All the power in the world and not enough battery!

Discussion in 'Amateur Radio News' started by VK6FLAB, Mar 2, 2019.

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  1. VK6FLAB

    VK6FLAB Ham Member QRZ Page

    Foundations of Amateur Radio

    All the power in the world and not enough battery!

    The transceiver you use to get on air and make noise needs power to operate. The traditional voltage for our amateur equipment is 13.8 Volts. Why not 12 Volts you ask. The short answer is chemistry, but let's move on, there is lots to cover.

    Generally that 13.8 Volt is specified with a +/- symbol and some percentage. For my radio it's 15%, which means that if I plug it into power that's somewhere between 11.7 Volt and 15.9 Volt, I'm good to go.

    Then when you look a little closer at the specification you'll see that my radio draws 22 Amp. That's a whole chunk of juice that needs to come from a power supply. Of course that means that you'll also need to deal with 22 Amp fuses, wire capable of dealing with 13.8 Volt at 22 Amp, and connectors that won't melt when you do that.

    If you look closer again, you might notice that 22 Amp is when you're using the radio at maximum power, that is, 100% duty cycle and 100% power, and only during transmit, in the case of my radio, 100 Watts for HF.

    So, if I'm using a digital mode, AM or FM, at 100 Watts on HF, my radio says it will draw 22 Amp at 13.8 Volts.

    Those numbers aren't correct of you're using CW or SSB. A rough number to work with for CW is 40%, that means if you're doing CW for a minute, that's the equivalent of key down at a 100% for 40 seconds and key up at 0% for 60 seconds.

    SSB is roughly 4 times as efficient as AM, about 25% duty cycle, but realistically it's more like 20%, since your power consumption depends on how much you're yelling into the microphone. If you take long breaths, 0% power, whistle into the microphone, 100% of SSB, or 25% of overall power.

    Now all this gets even more interesting if you consider that you're not just transmitting all the time. If you're only transmitting half the time, you need to take your power consumption down another 50%, so SSB might be 10%, CW only 20% and the digital modes 50%, from the perspective of the power supply.

    So you want to go portable and need batteries. Batteries don't come in 13.8 Volt versions. So 12 Volts. Get the number of amp hour and you're good to go right?


    Your battery doesn't just run at 12 Volts and then all of a sudden stop, it runs down, you've seen it in a torch or a Walkman when the tape got slower and slower. A 26 Ah battery should give you 26 Amp for an hour at 12 Volts, but if you actually do that, you'll need to buy a new battery, because you'll have destroyed the one you just exhausted.

    All of this then starts a conversation about chargers, which incidentally might put out 14.4 Volts. You might turn to solar panels, which at peak power operate at something like 18 Volts, then you stumble into the world of PWM vs MPPT solar converters or charges. Then there's the joys of over and under current, battery discharge rates, continuous versus intermittent charging, different battery types, battery safety, storage, weight, out-gassing and more fun than you'll want to know about on your morning commute.

    And I haven't even talked about battery isolation, HF interference from chargers and inverters, the differences between powering your radio straight from a battery or via a DC to DC converter, using 240 Volts, or if you're in the USA 120 Volts in the field, generators, compatibility with others and how much all this might cost and if you need to invest in lotto tickets to pay for this experience.

    One tool I stumbled across in my travels is the Four State QRP Group website which has the W1PNS / WA0ITP / AB8XA Battery Life Estimator, which in a single web page gives you the ability to say what mode you'll be using, for how long with what battery size and how much radio draw and it'll tell you how much more battery you'll need to get the job done. Very handy for a contest that you're hoping to operate portable from a battery.

    This all to say that power is a very deep rabbit hole and it will take you some time to figure out where your use pattern puts your requirements and budget.

    Here be dragons.

    I'm Onno VK6FLAB

    TL;DR This is the transcript of the weekly 'Foundations of Amateur Radio' podcast - for other episodes, see

    Attached Files:

    N2AMM, K3RW, W3MT and 5 others like this.
  2. AC7CW

    AC7CW Ham Member QRZ Page

    Very good! That calculator saves a lot of headache.
  3. KD6SM

    KD6SM XML Subscriber QRZ Page

    Nicely written and good food for thought! Thanks for taking the time to do this.
  4. N7WE

    N7WE XML Subscriber QRZ Page

    All true if you are running a lead-acid battery, but Lithium Iron Phosphate battery technology (LiFePO) has changed the rules.
    WA2LXB, W4EAE and AD0AR like this.
  5. W8JBM

    W8JBM Ham Member QRZ Page

    Nicely written and something we all need to take a look at from time time. Battery tech changes, solar is getting better, and the grid is in variable shape depending who you talk to and the same Mother Nature being stingy with the solar cycle is unpredictable at best. Batteries should be brought out here and there just so we remember what to do when needed and what shape they are really in.

    Again, good topic and 73!

  6. WN1MB

    WN1MB Ham Member QRZ Page

    CW is a digital mode.
    Last edited: Mar 3, 2019
    KU4GW, K7IQ, AA4MB and 5 others like this.
  7. KD5PUR

    KD5PUR Ham Member QRZ Page

    I had 4 Trojan J305H in series parallel and ran them for 2 weeks during hurricane Ike and only dropped .5 volt
    WA2LXB, K4FX, NL7W and 1 other person like this.
  8. G0WXU

    G0WXU Ham Member QRZ Page

    Interesting facts about batteries. I used to run the Regimental battery workshop while I was attached to the 1Bn KOSB in Osnabruck, Germany. I was R. Signals there. At least it got me off going on parades in the morning. With having that job, I was there before Morning Parade started. A good excuse was battery charging times. (Bullshit Baffles Brains). I had to prepare new batteries to put into service for both radio work and the vehicle batteries mostly for the 432 series of armoured vehicles that we used then. So the common battery was the 100 AHC 12 volt one run has a pair or multiple of that. It was an enjoyable job even though I was working with the REME workshop there. If I got any mouth from the REME guys I could always tell them that I was wearing a REME badge before they even joined the army, has a cadet from being 12 years of age.
  9. ZS6BV

    ZS6BV Ham Member QRZ Page

    I use 4 cell lithium polymer and 4 cell lithium ion batteries for SOTA activations. They are 14.8V batteries. The FT-857D likes 13.8V plus or minus 15%, as you specified, which gives a max of 15.87VDC.
    The problem is the 4S battery fully charged will have a voltage well over 16V. The solution I use is a stud mount bridge rectifier on a heat sink inline between my battery and radio which gives me a voltage drop through two of the diodes bringing the voltage below my radios max allowable.
    N2AMM, K4FX, WA1ZMS and 2 others like this.
  10. K4AGO

    K4AGO Ham Member QRZ Page

    I admire a man who uses his brain instead of just hooking it up because it has a + and a --
    K4FX, WA1ZMS and WN1MB like this.

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