Solar Basics: Batteries Part 2
In my last post we looked at an analogy to help explain solar power and batteries. There were several questions that came out of it that I thought we should address, so here is Part 2.
Q How Many Solar Panels Should I Have?
- Survival: 45 Watts; You can just barely charge your laptop, cell phone, and have one very small light.
- Barebones: 90 Watts; You can charge your laptop, cell phone, lights plus a fan and other devices.
- Adequate: 135 Watts; You can charge your laptop, cell phone, lights, as much as you want plus maybe have a compressor refrigerator.
- Comfortable: 200 Watts; This is what I recommend. It gives you plenty of power and you can usually buy a 200 watt panel for less than the price of any of the smaller panels You can charge your laptop, cell phone, lights, fans, refrigerator, microwave, rice cookers, TV and almost anything a frugal person would want. I am not frugal. In the winter it was not adequate to run my Satellite TV dish and receiver. But it had always been enough up until then.
- Abundance: 300 Watts; basically, you can use as much power as you want within reason including an electric bike.
Q: Can I Use Solar Power to Have Air Conditioning?
A: The short and simple answer is no, it just is not practical.
Q What Kind of Batteries Should I buy?
A: Slightly oversimplified, there are three kinds of batteries:
- Starting Batteries: Sprinters; they run very short distances as hard and fast as they can. If you use a starting battery for your solar system, it will die very quickly, maybe in a few days or weeks
- Marine Batteries: Marathoners; they run 26.2 miles pretty fast. A marine battery should last fairly well 2-5 years. They are a hybrid and do two jobs; they do them both fairly well, but neither great. I recommend you replace the starting battery of your van with a marine battery.
- Deep-Cycle Batteries: Ultra-Endurance Runners: 100 miles or more. A true deep-cycle battery should just keep going like the Energizer Bunny and last 5-10 years if treated well. One good example of a true deep-cycle is 6-volt golf cart batteries. But there are 12 volt deep-cycle batteries as well.
Q: Should I buy a Regular Wet Cell or an AGM Battery?
A: There is no short and simple answer to this question. Essentially you have two choices: (gels are not a good choice and I don’t recommend them)
Flooded wet cells: These are batteries with caps on the top You must periodically remove the caps and check how much water is in them and add it as needed. They off-gas a corrosive and explosive gas. It is nearly universally recommended that you NOT have them inside a sealed area because of the corrosion and possibility of explosion. I consider the risk from them as minimal, so I have always had them inside my camper with me. But I DO NOT recommend you do the same. Do as I say, not as I do. So if your batteries have to be inside with you, conventional wisdom dictates you must buy AGM.
AGM” These are sealed and maintenance-free therefore they do not off-gas. That means you can have them inside with you and you don’t ever have to check the water. In fact, it is impossible to check the water. You can store them on their side, upside down, or in bed with you! BUT (you knew there had to be a “but”) they cost twice as much as flooded wet cell.
For most people, I think you are better off with flooded wet cell. But you need to be honest with yourself, will you regularly check the water in your house batteries? If the truth is you will not, then AGMs may be better for you and well worth the extra price. On the other hand, remember we said you could never run your piggy bank down below a certain amount of money or it would make the piggy sick? Well, if you run your battery down below 12.2 volt (which is a half full battery) then you are damaging the battery and if you do that often enough, the battery will die prematurely. So you also have to be very honest and decide if you will regularly run your batteries down; if you will, then you are better off with flooded wet cell because you will quickly ruin your expensive AGM batteries and why pay double just to ruin them and have to replace them?
Q: What is the Best Ratio of Solar Panels to Batteries?
A: Conventional wisdom is a ratio of 1:1. That means if you have 100 watts of solar, you should have a 100 ah battery. But I think that is greatly oversimplified and there are too many variables to give an easy answer. For example, I have 190 watts of solar and 220 ah of batteries, but my batteries float very early every day and spend most of the day not being charged. The reason is I am very conservative with my use of power, so for me I need 440 ah of batteries. I know some of you didn’t understand a word of that so let’s slow down and go back to the piggy bank analogy.
The elves in my 190 watt panel are making $16 an hour and putting it into my piggy bank that holds $220 dollars. But I am such a cheap bastard that the night before I only spent $32. So my busy little elves start to work as soon as the sun comes up and in two hours they make the $32 I spent the night before. But remember, we said our piggy bank can only hold $220! It physically can NOT hold more, and if we try to put in more the piggy will get very sick and maybe die (the technical term is boiling over). So the Foreman (the Solar Controller) steps in and says “Stop Working, no more MONEY!!!” and all our wonderful little elf workers put down their tools and take a break. Our elves call it taking a break, but the technical term is FLOATING. And you can see that it is actually a good description of what our elves are doing.
So when your battery is floating early in the day, you have too much solar panel, and not enough batteries. I paid a lot of money for my solar panels/elves and it isn’t working half the day. If I add a second $220 piggy bank, the elves would keep working all day and at night I would have $440 to spend. Plus, the elves would be much happier! Elves are born to serve (as we all are, most of us just haven’t learned it yet) and they are happiest when they serve (again, we are all happiest when we serve!!).
Here you may say, “But you are a cheap bastard and you are only spending $32 a day, why do you want all that money you cheap bastard!!?” Simple, to be ready for a rainy day; and a rainy day is coming! If a storm blows through and I go a week without sun, $220 won’t be enough and I will go broke, but $440 will last me till the sun comes back. Plus, in the summer my elves may make $16 an hour and I may only use $32, but in the winter my elves will only make $12 an hour and I may use $48 a night, so I will need all the power I can get.
Now let’s change the example and pretend I am a power user. Let’s say my elves are still making $16 an hour, but I spend $96 a night. That means my elves spend 6 hours a day filling my piggy bank back up and if I use much during the day it will just barely get full. In fact during the winter it may not get full at all because I will use more and they will make less. So a heavy power user may be better off with more panels than a 1:1 ratio.
Here is the bottom line. I recommend you buy as many panels and batteries as you can afford using the 1:1 ratio and try it out. If you regularly find it floating early in the day, you probably need more batteries. If you don’t float till late in the day or not at all, you may need more solar panels.
Q: How Do I Know How Full My Batteries Are?
A: Just like your gas gauge tells you how much gas is in your tank, a voltmeter tells you how many volts are in your battery. The voltmeter will give you a read out of its voltage. The best way is to buy a solar controller with a built-in voltmeter. It will be the most accurate and easy to read. Use the table at right to know how full the battery is. By knowing the voltage of your battery you can know how many amps are left in it.
If you don’t have a solar controller, or if it doesn’t have a digital read-out of your voltage, the simplest solution is a volt meter that plugs into your cigarette lighter plug like this one:
Equus 3721 Battery and Charging System Monitor