DIY: Solar Power – Part 2
Where we left off
If you recall, we need our battery to supply 750 mA continuously over a 24 hour period. I arrived at that number by taking the maximum current requirements of an Elecraft K2 transceiver and cutting them in half. That is, 1.5 A * 50%, which gives me 750 mA.
Please note that this is for a transceiver! That is, it is transmitting 50% of the time, and receiving 50% of the time. If I had a non transceiver device, such as a WiFi access point, I would want to assume that it was drawing it’s rated current 100% of the time. I bring this up because it is something you will have to consider.
Think of a light. You are probably only going to use it for only a few hours a day. In the summer, you will probably use it less. In the winter, you will use it more, however, the chances of using a light 24 hours a day are pretty slim in most cases. This is something else to really think about as well. When we were kids we were taught to turn off lights when you leave a room. When we are dealing with a limited resource, such as battery power, we want to make sure we are practicing those same traits – when we are not using something, turn it off! It will save on the size of the solar system we are building, and hence, will save money.
Sorry if I seem to be digressing a lot, but these are extremely important things to think about.
Computing Battery Capacity
The easiest way to compute battery capacity is to do some simple math. In my case, I take 750 mA and multiply that by 24 hours. so, 750 * 24 = 18000 mA = 18 A.
Therefore, my battery must be at least an 18 Ah battery over a 24 hour period. You are probably thinking to yourself that I would need an 18 Ah battery at this point. Well, it just so happens that deep cycle, including gel cell batteries, are rated at a 20 hour rate. So, if a battery says that it will supply 18 Ah, that means it will supply 18 A over a 20 hour period. We want to go for 24 hours, which is longer than 20 hours obviously. We would end up having a problem if we used a 18 Ah battery! Therefore, we need to increase our battery capacity.
The easiest way to do this is to simply add another four hours, or, 750 * 4 = 3000, or 3 A. Now I add that 3A to the 18A and come up with 21 A. Now I know that if I want to power my Elecraft K2 for a 24 hour period, I would need a 21 Ah battery.
Overkill can be a good thing
You might be thinking at this point that adding some extra capacity to our battery bank would be a good thing to do. And you would be absolutely correct in thinking that, especially if you are aiming for a power source that needs to supply current constantly for a long time. Feel free to actually double the capacity. I suggest this because there are times that the sun does not shine as much as we want it to. This does not seem to make that much of a difference, if you want to know the truth, as even on the cloudiest of days I still see around 75% output from my solar panels, but if the system you are powering is critical, you will want that extra power available when you need it. It is always best practice to assume that Mr. Murphy is going to visit as he always does at some point in time.
Tags: battery, battery capacity, battery power, Elecraft, elecraft k2, gel, power, Solar