILl still perform within spec down to 12 Volts, albeit often with slightly lower power output. But many will begin to malfunction at a point just below 12 Volts; thinking a radio will or should function down to 10 Volts or so is completely erroneous. Typically, most Amateur equipment will fail to operate below 11.5 Volts; many will fail to operate if the voltage falls below 12 Volts. (And that must take into account the voltage drop when the radio is drawing transmit current; an inadequate supply or wiring harness may provide acceptable voltage when in receive mode, but will shut down on transmit due to the voltage drop when drawing maximum current in transmit.
  14. ZL1UZM

    ZL1UZM Ham Member QRZ Page

    This is nonsense! A violation of Ohm's law.
  15. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Right. Actually when you don't have enough potential (voltage), the current the rig draws is reduced, not increased. This results in less heat, but also less of everything including output power if the PA is powered directly from the source (unlike HT's and such where the PA might be operated at 6-9V, so any connection to external power is actually a regulator that drops the voltage down to something lower by design).

    The "100W SSB rigs" designed to operate at about 14V can start to become non-linear at 12V. They might still produce about the same peak output power, but may be under compression at lower voltage, which creates needless distortion.

    For FM rigs, this isn't as critical (although they peak draw current continuously, unlike SSB rigs, so a 50W FM rig can drain a battery faster than a 100W SSB rig might) because the transmitter stages don't need to be linear.

    How low you can go without having the rig stop working properly varies by rig. There's no standard for this that I know of.
  16. K7JEM

    K7JEM Ham Member QRZ Page

    Ohm's law would only apply if the load was a constant resistor, which it is not. Most radios have automatic power control which limits their output to some specified level. The power amp is usually capable of producing more power than that specific level (sometimes not). If you take a modern TX and reduce the voltage, the current will go up in most cases. The APC will force more drive to make the same power out as at the higher voltage. Of course, at some reduction in voltage, the power out will not be met, and the current will start reducing.
    Most modern FM radios will work fine down to about 10.5 volts, some below that. Their max power will drop, but usually if you are running less than full power the APC will hold that level.

    I tested an FT-1802 down to around 10v, and it worked OK. Below that, there were problems. The power output held stable on the lower powers positions, but dropped off somewhat on high power.

  17. K8MHZ

    K8MHZ Ham Member QRZ Page

    I don't have a modern rig to test, but when I get time I will see what happens with my IC-735. The PS I am using has both voltage and current meters. It would be a quick and easy test but my radio is buried under a bunch of other projects and I am in between antennas.
  18. ZL1UZM

    ZL1UZM Ham Member QRZ Page

    All the power output charts vs Voltage I have ever seen show a reduction of ouput when the voltage is reduced. Prime example is the IC-7000. At 13.8V output is 100W. At 12V only about 35W.
  19. K8MHZ

    K8MHZ Ham Member QRZ Page

    Current rises with voltage drop in electric motors. No violation of Ohm's law, it's the reduction of REMF which changes the impedance of the motor. The drop in REMF causes a drop in impedance and a rise in current.

    I can't think of anything in a radio that changes impedance based on a drop in voltage. That's why I asked for proof.
  20. N7BUI

    N7BUI Ham Member QRZ Page

    My TS-2000 at 13.8VDC from a power supply puts out almost exactly 100 watts. Same setup on 12.0VDC from solar electric is 75 watts. Yaesu FT-897 doesn't seem to care. It's output doesn't drop until the voltage reaches 11 volts.
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