December 5, 2019

Solar Pantry Project: Batteries

Continued from "Burying the Solar Cable."

Step by step we're making progress toward getting our freezer (and maybe a chest fridge) on solar. Each step has presented challenges! Choosing batteries, for example. Solar panels only produce electricity in the sun, so batteries are a necessity to keep things running when the sun goes down. However, there are so many choices. Here are the factors we looked at and how they influenced our decision.

Flooded lead acid batteries. Of the various kinds (flooded, sealed, lithium), these are the most economical. Considering this is our first solar set-up and it's possible we'll make some learning mistakes, it makes sense to keep it affordable. Life expectancy for this type of battery is only about five years, and they do require maintenance and need ventilation. But also, we can buy them locally, another plus.

Local availability. The reason for this is simple—shipping costs! Shipping for batteries starts at $250 to $350. Because of our budget, it makes more sense to buy locally. But also, if there's a problem it will be easier to return them to the store rather than ship them back.

Deep Cycle. These are different from car (cranking) batteries. In a vehicle, the battery has to supply a brief high demand to get an engine started, i.e., rotate the crankshaft. A cranking battery is built to bear a high load for a short duration. Deep cycle batteries are built for a low continual draw with repeated discharging and recharging.

6-volt batteries. We took this recommendation from our go-to book, Prepper's Total Grid Failure Handbook (my book review here.) They are similar in size to 12-volt batteries, which means they are more heavy-duty than their 12-volt batteries and should last longer.

Amp-hours (AH). Starting batteries list cranking amps, while deep cycle batteries list amp-hours. This isn't a specific time measurement, because factors like temperature make a difference. Basically, the higher the AH, the longer the battery can last before it needs recharging. Of course, the higher the AH, the higher the price.

Weight. Three popular sizes of batteries for solar battery banks are golf cart (GC), L16, and fork lift (industrial) batteries. The L16s and industrial batteries offer the highest amp-hours, but are heavier (and more expensive). In general, golf cart batteries typically weigh less than 100 pounds, L16s weigh more than 100 pounds, and industrial batteries weigh 800 pounds or more. Handling them is a consideration!

After weighing our choices, we decided that golf cart batteries would best meet our needs and our budget. We ended up with two options, both are the same size (GC2), weight (65 pounds), and AH (235).
  • Duracell from the battery chain store. $133 each + $21 core charge = $154 per battery
  • Rolls Surrette from a small business. $130 each + $16.67 core charge = $146.67 per battery

Price plus brand reputation determined our choice:


My original estimate was four batteries, but we were able to get six for $880. And that still keeps us within our original budget of $1500!

How much energy will these actually give us? Well, do you remember my explanation of series versus parallel wiring in "Wiring the Solar Panels?" We'll apply that to our battery bank, connecting some in series and some in parallel.

Blue - series connection, neg to pos. Adds voltage. 6v + 6v = 12v
Green - parallel connection, neg to neg & pos to pos. Adds amperage.
235 amp-hours + 235 amp-hours + 235 amp-hours = 705 amp-hours

This set-up will give us a 12-volt, 705 amp-hour battery bank. We need less than 1000 watts, so 12 volts will work well for us. If we wanted much more than 1000 watts, we'd need a 24-volt system, or even 48 volts for higher usage.

Hopefully, 705 amp-hours will give me a two-day back-up if the skies be dark and sunless. That's a definite possibility, especially this time of year. In that case, the options are plugging back into the grid or finding an alternative way to recharge the batteries (like a generator—something on Dan's wish list!) Adding more batteries at a later date is not an option. This is because the batteries in the bank need to age together, with the same number of discharges and charges. Adding new batteries would create an imbalance that would cause more problems.

Next step is building a box for them. While Dan's working on that, I'm figuring out cable sizes and DC circuit breakers. We're getting closer!

17 comments:

  1. Golf cart batteries are what we've come to think is best in our system, too. Who would have thought setting up a simple (ha!) alternative energy system would be so complicated? Not to be discouraging because every system is different for a variety of reasons, but we've had ours in place for over twenty years now and it seems there is always something (sigh) that needs tweaking or makes the system inoperable for a period of time. But if there ever comes a time when grid power is unavailable, all the hassle will be worth it. So kudos to you and Dan for doing the research, hard work, making the financial outlay and having the patience to work toward your goal.

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  2. Gorges, I hope that means I did a clear job explaining myself. That's always my aim!

    Mama Pea, it seems that the more complicated things are, the more problems they have. You and Papa Pea should write a troubleshooting guide! That would be very helpful for beginners like us. :)

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  3. It will be interesting to see how long they will last. My completely off the grid friend in Arkansas always has plenty of power in the summer months but in the winter like when I saw them last weekend where there are gray skies aplenty, if feels like a bar in their house because they can't keep many lights on for long periods of time. I know they have gotten used to it but for me, I just want to fall asleep.

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  4. Ed, that's something Dan and I are curious about as well. I'm kinda glad we're putting it together in winter, because that will give us the worst case pretty quickly. Your comment also points to the lifestyle changes that must accompany solar (or any alternative) system. In that regard, I'm also kinda glad we aren't going completely off grid. We've cut down on electric usage quite a bit, but I'm sure we'd have to make more adjustments if we were completely solar.

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  5. Golf cart batteries, we never thought of that, but then we were looking powering over 1000 watts too. I'm looking into resale of batteries from cell towers. They replace them every year and still have tons of life left in them as a cheaper alternative to be totally solar powered. Like you said, adding new to old creates an imbalance.

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  6. Jo, the cell tower batteries are an interesting idea. Have you sourced where to get them? You can still make a 24-volt system with golf cart batteries. Just wire four in series and then wire two strings in parallel. You could have a lot of watt-hours that way.

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  7. My opinion is you've made a good choice and they will serve you well for years to come.
    Also, you haven't said anything about Dan's hand. Is he having any trouble with it?

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  8. Mike, thanks for asking about Dan. The end of that finger will always be wildly crooked and he says that sometimes it gets in the way. But he doesn't let it slow him down. He compensates and presses on.

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  9. I admire you so much for what you are doing here. Your explanations are so good! Is it possible that you will be generating power by Christmas? Wouldn't that be a great gift?

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  10. Mary, it's possible, but I know better than to wrap the project in a time frame. :) Dan's almost done with the battery box and the last of the battery cables and circuit breakers arrived yesterday. So we've got all the components except for grounding. Reading through the installation manuals for the panels and charge controller is a job in itself, lol.

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  11. I think I would benefit from solar but I don't have the money or the knowledge or the physical wherewithal to carry it out at my age. I wish I would have been wiser when I was younger! LOL!

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  12. Sam, there is both an investment and a learning curve. I think it's been worth it for us. :)

    R, welcome! And you're welcome. :)

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  13. Hi Leigh :) Thanks for the descriptive post. There is SO much to take into consideration...I think you guys made the right choice for your budget. I didn't know that about adding new batteries either! I appreciate that you are being so up front about the set up!

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  14. Rain, thank you! It really helps to write everything down in a blog post. :) Helps me remember and makes an excellent record of things we need to remember.

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  15. Wow! You're making great progress.

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