Battery Showdown: Lead-Acid vs. Lithium-Ion
Which is the best battery for an off grid energy system?
This is an except from the new book: Off Grid Solar: A handbook for Photovoltaics with Lead-Acid or Lithium-Ion batteries.
I have seen so many off grid energy systems with defunct lead-acid batteries. They work as designed for a few years if you are careful, and then they lose their depth of charge. I’ve been working with Lithium-Ion batteries over the past year, and while significantly more complex, they have some amazing advantages. Based on my experience, I predict that in a few years, we’ll move away from lead-acid batteries.
I believe right now we are at a clear crossroads when it comes to choosing a battery type for energy storage for off grid energy systems. We are at the transition between lead-acid batteries, the tried-and-true technology used for decades, and lithium-ion’s promise of higher density, improved resiliency, and longer cycle life.
Recently we’ve seen companies successfully gain traction with cost effective lithium-ion technologies. Li-Ion batteries are selling for as low as $400/kWh. Tesla, LG Chemical, Sonnen, and Simpliphi Power all have reliable products at reasonable prices. And recently the German car manufacturer Mercedes-Benz announced that they are also getting into the stationary storage market with Mercedes-Benz Energy.
But what about lead-acid batteries? They’ve been around for so long that they are practically a commodity. Their quality ranges significantly depending on the manufacturer, but the technological differences are minimal. Manufacturers like Trojan, Rolls/Surrette, and MK/Deka have been in the game for decades. Why disrupt this two-century old technology? And what makes lithium-ion a game changer?
Advantage № 1 — Superior in Size & Weight
While superior energy density is not critical for stationary applications, it is essential for electric vehicles and portable electronics. Increased energy density greatly improves the deployment and installation process for stationary energy storage.
The graph above illustrates how lithium-ion batteries are about a third of the weight and half of the volume when compared to lead-acid (flooded, AGM, and gel). Lithium-ion batteries are in a league of their own when compared to all other battery types since they are significantly more energy dense.
Advantage № 2 — Superior Resilience
All batteries are vulnerable to damage from excessive discharging and extreme temperatures. Lead-acid batteries are generally less resilient to this kind of abuse and are harmed if discharged too quickly or deeply. Lead-acid batteries lose potential cycles if they are discharged below 50% of their State of Charge (SOC) or if discharged faster than C/8. On the other hand, lithium-ion batteries can be discharged to about 80% SOC and at a rate of C/2 without any long term damage. The table above shows common characteristics of the three types of batteries; flooded lead-acid, valve-regulated lead-acid, and lithium-ion.
A helpful way to think of charging and discharging a battery is to imagine it like a balloon. If you repeatedly inflate a balloon to its maximum capacity and then completely deflate it, the balloon material might fatigue from the excessive stress. Now imagine with another balloon you repeatedly inflate and deflate it from 50% to 90% full, the material will experience less stress and will last longer than the first balloon. The plates inside the battery undergo a comparable stress as the balloon material. In this example, lithium-ion batteries are simply made from a better, stronger balloon material when compared to lead-acid.
Besides the Depth of Discharge benefits, lithium-ion batteries also have a longer useful life; they can cycle more times without significant loss of capacity.
Advantage № 3— Superior Lifecycle
Yes, it’s true that the initial cost of lithium-ion batteries is MUCH more expensive than an alternative option. (See Initial Cost per Battery Capacity graph.) Since lithium-ion is a newer technology, it’s likely that it has the potential to catch up and reduce that cost gap compared to these matured battery technologies that have been around much longer. Lithium-ion batteries will reduce in price and will drop below $400 per kWh in the near future. But is it fair to compare batteries by the initial price tag and the rated capacity? Absolutely not.
The above graph can steer someone in the wrong direction about very different battery technologies. The initial cost of a battery is important when budgeting for the system, but it can be shortsighted to only focus on keeping the initial cost down when a more expensive battery can save money in the long run.
This Total Lifecycle Cost graph is a better comparison because it takes into account the depth of discharge and the typical lifecycle. In this case, flooded lead-acid batteries have the lowest lifecycle cost, but that is assuming they are properly maintained and are not abused. This is a best-case scenario. If they get discharged past 50% frequently or if the maintenance gets neglected, then they won’t last as long, thereby increasing their lifecycle cost. Lithium-ion batteries require little maintenance and are more resilient to irregular discharging. When taken together, these factors make the lithium-ion battery more appealing for an Off Grid solar energy system. Bottom line, lithium-ion have about six times the number of cycles compared to a lead-acid.
The Incumbent: Lead-Acid
For now, your safest bet is to use the tried-and-true lead acid battery for your off grid energy storage. The solar charge controllers and inverters available on the market today are designed to work with lead-acid and have years of field testing, which has worked out all the kinks. The initial price tag is still a significant savings over lithium-ion. If lead-acid batteries are maintained properly, they will function at 80–90% efficiency, but good luck with that maintenance.
The two main types of lead-acid batteries are flooded and VRLA (Valve Regulated Lead-Acid). Flooded lead-acid batteries are the least expensive but also the least resilient to damage requiring significant care and maintenance. If you want to avoid the need of maintenance then VRLA might be good but they are typically twice the price and have a shorter cycle life compared to the flooded type. Commonly called “sealed” batteries, VRLA are not technically sealed but are valve regulated to allow for off-gassing. Check out my other article, Choosing a Lead Acid Battery for Off Grid Solar.
The Newcomer: Lithium-Ion
For decades lead-acid batteries have been the dominant choice for Off Grid solar systems, but with the growth of electric vehicles, lithium-ion battery technology has improved and become a viable option for Off Grid solar.
In 2016, lithium-ion batteries were just beginning to be used for large-scale solar systems, but they have been used for portable and handheld solar systems for years. Due to their enhanced energy density and ease of transport, you should seriously consider using lithium-ion batteries when planning a portable solar energy system.
About the Author
Joe O’Connor, author of Off Grid Solar: A handbook for Photovoltaics with Lead-Acid or Lithium-Ion batteries. Seeking to amplify his impact after over a decade of traveling to remote places to install solar, Joe decided to share his knowledge and experience in solar energy systems by writing a book. Joe has built off grid solar energy systems in the U.S., Haiti, Nepal, Portugal, Guatemala, and most recently in Virunga National Park, Democratic Republic of the Congo. To learn more about his work in Congo, click here to read SolarCity, Tesla, and Virunga, Building Solar Micro Grids for the guardians of Africa’s oldest and most biodiverse national park.