More power Mr. Scott!
Though it is very far away (almost 4 months) we have been talking about our plans for the Lovell's Island excursion and the RSGB IOTA contest. We have radios. We have antennas. We're a bit puzzled about power sources. Last year I was able to run 6 hours in the bright sunshine with my 15 watt solar panel and a couple of batteries. This year I'd like to run for 12 hours, with some of the operation after dark (or at least after the Sun has set low enough that the solar panel will be useless). What are my options?
If I run QRP with my K2, I draw about 35mA on receive. I can run practically forever on any reasonable battery if I never transmit. Transmitting draws about 2.5 amps (a rough estimate). So, a 7Ah battery may last 2-3 hours under those conditions. For planning purposes, I'll use the 2 hour figure. I have two such batteries providing 4 full hours, perhaps 5, of operation. That's not enough for a 12 hour effort.
I had this problem last year which is why I opted for a solar panel to supplement my power. Not knowing how much this would get used, how effective the approach would be, and feeling uncharacteristically unadventurous, I selected a modestly-sized panel that I discussed in a previous blog entry. The 15 watt Global Solar unit purchased from The Alternative Energy Store provided enough juice that I could run all day on the island without fear of running out of juice before I needed to run to catch the last ferry. That was enough for last year's 6 hour effort. Though I was drawing down the batteries, the solar panel provided enough energy that the batteries were not exhausted.
This year's effort of 12 hours requires a rethink of this approach. Once the Sun sets, I'll have nothing but batteries for power. Ideally, I would like to have two fully-charged batteries to start the night shift. That means generating enough power during the day shift through solar panels to not only power the radio but to ensure that the batteries are "topped off" as well.
I have by no means made any decisions, but I'm looking closely at the 48 watt "big brother" of the panel I currently own. This offering is also from Global Solar and sold by The Alternative Energy Store. The panel provides 12 volts and approximately 2.5 amps. (Of course, these specifications are for optimal conditions.) I am speculating that this would be enough to run the K2 and keep the battery fully charged during daylight hours.
Consider the alternative in weight to accomplish the same thing with batteries. Let's say the panel does a good job and produces 2 amps of continuous current during the day. Every hour that passes is another 2Ah "in the bank". Four hours later, I've generated more energy than is stored in one of those 7Ah batteries. Eight hours later, and I've matched the energy I've brought in both my 7Ah batteries. The solar panel weighs under 4 pounds. Those batteries weigh 7 pounds each!
Chris Drummond had purchased a Brunton Solaris 26 panel for the Montserrat trip and had made contacts on an IC-703 using only the panel (no battery). This is possible, of course, but not recommended. I'd use a charge controller in there to ensure spikes didn't fry my radio. Still, it was an interesting experiment. One of many that we did while down there!
Again, no decisions on all this yet. The panel under consideration is expensive (and haven't I spent enough money lately?!) and I won't be making any decision for a while. I need to think about this a little more. But, operating portably for many hours at a time for either a contest like the RSGB IOTA event, for Field Day, or indeed for a weekend sounds like lots of fun. Like I said, I've got the radios, I've got the antennas, now I need to solve the power problem in a way that doesn't cost me too much money or too much weight in the backpack.