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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!