Thursday, January 12, 2017

Measurements vs. calculations (and a give-away)

One of the most useful features in Elecraft's KX2 is also one of the least mentioned: the ability to measure amp-hours consumed.

Using it was an eye-opener for me in terms of how long a small battery would power a 10-watt rig.

I recently had the need to power another 10-watt rig in such a way (and with so many variables) that calculating the needed battery capacity would have been an exercise in frustration. Here are the details:

A 2m APRS transmitter (10 watts RF out) and a GPS receiver, programmed to transmit my position
Byonics MT-RTG
 every 15 minutes for 48 hours. Current draw of the TX-GPS combo on receive is 50 mA - on transmit, it's about 1.3 amps. Transmission time lasts one second. Since the rig was outdoors, ambient temp at night dropped to -5C (almost record-breaking for south Texas - but two days later it was 23C). All these varying variables interfere with a formulaic solution.

How much capacity (in amp-hours) would be consumed in a 48 hour period? Keep in mind that battery voltage (from a 5AH LiPo) drops during TX and more significantly so as the battery further discharges. The constants aren't constant.

If anyone wants to take their best guesstimate of the amp-hours required to power the set-up described for 48 hours and post your value as a comment (no email, por favor), I'll send the the winner my 20m PSK31 QRPver, postage paid anywhere in the world. But - the winner must get within 20% of the correct value. Deadline is 0000Z on 19 Jan.

The prize: 20m QRPver (PSK31)
To rescue me from the mathematical uncertainties and complications of a non-standard duty cycle is the Q-PWR-150. It displays voltage, current and power at all times as well as a scrolling display of amp-hours, watt-hours, peak current and peak voltage of the device connected to it.

As with the KX2's built-in amp-hour meter, this one absolutely removes the guesswork of calculating power usage, not only in my APRS application, but for equally variable duty cycles and operating styles of using CW and SSB with any rig (even QRO).

I tested it for accuracy, comparing its displayed voltage and current with that measured by a Fluke 87. It was spot-on.


  1. John I have thought about the idea of using a solar panel/supercapacitor combination to sustain the APRS transmitter. But I have done no calculations. 72 N4KGL

    1. Hi Greg, I eventually want to use a solar panel to sustain the battery that will power this APRS set-up. I'll get to the reason for that in a later posting...

      I did look into super-capacitors after you mentioned it in a previous comment. It is a neat concept, but for now it is cost prohibitive. Batteries keep getting cheaper and better with higher power densities.

  2. Hi John, a bit of rough calculating. Considering temps, voltage drop, internal R and switching peaks brings me at 2,66Ah. 73, Bas

  3. I think about 3,5Ah 73 Arnold PE1OWG

  4. The correct answer was 2.736 AH. Thanks for the estimates from those who submitted them. The QRPver will be on its way to Roodeschool Netherlands tomorrow...

    I was surprised at the measured result and expected it to be higher, especially given the temperatures we had during the two nights of the test period. As it turns out, the capacity could almost have been calculated simply by looking at the receive current draw and igoring the transmit time/current altogether. Congrats Bas!

    1. Considering my rough calculations it was very close. The 5Ah LiPo was probabely not exactly 5Ah but 5,076Ah that explains my error ;-) Thanks voor the QRPver John I have received it well and will be testing it soon. 73, Bas