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Solar Basics: Batteries Part 2

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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?

This is a friends solar setup. He has two 140 watt panels and he never lacks for power!

This is a friends solar setup. He has two 140 watt panels and he never lacks for power!

A: Most people will give you formulas and tables and tell you to figure out your usage and then buy enough to meet those needs, but that isn’t my suggestion. My standard answer is you should have as much as you can afford! You can’t have enough! While vandwelling revolves around the idea that less is more, this is one of those rare cases where the exception proves the rule: With solar, more is much better!! But I know you want more information than that so here are my recommendations:

  • 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:

  1. 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
  2. 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.
  3. 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.
This is another friends battery bank. He has eight  6 volt golf cart batteries and 500 watts. He lives just like he was in a house!

This is another friends battery bank. He has eight 6 volt golf cart batteries and 500 watts. He lives just like he was in a house!

When you read that description, the first thing you need to understand is that under NO conditions do you use a starting battery in your solar power system. That is just throwing money away. Your second thought might be that the only good choice is a deep cycle battery, but that isn’t always true; there are situations where a marine battery might be the better choice. The problem with deep cycle batteries is that they are more expensive. If you take good care of them and they last longer, then they are actually a much better deal in the long run. But if you don’t take care of them they won’t last any longer and the extra money you spent on them was just thrown away. So if you are brand-new to off-grid, solar power living, you may be better off with a pair of WalMart marine batteries. They are a good battery at a good price and they have a great warranty. If you ruin them (which is likely) you might can get all your money back. Better to learn on the cheapest battery you can. On the other hand, if you are experienced enough to baby your batteries, then spending the money on a top-quality, expensive battery makes sense. In the long run it will cost you less.
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)
6_Volt_Series-480x359Flooded 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?
During the day when the batteries are being charged, they should read over 13 volts, and up to 14 volts.

During the day when the batteries are being charged, they should read over 13 volts, and up to 14 volts.

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?
Battery_VoltageA: 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


  1. Laughing Richard

    This is one of the best basic explanations I’ve read on how to decide on number of panels and batteries! You paint a very easy to understand picture.
    Thank you,

    • Bob

      Laughing Richard, I wish I could take credit but from simple minds come simple ideas, and I have a very simple mind!

  2. Al Christensen

    When you have two, say, 100 Ah6V batteries wired in series, do you end up with the equivalent of a 100Ah 12V battery or a 200 Ah 12V battery?

    • Frank

      You would have 100ah @ 12volts two 12volt 100ah batts. hooked in parallel would give you 200ah @ 12volts

    • Bob

      Al, Frank answered your question properly. In working with solar power, there are several times you will be changing voltage so it’s important you understand that it is a general rule that if the volts change, the amps have to change also and in the opposite direction. So if the volts go up, the amps go down, and if the volts go down the amps have to go up. Since we doubled the voltage from 6 to 12, we have to cut the amps in half from 200 (two 100 ah batteries) to 100 (two 50 ah batteries). Here is another example: when you use an inverter to change the voltage from your battery (12 volt) to 120 volt household power, you increase the voltage by 10 times which means the amps go down by a 10th. That explains why your cables are so small for 120 volt appliances and they are so large for 12 volt appliances; 12 volt appliances have to carry 10 times more amps. So the cable that goes from your battery to your 2000 watt inverter is the size of your thumb, but the cable that goes from the inverter to the microwave is fairly small.

  3. Blars

    Float does not mean the battery is full, it means it is 80% full and cannot safely accept charge at the highest rate. A lead-acid battery takes 6-10 hours at float to be fully charged.

    • Bob

      Hi Blars, this is one of those things that means different things to different people. Here is a quote from Wikipedea:

      One MIT report[3] defines float voltage as the voltage at which the battery is maintained after being fully charged to maintain that capacity by compensating for self-discharge of the battery.
      That matches my understanding that floating is when the battery is full and the controller only allows in enough to compensate from any new drain. But, I am no expert!

      • DougB

        Uh Bob, apparently you are. My Morningstar MPPT controller manuals say the same thing. Bulk is flat-out max-rate charging, Absorption is throttled back and held to attain a full state of charge, and Float “provides a very low rate of maintenance charging” and “is to protect the battery from long-term overcharge”. In my particular installation, with a sunrise at 6AM, it will flop around indecisively until 7:30 or 8 when voltage ramps up to max. Once it goes into absorption, it will hold that voltage for 2-1/2 or 3 hours. It will generally drop into float mode no later than 2PM, and coast. Left alone overnight, voltage will slowly drop to 12.8, which with my batteries, equals 100% charge. With the exception of my CPAP’s battery charging system, I’m using the 1:1 ratio you talked about. Curious to see how things go when I return to the cloudy Upper Midwest. Piece of cake down here!
        Very nice post by the way, though I do not see any money pouring out of my solar system. Maybe I misunderstood that part. I do see the elves, though…

        • Bob

          Doug, your solar panel doesn’t have money pouring out of it, I thought they all did!! Yeah, the elves in my system shovel the money into little tiny wheelbarrows and take it down the red wire and dump it into the battery, then they go back to the solar panel on the black wire. That’s why they are color-coded, so the elves don’t crash into each other!
          I thought everybody knew that!!!! I hope your elves don’t get too fat from not working in the mid-west!

        • Bob

          Sure, it’s a piece of cake when you have 800 watts of solar!!! I would think you must be making a pretty good amount of power even on a cloudy day.
          We have a great camp in Prescott, but the road is a little rough. I think you could make it fine though. Also, if it ever rains or snows hard, the road will be impassable until it dries. Hope you can join us again!

  4. Cyrus

    Great post Bob! I’ve read a lot about batteries and solar panels, but they are usually over my head. I liked your style of breaking it down.

    • Bob

      Cyrus, thanks for your kind words! Being aable to make things simple is one of the few advantages of being such a simple person like me!

  5. Dennis

    Great info Bob and you have broken down the battery issue very well.
    What I’m wondering is…are all solar panels equal in quality and performance?
    I suspect they are not but how do you select the best panel for a given price or application? Thanks!

    • Bob

      Dennis, to be honest, I don’t really know. I can’t see any difference in the panel quality of any of the panels I have looked at. My first two panels were made by Kyocera which is one of the very best solar panels. My last panel was a manufacturer I have never heard of, but I bought it from a home solar installer in Victorville, CA. He said he had installed many hundreds of them and they were trouble free. So I bought one. Solar panels have become commodities and I think they are all pretty good. My best advice is to buy from someone you trust.

  6. m.a.

    Bob – you are just a walking battery info source! Another great post. I’m sure you’re helping so many of us get this stuff into our brains so we understand it. Thanks once again.
    ps – re: your new camp – I’ll just bet it’s a little chillier! From the facebook posts I’m getting I think ALL the vandwellers are getting slammed by the storm systems moving through. Snow here in Idaho. 🙁 Stay warm! Spring WILL come.

    • Bob

      Thanks, m.a. every time I make a new vandweller friend and I try to learn as much as I can from them. Then I collect that info, and pass it on!
      Yes, it is much cooler! We moved from Wickenburg to here which gained about 3000 feet in elevation and then this storm hit today and it has been quite cool and very windy. The overnight lows are going to be in the low 30’s. I had to put on my wool socks already and I’ve worn a light coat all day. But it will be back into the 70s on Wednesday which is perfect! Next year you can be camping with us and avoiding that snow!

  7. m.a.

    ps again – question: do you make a little if we buy something on Amazon linking from your site?

    • Bob

      m.a., yes I do. It’s around 6%, but it doesn’t cost you anything, you won’t pay more or less because you linked to it from my sites.

  8. Hunter

    Great post Bob
    You took away all the unnecessary detail. Genius, like a Picasso.
    Born to serve reminds my of Bob Dylan. “You got to serve somebody.”
    Hunter in Indonesia

    • Bob

      Hunter, wow! Thanks for the very kind words! The best things in life usually come from stripping away everything but the essentials!

  9. Zora

    So I am looking to run a dometic fridge/freezer combo that supposedly sucks 3.3 amps (scored it new in box for $200 on craigslist! it is one of the ones with separate freezer, looks like minifridge, goes for about 800 off the shelf). I wanted to get 2 AGM batteries that are 125 each (I don’t want to worry about checking levels). Then a panel- 135 watts? Does this sizing make sense or should I do something different? The fridge is really all I want to run, though I don’t mind extra $ in the piggy too 😉

    • Bob

      Hi Zora, I think that will work just fine! I have both a Dometic and a Whynter 12 volt compressor fridge and I have run either one (not the two together) on my solar and I have 190 watts total on my trailer. I moved the Dometic out into my van (I live in a 6×10 travel trailer) and it is working off of a single 140 watt panel charging 2 golf cart batteries just fine.
      My Dometic also does draw only 3.3 amps. I added a lot of extra insulation to mine but I’m not sure if you can do that with yours. I would put some underneath it and on top of it. I would use Gorrilla Glue and get a sheet of polyiso foam insulation from Home depot (R6). Cut it into panel pieces to fit top, bottom, door and sides. Then glue them on. The thicker the better using multiple layers. Be very careful to not interfere with its cooling in any way!!

      • Douglas

        I also drive my truck more regularly than most people, so I can help charging that way, but I prefer not to drive it much, even with gas going down. I drive for going to job sites and such, although I don’t do that full time. My warehouse job is the full time job.

        • Bob

          The one advantage of driving a lot is keeping the batteries charged up.

  10. Douglas

    Because I live in the desert during the summer, silly and i know it, I thought about trying to have ac in the unit. I would have to have at least 6 100 watt panels as well as at least a 600 amp hours to make sure i had enough battery.

    • Bob

      That sounds about right for a small window unit.

  11. gail callaway

    I live in ft. worthy and would like to spend the winter in quartzite. I will be traveling in my basic cargo van. anywhere I can get batteries and solar panel installed on the way or should I just wait til I get ther? I have not bought anything yet cause I don’t know what to order.

    • Bob

      Gail, I’d wait till you get here. There are many good shops here to buy the parts and we can help you figure out what you need.
      See you soon!

  12. Bill

    Bob I know this is a old blog and I hope you or someone sees this and addresses my question. I am ready to buy the batteries for my solar system and I can get a great deal on some big marine 12 volt batteries that are maintenance free. Can I use these? There is no way to add water or check them. Also since they are sealed what should my voltages for the controller be set at, gel, sla, or flooded etc? Can I use the equalizer method on these of 15.2 volts for 2 hours? Thanks, Bill n Sadie plus Mic

    • Bob

      Hi Bill, sure you can use a maintenance free battery in a solar system no problem. They are either gel or AGM and AGM is far better. Fairly often your controller will will give you the choice of FWC, gel or AGM. I’d suggest you do a google search on the battery and find a manual for it or search for the manufacturer and find their information on their site. It’ll tell you which setting to use and that batteries preferred charge rate and float point.
      Nearly all sealed batteries must NOT be put on an equalizing charge and can be damaged by it. A few can be but unless you know or sure it’s safe don’t do it.

  13. Steve

    Hi Bob, So how many watts (panels) and storage batteries are you running now? I’m looking at your recommended 200 watt system on Amazon. What batteries do recommend? ……. Steve + Cheryl
    Trapper Springs

    • Bob

      Good to hear from you again Steve! You can’t beat a pair of 6 volt golf cart batteries. You can get good prices on them at Costco or Sams and they are good batteries.

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