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Concentration of CO2 in the Atmosphere

Batteries for Backup or Off-grid storage

The times they are a-changin’.” – Bob Dylan

Before: System with lead-acid batteries in need of replacement.

Before: System with lead-acid batteries in need of replacement.

By N. R. Mallery and George Harvey

Until recently, the choice for battery storage of renewable energy generation has mostly been with lead-acid options. Today, with all the new battery technologies touted as the greatest and the latest, many are asking, “What is the best battery for my application?”

In the past, if you wanted a battery backup system for on or off-grid storage, the answer was clearly lead-acid battery options. Today, however, battery technology is one of the most heavily researched subjects of science. Scientists constantly look for less expensive, lighter, and safer systems. It leaves many of us not knowing which way to turn.

Nevertheless, one of the interesting things about this is that old, reliable technologies often still have their places. Perhaps a much better question should be, “How can I tell what the best battery for my application is?”

After: The same system with new lithium batteries. Courtesy photos: John Hassell.

After: The same system with new lithium batteries. Courtesy photos: John Hassell.

John Hassell, of Be Green Solar, in Benton, New Hampshire, helped us out on this. “There are a lot of things you need to know about,” he said, “but four are at the top of the list.”

  1. What is the battery capacity? This is expressed in amp-hours. By multiplying the number of amp-hours times the voltage, we can find the amount of energy the battery can store in watt-hours.

  2. What is the discharge rate for the stated capacity? This is the rate of how fast the batteries can be discharged. This is important, because you need to know that the battery you have will support all the loads you want it to power.

  3. What is the depth of discharge (DOD)? This tells what part of the total energy the battery holds can be used before it should be recharged.

  4. What is the expected life of the battery, which is measured by cycles of charge and discharge? Also, what are the warranty terms?

Knowing the answers to these questions will not give you all the information you need to make decisions, but they are a good start. Other things to consider are often related to the place the battery system will be installed and the attention it will require. They include the weight, physical size, operating temperature range, need to vent for generated gases, cost relative to capacity, need for maintenance, the ability to hold a charge when not in use, and more.

The biggest recent changes for home energy storage are due to new lithium battery technology. Most famously, Tesla has introduced its PowerWall 2 battery, which cut the price of lithium-ion batteries. Though they are great batteries for many circumstances, they are not perfect for all. They are not recommended for off-grid applications, for example. Part of the reason for this appears to be that the lithium-ion technology they use requires active cooling. Without cooling, temperature control becomes an issue and must be managed by the battery’s BMS.

There are other outstanding lithium technologies available, however, without concerns of overheating. There are many to choose from, but we are most familiar with RELiON, SimpliPhi, Sonnen, and Iron Edison, which are all brands that we recommend. They all offer lithium-iron-phosphate batteries (called LiFePO4). These batteries are readily available and have some significant advantages for a home or business. They are safe, reliable, and not ecologically hazardous, and require a considerably smaller space than lead-acid options for the same amount of storage.

After much research, co-author N. R. Mallery is currently in the process of replacing her current bank of 24-2V lead-acid batteries for the third time since her system was built in 2002. The new bank will consist of just two to four lithium batteries. Their initial higher cost will be more than compensated for due to their 16 to 25 year lifespan alone. During that time, they will need no maintenance. Where most lead-acid batteries can only be discharged 50% without damage, the lithium batteries can be discharged to 70% to 80%, or even completely with some brands. Unlike lead-acid batteries, lithium batteries maintain a relatively flat voltage as they discharge. These features are similar across the brands of lithium batteries we recommend.

John Hassell has been suggesting SimpliPhi batteries for his customers. Co-author George Harvey has been struck by the social efforts Sonnen has made to help people in Puerto Rico. N. R. Mallery is considering RELiON batteries to replace her bank of lead-acid batteries. Clearly, all of these companies and batteries have their good points.

We should address whether lead-acid batteries are still a good option. The quick answer is yes, they do have their place. The low price of lead-acid batteries makes them attractive for many applications. The flooded batteries require regular maintenance, and it can determine how long they will last, though somewhat higher-priced gel and AGM batteries are nearly maintenance free. Solar installers might tell you to expect them to last six to seven years, but there are many folks that we have talked to whose batteries have been lasted much longer.

Trojan batteries in particular, the ones most old-time off-gridders are familiar with, are known for their ability to handle daily charging and discharging cycles. Our eyes are definitely on the new renewable energy offerings from Trojan Battery that have a much longer life expectancy than we have seen in the past. Check out their ad on this page.

Another tried and true standard is the Rolls Battery that many solar installers in our region recommend. RAE Battery Storage is the go-to company for Rolls and offers some of the best service around the northeast. (See ad on page 13) The owner of the company, Roy A. Early has been working with lead-acid batteries longer than most of us have been alive. He is happy to share his deep knowledge of these batteries with anyone considering them. He tells a story of batteries in the Philippines that have been continually used for over 35 years. The island depends on the batteries for their energy storage and have performed extremely high levels of the proper maintenance that they learned from Early.

Many customers still favor batteries from Trojan or Rolls Surrette with good reasons that start with affordability and a solid history of performance. They are, however, different from lithium batteries. You should do a side-by-side comparison of features before buying.

Another technology that deserves mention is nickel-iron. Iron Edison has said that customers who buy nickel-iron battery systems will probably never have to replace them. As we said above, they also offer Lithium Phosphate LiFePO4 options.

We hope this information is helpful to you. If you have questions, do call the companies that offer them to get answers that are particular for your situation. Contact info for our recommended brands are: RELiON Batteries:; SimpliPhi:; Sonnen: Iron Edison: And do take note that battery systems qualify for a 30% federal tax rebate!

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