Lithium-ion vs. AGM Batteries: Which is best for solar?

Written By Reed Crossley

On January 28, 2020

At a Glance:

Consumers are beginning to justify the higher cost of lithium-ion over lead-acid for the added value it delivers in longevity and robustness for residential and commercial energy storage systems. But which is the best type of battery for solar?

The rise of lithium-ion batteries over their lead-acid counterparts happened first in mobile applications, such as laptops and phones. Armed with a significantly higher energy density, lithium-ion batteries’ ability to offer the same amount of power with less weight made it the preferred portable choice. Now consumers are beginning to justify the higher cost of lithium-ion over lead-acid for the added value it delivers in longevity and robustness for residential and commercial energy storage systems.

So which is the best choice to pair with solar? Lithium-ion batteries beat lead-acid in several key metrics but are more expensive, which means the answer can be dependent on the application. Let’s breakdown each type in detail and explore the pros and cons.

Lead-Acid Batteries, Specifically Absorbent Glass Mat (AGM)

There are several categories of lead-acid batteries, including flooded, gel, and absorbent glass mat (AGM). AGM is the newest of the three and the best suited for solar applications because of its zero-to-low maintenance, so it gets the spotlight here. AGM is named for the woven fiberglass mat that separates the positive and negative electrodes and holds acid absorbed in its fibers. Unlike flooded batteries, AGM is unspillable and requires no maintenance. The same can be said for gel batteries, but AGM is more forgiving during recharge, giving it the edge in solar.

Lithium-ion Batteries

The beginnings of lithium-ion batteries can be traced back to the 1970s oil crisis when researchers were looking for energy storage methods to support fossil-free technologies. Their work led to a battery that exchanged lithium ions back and forth between electrodes instead of the conversion of water to sulfuric acid that occurs in lead-acid batteries. This new battery entered the markets in 1991 and has proven to be a revolutionary force within energy storage for reasons that will be discussed below. In recognition of the global impact of lithium batteries, the 2019 Nobel Prize in Chemistry was awarded to three of the scientists key to its development. Lithium-ion use continues to grow, and within the last three years has begun to compete for use in large-scale solar systems.

Benefits of Lithium-ion Batteries

Depth of Discharge: This refers to the percentage of a battery that can be discharged without inflicting damage. Lithium-ion can generally discharge around 86-98% while lead-acid can handle closer to 30-50% discharge. This means at any given moment, a lithium-ion battery has over a 30% larger operating range. It also means lithium-ion batteries are less prone to the damage that comes from exceeding the depth of discharge.

Lifespan: All batteries degrade over time, but lithium-ion batteries consistently outlast AGM. Factors like a larger depth of discharge make lithium-ion batteries more resilient and help these batteries go up to six times the life cycles of AGM alternatives. Depending on the chemistry and usage habits, lithium-ion batteries can degrade at less than a percentage point year-over-year.

Capacity: While not as crucial for stationary applications, it is important to note that the higher energy density of lithium-ion means that it can store more energy per mass than AGM. This means systems can be smaller and lighter while providing the same amount of output.

Warranty: A lithium-ion battery can typically be warrantied for up to 15 years, whereas other battery types generally are warrantied for only a maximum of 1 year.

Embodied Energy: This may be an important factor for every business or homeowner, but lithium-ion batteries have a smaller energy footprint to build than lead-acid. A Stanford study examined several different grid-scale storage options and found that lithium-ion is five times better than lead-acid when considering the energy needed to make the batteries compared to the energy stored over its lifetime.

Sun Xtender AGM Batteries
ground-mounted solar system

Cost: The lower upfront cost of AGM batteries is the primary factor keeping them competitive with lithium-ion. For those with restricted capital, an AGM system may be the best choice. It is important to note that when adjusted for cost over the lifecycle, lithium-ion batteries are at par or cheaper than AGM.

History: Since AGM and other lead-acid batteries have been around longer, they have a greater history of field testing. There is simply more experience with these batteries in how they work with other energy system components in various applications.

Which Is “Better” For Making The Switch To Solar?

The answer hinges on your goals, how much of an upfront investment you are willing to make, and the duration and frequency the battery storage will be used. For example, if you are planning a home solar array for living fully off the grid for decades to come, then a more expensive investment in lithium-ion batteries will pencil better over the lifecycle of the system.

Let’s now say that the battery is only being used as a backup power system only when the power goes out for a home or business that is connected to the grid. Since these batteries will only see occasional use and therefore will not degrade as quickly as a battery for daily use, choosing an AGM battery bank will meet your energy storage needs while saving on cost. As a clean, maintenance-free alternative to a generator that you can also take a few tax incentives and rebates on, it may be your low-cost key to resilience.

Now, with utility policies like Time-of-Use and Demand Charges increasing in popularity across the country, it is financially advantageous to cycle your battery system every day to reduce the added costs at various times throughout the day.

For now, the price difference between AGM and lithium-ion is a key differentiator, but the gap is narrowing. A 2019 report by the research company BloombergNEF showed lithium-ion batteries levelized cost of energy (the cost to deliver energy over the technology’s lifespan) dropped 35% from early 2018. This metric does not perfectly correlate to the current pricing of lithium-ion batteries, as manufacturing is rushing to scale up to meet the new boom in demand.

The increasing reliance on renewable energy means that the battery storage systems have never been more critical. The success and growth of renewable energy are inextricably linked to the batteries that allow it to be accessible at any hour of the day in any climate condition. Home and business owners should take stock of the goals for their solar power project and use that to make an informed decision as to which battery storage system is right for their situation.

sonnen Battery
Reed Crossley

About the Author

Reed Crossley is a Digital Marketing Strategist living in Boulder, Colorado. Reed works with a variety of energy, sustainability, and social impact organizations to do groovy things with digital technologies and achieve meaningful organizational goals. He holds a Master’s degree in Environmental Management from CU Boulder and volunteers as the Marketing Coordinator for the Boulder chapter of the Colorado Renewable Energy Society (BCRES). Ever quick with a movie reference or fun fact about the cleantech industry, Reed can be found hiking or kayaking in the mountains in his spare time. Connect with Reed on LinkedIn.

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