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Foundations of Amateur Radio #195:

from Onno VK6FLAB on March 3, 2019
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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

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