In 2023, your practical and proven battery choices for solar backup are between lead-acid and lithium-iron-phosphate (LFP) batteries. In brief, lead-acid batteries are cheaper up front, but LFP batteries have a lower overall life-cycle cost because they last longer. Gel or AGM sealed lead-acid batteries are safer to use in living spaces because of the fire hazard associated with lithium batteries.
Your Choices are Between Lead-Acid or Lithium-Ion
Lead-acid Flooded, AGM, and Gel
Lead-acid chemistry batteries are available as standard flooded, absorbant glass mat(AGM), and Gel. All three types have the same basic lead-acid chemistry.
Flooded Cell
Flooded cell lead-acid batteries have a liquid electrolyte covering lead plates. They are the original wet-cell battery design that continues almost unchanged since being invented by French physician Gaston Planté in 1859. Most combustion cars use lead-acid batteries. Starting a car engine requires a short burst of 400 to 600 amps, and flooded cell batteries are excellent for this purpose.
This battery type is the least expensive and has a service life of 200 to 300 cycles, the shortest of all battery types. The service life is defined as the number of charge/discharge cycles until the battery capacity decreases to 80% of the original. The battery is still usable after its normal charge cycle life especially if it was oversized in the first place. However, it will hold a smaller total charge and may be marginal for engine starting. A flooded cell battery lasts about 3 to 5 years.
Flooded cell batteries tolerate a depth of discharge of about 50%. The depth of discharge(DoD) is defined as the percent of the rated capacity that a battery can be discharged and recharged without damage. In other words, a flooded cell battery rated at 100Ah has a usable capacity of 50Ah. Taking a battery deeper than 50% DoD can shorten its life.
Gel
Gel batteries are a newer design with the same basic chemistry as a flooded cell type. In this case, the liquid electrolyte is modified into a semi-solid gel by the addition of silica. Gel batteries tolerate deep cycling well but are not suitable for use as engine starting batteries. They are spill-proof and vibration-resistant but do not perform at low temperatures. Gel batteries have the lowest self-discharge rate of all lead-acid batteries. They also have the longest life of all lead-acid batteries.
Gel batteries must be charged slowly and are easily damaged by overcharging. They require properly calibrated chargers. Make sure that the charger you are using can handle a gel battery. To learn more about solar chargers, click here.
Gel batteries are the most expensive of all lead-acid type batteries. Because they are deep cycling, long-lived, safe, and maintenance-free, they are the best solar backup battery in the lead-acid class.
Absorbant Glass Mat (AGM)
Instead of being filled with liquid electrolyte, AGM batteries have a fiberglass mat between the metal plates that are saturated with electrolyte. Think of a sponge saturated with water compared to a glass of water.
AGM batteries give more starting amps for their size compared to flooded cell batteries. They also have a longer service life, although not as long as lithium batteries. They are vibration-resistant, perform well in extreme temperatures, and are maintenance-free. AGM batteries are commonly used in boats, racing cars, and aircraft.
AGM batteries are more expensive than flooded cells but still much less expensive than lithium. They have a lower Ah rating than flooded cell types for the same size and weight. This means that they are good at starting engines, but will have less total capacity as a house battery for running your appliances. While they can be charged faster, they are more easily damaged by overcharging. AGM batteries can be damaged by chargers that are intended for standard flooded cell batteries. They can be used for both starting engines and as a deep-cycle house or solar backup.
Lithium-Ion
In 2023, some of the most common types of lithium-ion batteries are lithium-iron-phosphate(LFP), lithium-cobalt oxide(LCO), and lithium-manganese oxide(LMO).
All types of lithium batteries perform better and are more expensive than lead-acid batteries. However, lithium batteries are potentially more dangerous because of the possibility of thermal runaway.
Understand Thermal Runaway
In thermal runaway, the battery releases energy without control causing higher and higher internal temperatures and the risk of fire and explosion. It can occur when the battery is defective, damaged, overheated, or overcharged. This is why lithium batteries are restricted on commercial airlines.
Fortunately, in 2023, lithium batteries are much safer than they used to be. Consider that they are in common use in all types of consumer devices and in electric cars.
Lithium batteries need special protection circuits to monitor their power output and temperature to protect against thermal runaway. To maximize safety, a solar system using lithium batteries needs to be properly designed in terms of battery storage, overload protection, and charger selection and set-up.
Lithium-Iron Phosphate(LFP)
Right now, LFP batteries are the best choice for solar energy backup. In case you are wondering why it is abbreviated LFP, it is because the chemical symbol for iron is Fe, as in the word ‘ferrous’.
LFP batteries are one of the safest types of lithium batteries. They have a specific energy of about 90 to 160 watt-hours per kilogram(Wh/Kg). While this is only a moderate energy density compared to other lithium chemistries, it is much better than lead-acid, which has a specific energy of about 30 to 50 Wh/Kg.
LFP batteries are cobalt-free. Because cobalt is unstable at higher temperatures, eliminating it from the battery chemistry improves safety. Also, cobalt is very expensive and has a large environmental impact. So being cobalt-free makes LFP batteries safer, less expensive than other lithium batteries, and better for the environment.
LPF batteries have a charge/discharge life of up to 4000 cycles, which is longer than other lithium chemistries and much longer than lead-acid batteries. They can be run to 80% DoD without reducing their lifespan. Expect an LPF battery to last about 15 years.
LFP batteries require a battery management system for safety and best performance. They need to be protected from extreme temperatures, especially during charging.
Lithium-Cobalt Oxide(LCO)
LCO batteries are commonly used in cell phones and small electronic devices. They have a high energy density and provide small currents for a long time. They can be recharged quickly.
LCO batteries can easily overheat if they are overloaded. You have likely noticed your phone or laptop getting warm during use. The chemistry does not lend itself to power applications or larger sizes because of safety concerns. Also, they are difficult to recycle. The cobalt used in LCO is a rare and expensive material and is one of the most environmentally damaging substances.
Lithium-Manganese Oxide(LMO)
LMO batteries are often used in portable power tools, electric bikes, and portable power applications. Compared to LCO batteries, LMO batteries provide higher currents but for shorter times. They also have a shorter life than LFO batteries, only about 1000 cycles.
There are some other types of lithium-ion batteries in commercial use, but they are not generally applicable to solar storage.
Conclusion
To summarize, in 2023 your choices for a solar backup battery are between one of the lead-acid types or a lithium–iron phosphate battery. The least expensive battery to buy is a flooded lead-acid battery. While the lithium-iron-phosphate(LFP) battery costs much more upfront, over the total lifespan, it is cheaper because it lasts so much longer.
Battery Parameters
Before purchasing a battery, it is important to determine the amount of electricity your system consumes daily. For more information on sizing solar energy systems, please click here.
Before your buy a battery, make sure you understand the following properties:
Rated Capacity and Depth of Discharge
The rated capacity is simply the named size of the battery in Ampere-hours. Keep in mind that energy calculations are in watts, which is amperes times volts. To find out the usable energy in the battery, you have to multiply the rated capacity by the Depth of Discharge(DoD) percent. For example, if the recommended battery Dod is 50%, then the usable energy in a 100Ah battery is 100 x .5 = 50 Ah. Many flooded-cell lead-acid batteries have Dods of 50%.
Maximum Current, Deep Discharge, or Multi-Purpose
Batteries can be designed to optimize the maximum current for starting engines or operating other heavy-draw equipment. Alternatively, they can be designed for deep cycles. Boats often have separate starting and house batteries. The starting battery provides the maximum cranking amps, and the house batteries keep the cabin appliances and lights going all night. Some AGM batteries are advertised as dual-purpose. Be sure to check the specs.
Charge-Discharge Lifecycle
Basic lead-acid batteries last about 300 cycles until they reach 80% capacity. AGM and gel types may last into the high hundreds of cycles. LFP batteries last for thousands of cycles and usually have an 80% Dod.
Battery Management
Makes sure you know the battery storage and operation requirement. Flooded cell, AGM, and gel batteries have different requirements for storage and charging. Overall, all types of lead-acid batteries are more simple to manage and safer than lithium-ion batteries.
Most lithium batteries have a built-in management system that monitors battery temperature, voltage, output, and charge state. Charge controllers for lithium batteries should be matched to the specific battery, and include a remote battery temperature monitor.
Safe Installation
Installation requirements vary depending on the battery. Basic flooded cell lead-acid batteries require ventilation. Many batteries must be protected from extreme temperatures. Make yourself aware of the installation guidelines and regulations for your specific application.