Nine Months: Portable/Solar Thoughts

SolarPanel

From a patient reader, who did the work and then tolerated graciously the editor’s “Squirrel!” tendencies:

I can go only with what I did, as well as some lessons-learned from doing some calculations with a mutual friend (re batt consumption).

The first step really must come first. One must learn the radio and what it consumes in an average duty cycle. Basically that asks how much power is it consuming when transmitting in your preferred mode (pick the one with the most overhead as a SWAG). Typically the manual will tell you this if you actually read it. This is part of simply nugging out where to plug in the variables, because this is a math problem at its core. This consumption will drive how you size your battery solution, therefore your solar panel, and the charge controller you use. My personal average duty cycle is about 80/20; 80% receive and 20% transmit. It’s not a bad rule of thumb, but your own use will dictate this. If you’re acting as a relay for 2 other stations, or as NCS (net control station) of some other dispersed folks out in the bush you may (or not) want to adjust that ratio.

One important note for those who have mobility of the wheeled variety & will simply cart along their vanilla 100w base station, having the ability to feed it with large batteries. Just because you turn the power down doesn’t reduce the transmit consumption in a linear fashion. There is a floor where the radio will still consume a lot of power to run its other stuff like big power supply, fancy displays, button functions, programming retention, etc. A base that consumes 22A at full 100w transmit at home won’t consume 11A running at 50w output. You can typically find most of these will “bottom out” at about 12-16A, even if you turn its output down to QRP levels & turn off some of the other functions (you can get an FT-817 down to really being a miser by just disabling some usually un-needed functions). Lots of stuff behind that box & and it’s power hungry.

So let’s take my particular radio, run some numbers and size the battery I settled on as well as the panel & charge controller:

The rig draws 2A at full transmit & .6A in receive. In terms of amp-hours needed from the battery .2*(2A) + .8(.6A) = .88 amps. I’m kinda conservative so I call that 1 amp-hour (aH). Now comes the completely subjective assessment of how many of those aH’s do I need. For me, 4 to 5 is plenty & would mean I’m on the radio alot vs. doing other stuff I SHOULD be doing like setting camo or watching my backtrail or taking a nap (naps are good). Rather than a really hi-speed low-drag solution I decided to go with a simple 5aH sealed lead-acid (SLA,or “gel cell”) battery available in nearly any hardware store that sells outdoor lawn or farm equipment. Its form-factor & weight meets my needs. So I’ve got a couple.

As to sizing the panel it comes down to things like how much sunlight am I likely to have, and how quickly do I want the idle battery replenished? In northern climes during winter there is less sunlight available so I decided a few hours (vs overnight if you’re in Alaska during the summer) would be all I wanted to allocate, assuming sunlight. Following this point I then proceeded the old-fashioned way: I asked someone, who is neck-deep in this stuff all day every day, for help, That turned out to be a source for me. I will give that link with the disclaimer that I’m not hooked up with them in any fashion; they just know what they’re talking about, eat this stuff up, and are easily engaged in ferreting out a solution so you’re happy.

Charging has a rule of thumb of about 1aH = 12 watts; my source said in the case of solar methods go to 15 to be on the conservative side. It is also a good rule of thumb not to hit SLA batteries with more than 25% of their end-charge; your battery/mileage may vary. With a few hours allowed for the charging cycle, a 15w panel is fine. But you should have a small smart charge controller along with it that regulates the feed from the panel to the battery. This does 2 things. First, it prevents over-volting the battery and, second, they typically will do that final trickle “top off” of the battery so you get the most from it when you put it in use. So I want something slightly less in terms of output from the controller so it isn’t slamming the battery with the full output of the panel. 800mA or so works great and will still get that battery up to a nice topped-off state in about 4-1/2 to 5 hours. Can you size the panel bigger? Sure. You just need to expand that effort to consideration of the controller as well. In my case, this was meant to be a solution where everything, including the KX3, could go in an attachable pouch on the ruck.

One thing regarding connectors, and this is where you can do whatever trips your trigger. Much is said about Power Poles, etc. and in my mind they’re great – as long as you don’t have to make them on the fly or troubleshoot them. You can see in the pic of my panel that the physical interface with the panel looks like a typical old-school trailer-light connector (or part of it). I picked that as my interface because they are easily wired, easily cleaned, you can even wrap wire easily & directly around the conductor, and they are available at any parts store, RV place, truckstop, etc. Just because all they have is a 3 or 5 prong pigtail doesn’t mean you have to use all of them; just terminate what you want, ez-peezy. The other advantage of this is any hookup wire will do if you want to extend the connection. If you look at my panel you’ll see grommets along the edges. Very handy for running some paracord or braided fishing line & putting the thing up a tree to get best sun or on the edge of a vegetated area while you lie in the shade, out of sight attending to other camp chores, like taking that nap or making breakfast.

Here’s the link of where I located my panel stuff, and their personnel are great, fine service, good advice. Whoever it is CALL the people you’re dealing with and let them in on the end-state of what you’re after. If they’re worth anything, they will be all over helping you.

Link: http://www.batterystuff.com/ ; and at the time I dealt with a gent named “Steve”.

One other “general” lesson-learned regarding battery use, whether for your HF rig, your handhelds or anything else. If you’re in a field environment – or charging the battery to stash it away – DO NOT connect it to the radio you’re going to be using it with. If you have little HT’s with charged batteries that you stash around in various “go-kits” – take the battery off, put it in its own ziploc, with the radio’s ziploc or whatever. Batteries will leak; it’s what they do, it’s in their DNA, which is governed by principles in physics, the first of which is: “Electrons are lazy.” They will go to wherever they have more space to kick back & be, well, lazy – whether that’s something else conductive like an antenna, or just the chassis of your Baofeng handheld.

At any rate, this is a math problem. One can do the homework & crunch it out. Laying out the variables first, it is not as daunting as one may think.
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See also this related CSG post.

10 responses to “Nine Months: Portable/Solar Thoughts

  1. Romeo Foxtrot

    Outstanding article!! Nothing like live, real world experience….
    Here’s another article in the same vein…
    http://quietsurvivalist.com/guest-post-simple-solar-power-for-amateur-radio/

  2. Grenadier1

    These guys are a bit pricey but by all accounts I have heard its high quality with little need to think about it.

    http://www.portableuniversalpower.com/juicebox-r2/

  3. Grenadier1

    Check out this one for a little smaller form factor.

    http://www.portableuniversalpower.com/revolt-g2/

  4. Please restate; what does this sentence mean?
    “It is also a good rule of thumb not to hit SLA batteries with more than 25% of their end-charge”
    Good piece, appreciate the experience shared with us.

    • SLA is sealed lead acid. If it is a small 10 amp hour battery, do not charge with more than 2.5 amps of current or 25% of the rated capacity. Too fast of a charge rate produces too much heat and gas pressure inside the sealed case. It can rupture.
      Batteries like in a car are different than deep cycle batteries and are designed for rapid discharge and recharge. It has to do with the internal resistance of the design.

    • I believe that he is talking charge rate: C/4 as max charge rate. For a 12ah battery, that would mean 3a charging rate. Basically batteries will last longer when they are charged at an appropriate rate which differs for different battery construction.

      I have some small folding solar panels as well as a couple of 80w 12v panels for small battery charging. I am beginning to assess true charge rates and capacities for different setups. Portable comms are VERY important.

    • It tells you what SLA is in the paragraph above… Sealed Lead Acid…

  5. Less portable, but highly capable: Morningstar MPPT 15A controller http://www.morningstarcorp.com/products/sunsaver-mppt/
    The manuals are highly informative for whatever kind of controller you get. External computer programming and monitoring is possible with this model.
    You can use higher-voltage solar panels that boost current into a lower-voltage battery and have a 12v battery pack that charges at up to 15A, or a 24v battery that charges at up to 15A. The controller is also protected from having about 50% more panel Watts than it can charge (so you get max current earlier and later, or when mis-aligned).
    If you have the space/weight capacity, sizing up battery pack so that it’s 20x your charging capacity is about max., so 20x 15A = 300A at either 12v or 24v (24v pack has 2x as many Watt-Hours to charge with from this controller, with less current lost in wires, but fewer devices run directly on 24v dc – some refrigerators-. 24v battery pack is a big advantage with inverters and 120v output). YMMv, and batteries are freakishly heavy.

  6. Alfred E. Neuman

    Reblogged this on MCS.