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I Bought a Battery

pc1100_straight_540x540The Odyssey batteries cam highly recommended from a friend who operates mobile in the boonies a lot.  Despite my earlier calculations, I decided on something with about 45Ah of capacity.  The folks at Odyssey provide a nice detailed technical spec.

I also liked this battery because it is small and light:

Length: 9.8″
Width: 3.8″
Height: 8.1″

Weight: 27.5 pounds

I plan to make an excursion up north in a few with for a trial run.

Actual Charging Capacity of 20W Solar Panel

Actual 20W Solar Panel Charging

In this photo you can see the actual rate of charge being supplied to the 12V gel cell battery by my 20W solar panel and charge controller.

Outside air temperature is about 60 degrees. The panel is pointed directly at the sun in a clear cloudless blue sky at about 9am.

The system (solar panel and charge controller) are producing 14.8 watts. That’s an efficiency of approximately 75% (I suspect that figure would be higher if the charge controller were MPPT instead of PWM).  You can see that the system is producing 1.14A of current, which is more that what is required to simultaneously operate the Elecraft KX-1 and charge the Kenwood TH-F6A.

Low Cost Solar Powered Ham Radio Station

I’ve been able to solar-power a small ham radio station for about two hundred dollars. The radios in this setup have relatively modest power requirements:

  • Elecraft KX-1
  • Kenwood TH-F6A 2M/440 handheld

Here is a diagram of the station’s power supply:

LOw Cost Solar Powered Ham Radio Station

EcoDirect-VLS-20W-2TSolar Panel

The solar panel is a small 20W panel purchased from EcoDirect.com for $56.95 (Model VLS-20W). It is large enough to operate the KX-1 and charge the TH-F^A continuously (when there is sunshine) and keep the battery fully charged.

Morningstar-SunGuard-SG-4-2T-1Charge Controller

The charge controller is the Morningstar SG-4 PWM 4.5A Charge Controller. This is a PWM controller so it is less efficient than an MPPT controller but in this low-power application it will be just fine.  It also cost only $25.75.

12V_7.5AhBattery

The battery is a 7.5Ah Gel CEll.  I purchased mine from BatterySpace.com for $18.80, but these can be found anywhere. In this application, there is nothing magic about the capacity (7.5Ah). You’ll reliably and repeatedly get 4.5Ah out of this battery. (For an explanation of why 4.5 instead of 7.4 Ah, read this post.)  It contains enough charge to operate the KX-1 for about 6 hours.

Why didn’t I use a Lithium battery, you ask? Lithium batteries, preferably Lithium Phosphate (LiPo) are much smaller and lighter.  However, most charge controllers are intended for use with gel cell batteries. Lithium battery packs also need charge-balancing periodically.

wattmeter power analyzerPower Meter/Analyzer

The Power Meter/Analyzer is from Turnigy. I bought it from Powerworx.com (the ANderson PowerPole people) for $59.99.

Why a Wattmeter / Power Analyzer? The meter operating inline continuously allows me to keep tabs on the battery voltage. It is really important to avoid discharging the battery below about 11.9V (Click here to learn why). It is also useful for measuring the actual current draw of the radios (as well as the output fro the solar panel and charge controller). This particular meter comes with Anderson Powerpole connectors already installed.

PP15_medConnections

All connections use Anderson Powerpoles. The more I use them the better I like them.

Radios

The Elecraft KX-1 draws about 700mA on transmit and 30mA on receive.  For the Kenwood TH-F6A, I use its own internal batteries and recharge them from the 12V Gel Cell.  The TH-F6A draws about 300mA while charging.

How Much Battery Capacity Do I Need?

The battery capacity you will need for your solar powered ham radio station depends upon several factors (I’m also assuming that you will be using a Sealed Lead Acid (SLA) battery.)

  • The radio’s current draw on receive
  • Percentage of the time you expect to be in receive mode
  • The radio’s current draw on transmit
  • Percentage of the time you expect to be in transmit mode
  • How long you intend to operate

Let’s say:

  • 2Ah on receive
  • 20Ah on transmit
  • 50% receive / 50% transmit
  • operating 4 hours per day

Since I’m in receive mode half the time, and consume 2Ah on receive, that means I’ll consume 1 amp in receive mode per hour.

Since I’m in transmit half the time, and consume 20Ah on transmit, that means I’ll consume 10 amps in transmit mode per hour.

My hourly current draw is the sum of those two, or 11 amps.

If I’m going to operate for 4 hours per day then I need 44Ah of capacity.

Expressed as a formula, it looks like this:

((Pr * Ir) + (Pt * It)) * Ho

Pr = Percentage of an hour in receive mode, expressed as a decimal from 0 to 1
Ir = Current draw in receive mode, in amps
Pt = Percentage of an hour in transmit mode, expressed as a decimal from 0 to 1
It = Current draw in transmit mode, in amps
Ho = number of hours of operation

Using our example of 44Ah of needed capacity, let look at batteries.  The following chart is from solarnavigator.net:

Battery State of Charge Battery Voltage
100% 12.7
90% 12.5
80% 12.42
70% 12.32
60% 12.20
50% 12.06
40% 11.9
30% 11.75
20% 11.58
10% 11.31
0% 10.5

What this chart says is that if your battery’s voltage is 12.2V then it contains 60% of its rated capacity. It’s important to note however that if you use up 100% of the battery’s rated capacity you have killed it, meaning you have drastically shortened its life.  In fact, to preserve battery life you never want to get into the yellow zone in the above chart. This means that if it says 20Ah on the side of the battery it really means that you can use only 14.4Ah (the top 60%) without permanently damaging it.

To be technically precise, this measurement should be taken when the battery is not under load and has been resting for 3 hours, but the chart is close enough for our purposes. Also keep in mind that the industry standard for battery ratings is to assume that the discharge takes place continuously over a period of 20 hours. Longer or shorter

Using our example and the above chart, to obtain 44Ah of capacity we need to buy a battery with a capacity of 44 / .6 = 73.33Ah.

Here is the formula to account for this:

(((Pr * Ir) + (Pt * It)) * Ho) / .6

If your soar panel is simultaneously charging the battery (and if you only operate on sunny days) then you can get by with a smaller battery. If the solar panel and charge controller can deliver 60 watts (12V * 5 amps) to the battery, then for each hour of operation you’ll be recharging the battery by 5 amps.  In four hours that’s 20 amps. Theoretically then you could get by with a 53.33Ah battery. Given system inefficiencies however that is probably optimistic. (I’ll update this post after I run some actual real-world tests.

And one final consideration: Batteries sitting on the shelf will gradually lose capacity over time.  Trickle chargers are inexpensive and the battery should be kept on one when not in use.

Antenna Tuning Day

pic1The temperature was in the sixties this morning and so it was time to erect the 40M inverted Vee I built a few weeks ago and trim it to length.

I could have done the job in my backyard but decided that it was a perfect opportunity to get out for a bit.  There is a nice park a few miles from my hows with great views. The ramada is down in a saddle between two hills so it is a lousy spot to try an make any contacts, but it is scenic nevertheless.

As you can see, the portable station fits in a plastic 50cal ammo can. Trimming the antenna for resonance at the bottom end of 40M went very quickly. (An aside: Antenna analyzers like my MFJ-269 are amazing tools. When I started in ham radio 35 years ago we had nothing like it.)

Did I make any contacts today?  No.  I didn’t expect to, given the limited time I had, my inexperience as a QRP operator, and my lousy location.  That will change.

Anderson Powerpoles

powerpoles_45_xlrgI am using Anderson Powerpoles throughout this project. I became a fan of crimp-on connectors (instead of soldered) a long time ago while building my airplane.

A crimped connection provides much better strain relief than a soldered connection. A crimped connection can be made miles from a source of power required to heat a soldering iron.

I started using Anderson Powerpoles in my ham shack a couple of years ago. It made sense to use them from the start for this project.

The Adventure Begins

Solar Powered Ham RadioHi.  I’m Dave, WB7OBG. I live in Phoenix, Arizona where there is no shortage of sunshine. The idea of a solar-powered ham radio station has always intrigued me.

We’re going to start out small, inexpensive and low-power.

I have an Elecraft KX-1 and currently power it from a 12V 7AH gel cell.  I’ll be using a small (20 watt) solar panel and charge controller to charge the battery.

My handheld of choice is the Kenwood TH-F6A. Keeping things simple, We’ll be charging it from the 12V gel cell.

From there we’ll see where solar powered hamradio it takes us.

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