For decades lead-acid batteries have been the dominant choice for off grid solar systems, but with the growth of electric vehicles (EVs), lithium-ion (Li-ion) battery technology has improved and become a viable option for off grid solar.
But why bother using Lithium-Ion Batteries if they are more expensive and more complicated?
In the past five years, 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.
While Li-ion batteries have their advantages for small, portable solar projects, I have some hesitation to recommend them for all larger systems. Most of the off grid charge controllers and inverters on the market today are designed for lead-acid batteries, meaning the built-in set points for protection devices are not designed for lithium-ion batteries. Using these electronics with a lithium-ion battery would result in communication problems with the Battery Management System (BMS) protecting the battery. That being said, there are already some manufacturers that sell charge controllers for Li-ion batteries and that number is likely to grow in the future.
Could Lithium Batteries be cheaper than Lead-Acid?
Lithium-Ion Batteries may have a higher upfront cost, but the long term cost of ownership can be less than other battery types.
Initial Cost per Battery Capacity
The Initial Cost per Battery Capacity graph incorporates:
The initial cost of the battery
The full capacity at 20-hour rating
The Li-ion pack includes BMS or PCM and other equipment so it can be compared fairly to lead-acid batteries
Li-ion 2nd Life assumes using old EV batteries
Total Lifecycle Cost
The Total Lifecycle Cost graph incorporates the details in the above graph but also includes:
The representative depth of discharge (DOD) based on the given cycle count
The round-trip efficiency during a cycle
The number of cycles until it reaches the standard end of life limit of 80% State of Health (SOH)
For the Li-ion, 2nd Life, 1,000 cycles were assumed until the battery was retired
All the data used for the two graphs above utilized the actual details from the representative data sheets and market value. I choose to not list actual manufacturers and instead use an average product from each category.
Initial cost of Lithium Batteries may be higher, but the lifecycle cost is lower.
Depending on which graph you look at first, you can draw drastically different conclusions about which battery technology is most cost effective. 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 the more expensive battery can save money (or trouble) in the long run.
Besides the cost, what are the other advantages?
Compared to lead-acid batteries, Li-Ions are also more resilient to damage from deep discharging, do not need to be brought to a full state of charge each cycle, function better in hot climates, and last up to three times longer. Lithium batteries major downfall compared to lead acid batteries that they require a Battery Management System (BMS) to function safely.
Lithium-ion batteries tend to perform better than lead-acid ones in both hot and cold climates. The graph below shows an example of how the different battery types will function in high and low temperatures. In the “Peak” performance range you can expect the battery to function as described in the datasheet; in the “Low” performance range the battery will function but at a reduced efficiency from “Peak”. It is recommended to never use your battery in the “Out of Specification” range, particularly in the high temperature range as this dangerously approaches thermal runaway temperatures.
TEMPERATURE Performance
Energy Density
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.
Battery Density Comparison
Battery Lifespan
There are a variety of factors that will reduce the expected lifespan of a battery and each battery type has its own weaknesses. Most lead-acid batteries get damaged if they are discharged below 40% of their State of Charge. When I describe a battery getting “damaged,” I am specifically describing a decrease in the battery’s State of Health (SOH), which just means the total capacity of the battery has decreased. A 100% SOH means the battery is functioning in a full SOH as the manufacturer intended and an 80% SOH is when manufacturers typically recommend retiring a battery, even though you can continue using them a bit longer.
Expected Lifespan per Battery Type
Lead-Acid (Sealed)
Starter (Car battery): 3-12 months
Gelled Deep-Cycle: 2-5 years
AGM Deep-Cycle: 2-8 years
Lead-Acid (Flooded)
Low Quality Deep-Cycle 2-7 years
Premium Deep-Cycle 7-15 years
Tubular Plate Deep-Cycle 10-20 years
Other
NiCd (Nickel–Cadmium) 1-20 years
Li-ion (Lithium-Ion) 5-15 years
NiFe (Nickel-Iron) 5-35 years