Solar Design Guide: Off-Grid Battery Selection

Batteries are the fundamental cornerstone of any off-grid energy system. Whether you are using solar, wind, hydro or generators, however fueled, your batteries will make the difference between success and failure. This guide is written to help you make the most of your alternate energy sources by making an intelligent and informed battery selection.

Before making a purchasing decision about batteries or any other off-grid component, you must first determine your requirements for continuous use. Be sure to read the previous article in this series for details about what loads are realistic with various power levels.

Battery Fundamentals

Many battery technologies are available to fill various needs. For most readers with realistic loads such as lights, security systems or essential appliances like freezers, the most cost-effective currently available solution for most people is the use of deep cycle lead acid batteries arranged in arrays to create a much larger virtual battery.

For longest service life, however, other technologies such as lithium iron phosphate (LiFePO4, or simply LFP) batteries are available, but these are currently more expensive for a given capacity. There are also even more capable battery technologies on the horizon, but are not yet available commercially at a reasonable price. As an example of LiFePO4 batteries, they may last ten years in a given application, where a similar capacity of lead-acid batteries may last two years. Since LiFePO4 batteries are currently more than five times more expensive than lead-acid, they currently cost more per year of service. Regardless, the installed cost of LiFePO4 batteries may be out of reach for most readers that only need to power their freezer in an emergency. Plus, LiFePO4 batteries are currently at the knee-of-the-curve point price-wise, and may fall dramatically in price in the next few years. For this reason, we recommend that our readers field commercial off-the-shelf lead acid batteries today, and then supplement or replace them with LiFePO4 when those prices fall.

Regardless of which battery technology is chosen, you must select a battery which is capable of deep-cycling. Look for batteries which are described as deep cycle, golf cart, or marine. Any of these will be suitable for off-grid use.

We do not sell batteries, but suitable batteries can be found at several retailers at reasonable prices. Keep an eye on retailers such as BatteriesPlus, sometimes they will run specials such as web discounts or first-time customer discounts. When they do, stock up.

Calculate the Right Capacity

 Load  Whr 
50 W2000
100 W4000
250 W10000
500 W20000
You must also select sufficient battery capacity to operate your system for the time that the sun isn't providing peak power. In this guide, we will use solar as our example for illustrating how to size your battery array. We will show you how to do the math, and then supply a calculator at the end of this guide to make those calculations easier, and allow you to compare battery prices in an informed way.

With solar, it is a rule of thumb that the sun provides power for 4 to 5 hours per day, and your batteries supply power for the remaining 20 or so hours. Fortunately, although battery capacity is given in many ways from brand to brand, most large lead-acid batteries are usually given ratings expressed as their 20 amp hour rating. This rating roughly means the amount of total amp-hours the battery can supply if discharged fully and uniformly over a 20 hour period. We will use this rating as our way of comparing batteries in our calculations below.

To determine the required total battery capacity, expressed in units known as watt-hours, simply multiply the desired continuous use wattage by 40. See the chart to the right for these values.

Why 40 instead of 20 since we are designing for a 20-hour battery day? To improve the cycle-life. If we designed the battery system to run dry (100% depth of discharge) just as the sun was reaching useful power, our batteries would be fully discharged and would wear out quickly. By multiplying by 40, the batteries will typically be at half charge (50% depth of discharge), and will last longer, much more than twice as long. Plus, you will also have reserve capacity in case you had an emergency and need to use a little more power than planned now and then.

Pick a Deep Cycle Battery

Choose from 6 volt, 8 volt and 12 volt options. Because we will be using these batteries to form an array, the specific voltage, with some exceptions, isn't as important as the overall energy available for the price. Some typical deep-cycle battery options are listed below, including their overall energy in watt-hours and price per watt-hour:

Model V 20-A-hr Wh Price Price per Wh Price w/ $15 core Price per Wh
Duracell GC262151290$84.52$0.0655$99.52$0.0771
Duracell GC881651320$99.72$0.0755$114.72$0.0869
Duracell EGC262301380$112.68$0.0817$127.68$0.0925
Duracell 27D12901080$79.88$0.0740$94.88$0.0897
Prices listed reflect typical retail at time of writing, compare retailers before purchasing. Core charges vary by state.

As you can see from the chart above, stringer price doesn't tell the entire story, nor does raw 20-amp-hour rating. Note also that adding core charges can change the relative pricing somewhat.

To have a fair comparison among batteries, always look for the 20-amp hour rating. Note that some batteries will also specify shorter discharge periods, such as five or six hours, and that the available amp-hours during these shorter periods is significantly lower, usually by about 25%. This is one vivid example why we want to use more batteries than just the bare minimum to supply the load for 20 hours. Not only will the batteries last much longer by using them more lightly, they can supply more energy overall with smaller currents.

Also, avoid the temptation to mix new batteries with old batteries, or to mix and match battery types. Ideally, your array should be composed of all the same type of battery deployed at the same time. If you expand your system later, consider employing new batteries on different charger and inverter circuits, perhaps. Old batteries in an otherwise new battery array will consume an inordinate amount of charger current as heat and keep the fresh batteries from ever reaching their full charge. Similarly, when discharging, the old batteries will force the new batteries to work harder and age prematurely.

Once you have selected a deep-cycle battery, you are ready to design your battery array. See also our handy Off-Grid Battery Calculator, which simplifies and combines the math for both this article and the next.

Next article: Designing an off-grid battery array ...


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