Now that you know how to convert between power (P) in watts, current (I) in Amps, and voltage (E) in volts, you are well on your way to having enough information to buy your first panel.
As we determined previously, I need my panel to generate an output of 1.89 Amps. I need to take my battery up to 13.7 volts for it to be considered fully charged. By using the Power formula, P = I * E, I can now plug in the numbers.
P = 1.89 * 13.7 = 25.89
So, I need at least a 25.89 watt panel. Truth be told, I could get away with only a 25 watt panel. And that’s just what I did. Kind of. But more on that later.
Why can I get away with a 25 W panel? Simple…. I turn stuff off when I don’t use it. That means, when I have to go get a drink of water, use the bathroom, take a shower, grab something to eat, etc., I turn the radio off. Simply removing power when the radio is not in use, even for a short time, will save us over the course of a day. That 0.89 watts will end up being something that we really do not need to worry about.
However, this 25.89 is actually the minimum size we want. Remember I noted earlier about 75% efficiency on a cloudy day? We really want to add 25% to the panel size to account for this. So, 25.89 + 25% = 32 Watts. Given this buffer, I would say a 30-35 watt panel would be just the ticket.
We can hook up a panel directly to a battery and let it charge. However, this is a really bad thing to do for a number of reasons. First off, batteries are not cheap. We want them to last a good long time. I have a set of 85 Ah gel cells that are currently on their 5th year of service. Why? Because I take care of them. They are around $200 each new. I want to be able to get as much use out of them as possible.
The second, and most important reason, however, is because batteries are dangerous. Overcharging a battery will kill it by causing gas to vent, and the liquid gel inside to end up evaporating, but it can also cause the battery case to melt which can lead to an explosion or fire.
We must therefore use a charging circuit on our battery bank!
There are quite a few chargers out on the market. Some are rather simple and some are super complex with all sorts of signal outputs so you can do remote monitoring of the battery pack, etc. These do work just fine. What I prefer to use though is a charger that came in kit form from CirKits.
The model I use is the SCC3, which is a 12V charger that will handle 20 A worth of solar panels. What I like about this controller is that it will pulse the battery when the battery is done charging. This pulsing seems to help keep the plates of the battery clean, thus helping the battery hold a charge after years of service.
Tags: gel cells, solar panel, solar powerMany moons ago I decided it would be fun to run my amateur radio station off of solar power. What started off on a small scale, quickly became overkill in some respects. However, I have had nothing but success from the endeavor.
My main goal in doing this at that time was to just run one low powered radio for at least a 24 hour period, with no interruption. Today, I am powering two radios with the goal of having them operate continuously with zero downtime because of no power. I also have a lamp, and am able to charge the batteries on two hand-held radios – all on solar power, and all for free. Well, kind of free.
If you decide that you want to undertake a project like this, you first need to sit down and figure out what you are wanting to power 24-7. Although this article applies mainly to ham radio, you can apply it to other things as well. For instance, a good place to start might be powering your cable modem and WiFi access point. Since those two devices are normally left on all the time, they would be a terrific starting point – but you will have to do some other things besides hooking up a panel to them and expecting them to work.
As a matter of fact, hooking up a panel to them would most likely result in their destruction. This is something that will be addressed later. So, for now, we will just be focusing on my solar powered ham radio station.
Any system like this needs a plan. As much as you might want it to, throwing together a batter and a solar panel is NOT the way to do things.
The first thing you MUST do is figure out what your needs are. The radio I first used was an Elecraft K2. The K2 has very low power requirements – 12V @ 1.5A for 5W output when transmitting and 12V @ 400 mA when receiving.
Now, one could hook up a solar panel directly to the device one wants to power, but this would be bad for the device. Solar panels very seldom put out 12 volts – the are most commonly putting out 18V and in some cases 24 and 48 volts, depending on the size of the panel. Most 12 volt devices have an operating zone they can tolerate of about 15%, so a 12V device could actually operate at (12 * .15) = 1.8, or somewhere in the 10.2-13.8 volts range.
But here is another kicker – a lot of things are designed to be operated in a car. The car electrical system may say it is 12 volts but in actuality it is 13.5 volts. This gives us a slightly larger operating range of +- 2.02 volts, or, 11.5 to 15.5 volts. It is best, however, to keep this voltage at around 13.7-14.2 volts. You will see why later on.
For now, we are wanting to try and figure out what kind of load we are going to be putting on the panel and try to size our solar panel accordingly. It would be really easy to just go buy a 150W panel and be done with it. But, we would end up spending money we probably do not need to. Add to this the physical size and weight of a 150W panel and you will probably want to do otherwise as well, especially if you are planning on taking your panels out to the field.
In my case of using the K2, I figured that I would be drawing 1.5A around 30% of the time. I spend an awful lot of time listening instead of transmitting. However, to be safe, I went ahead and figured out the system at a 50% load of 1.5A. I would rather have too much available to me than not enough, especially since if there were some emergency communications event going on, I would need to have my station running 24/7 for up to two weeks. Maybe longer even, if conditions so dictate.
Now that I know I am going to require 750 mA continuously (1.5A * 50%), I can now size the battery I will need. Why a battery? Because the sun does not shine 24/7, no matter how much I want it to! I will need my battery to supply 750 mA constantly over a 24 hour period.
The battery I will choose is what is commonly referred to as a Gel Cell. These are sealed maintenance free batteries. They are NOT car batteries, which are actually junk and are not able to do what we need them to. If you are thinking “I will just use a car battery!” think again. You will have ZERO success doing this. You must use some sort of a deep cycle battery. A gel cell is a perfect battery to use for this application.
One word about batteries before we continue much further. Batteries are extremely dangerous things. You are probably not going to be shocked by them (you can be, but not with this type of system), however it is extremely easy to do something really bad and start a fire or cause an explosion. When you are working around any type of battery, remove any jewelry you have on. Make sure nothing can come in contact with the battery terminals and cause a short. If you do cause a short, run like hell because something bad is about to happen. Do NOT take any chances with these things. I am very serious about this, and you should be too.
With that warning out of the way, we are now ready to start doing some more cypherin’…
Tags: amateur radio station, batteries, elecraft k2, solar panel, solar panels, solar power