What Is a Solar Battery?

Do skyrocketing electricity costs, ageing infrastructure, and the global increase in extreme weather events and blackouts make you think about switching to solar power?

You’re not alone.

Solar power is by far the fastest-growing renewable source of electricity production, both on a utility-scale and for individual consumers. 

Residential solar panel systems are falling in price and gaining in performance…

And generous government incentives have made switching to solar more affordable than ever before.

But you’ll need more than solar panels to achieve energy security and maximise your return on your solar investment. 

For off-grid and hybrid solar panel systems, a solar battery is essential.

But do you need one for on-grid solar?

And what is the best type of solar battery?

Read on to find out.

What Are Solar Batteries?

Solar batteries store direct current (DC) electricity produced by photovoltaic (PV)  modules — like solar panels and shingles — for later use.

Solar batteries are required in off-grid and hybrid PV systems because clean, renewable energy sources like solar power are intermittent.

Solar panels don’t work at night.  

On cloudy days, electricity generation is reduced.

During peak sunlight hours, batteries provide stored power as required when electricity consumption exceeds generation from your solar panel array. 

In grid-tied systems, solar batteries are optional…

However, on-grid solar power systems DO NOT work during a blackout.

Residential solar panel systems without storage connect to the utility grid through an alternating current solar inverter and bidirectional electricity meter.

During an outage, a kill switch automatically interrupts the connection between your home and the utility grid.

Allowing electricity to be transmitted during a blackout could injure or kill workers trying to restore power and cause further damage to the grid.

If you’re concerned about blackouts, an off-grid or hybrid PV system with solar + storage offers far more home energy security.  

How Does a Solar Battery Work?

Solar batteries come in numerous types, and the process behind how each one works varies significantly. 

However, the basic principles behind the role solar batteries play in photovoltaic systems are the same.

All currently available solar energy systems that produce electricity do so using the photovoltaic effect.

Photovoltaic (PV) cells are encased in a solar panel or other protective enclosure, typically comprised of a stainless steel or aluminium frame and a transparent surface like tempered glass.

PV modules harvest photons from sunlight and transform the energy into direct current (DC) electricity.  

In off-grid and hybrid PV systems, DC electricity is transmitted from the PV modules via cables to a solar charge controller.

The charge controller routes the DC electricity to a solar battery for storage and use.

Solar batteries are typically comprised of multiple battery cells regulated by a battery management system. 

All batteries store power as DC, which must be converted to alternating current (AC) electricity by a storage or solar inverter for household use. 

All the necessary components — other than the PV modules — required to produce usable electricity from solar power are collectively called a balance of system.  

As long as each part is compatible, the individual components of a PV system — including solar panels and batteries — can be purchased separately and from different manufacturers.

Many people prefer to purchase all-in-one solar power solutions, such as EcoFlow’s portable power stations or whole-home generators, for fewer headaches and better performance. 

What To Consider When Choosing a Solar Battery

Before we dive into the different types of solar batteries, it’s essential to understand the factors to consider when evaluating performance.

Here’s a quick guide to the terms and concepts to help you make the best purchase decision.

Battery Type

Battery type is the number one factor that determines performance.

Batteries are classified by chemistry and construction. 

The materials and processes used to store and deliver electricity are of paramount importance.

The type of battery determines and impacts all other considerations below — including the price. 

Storage Capacity

Capacity measures the total electricity storage potential of the battery.

Low-load electronics like phones and laptops typically measure capacity in amp-hours (Ah) and milliamp-hours (mAh).

Solar batteries for high-load applications measure capacity in watt-hours and kilowatt-hours (kWh) — just like on your electricity bill.

Cycles and Cycle Life 

One full discharge and recharge of a battery is called a cycle. 

Cycle life is a specification manufacturers use to estimate lifespan based on how many times   

a battery can be charged and discharged before losing storage capacity.

Depth of Discharge (DoD)

DoD is expressed as a percentage that measures how much electricity remains in a battery relative to its total storage capacity.

For example, a battery has 50% DoD at half of its total capacity.

A battery that’s down to 20% of total storage has a DoD of 80%

You can’t use the total storage capacity of rechargeable batteries without negative consequences.

The recommended DoD indicates how much of the battery’s total storage capacity you can use without causing damage or shortening its cycle life.

When it comes to DoD, read the fine print.

“Deep-cycle” batteries may claim to have an 80% depth of discharge but can only be operated at that level for short periods without causing permanent damage.

State of Charge (SoC)

State of Charge is the direct inverse of DoD.

The formula is SoC = 1 – DoD

It measures how much electricity remains in the battery relative to total storage capacity.

At 50%, DoD and SoC are equal.

Once you’ve used 20% of the total capacity, the SoC is 80%.

Energy Density

What Are the Different Types of Solar Batteries?

Rechargeable batteries and solar cells may be older than you think.

The first lead-acid rechargeable battery was invented by Gaston Planté in 1859.

The discovery of solar power dates back even further.

Edmond Becquerel first demonstrated the photovoltaic effect using an electrochemical solar cell in 1839.

Needless to say, solar cells and rechargeable battery technology have come a long way since then.

Nevertheless, lead-acid batteries are still common in photovoltaic applications today.

Here are today’s most widely used solar battery types, in ascending order from low to highest performance.

Flooded Lead-Acid Batteries 

Flooded Lead Acid (FLA) is the oldest rechargeable battery technology and is still widely used today.

All lead-acid batteries — including Sealed Lead Acid (SLA) batteries like AGM and Gel Cell — charge and discharge DC electricity through electrolysis.

They also share many of the same raw materials, though the battery cell construction varies significantly between types. 

Unless you’ve switched to charging a Tesla or other EV, you probably have an FLA battery in your car or truck.

FLA batteries are inexpensive and proficient at delivering short bursts of high-current electricity.

(Source: ResearchGate)

The essential components of an FLA battery are:   

• Cathode (positive electrode): Lead-dioxide plate
• Anode (negative electrode): Metallic lead plate
• Electrolyte: Sulfuric acid diluted with distilled water (H2SO4 + H2O ) 

FLA batteries’ ability to provide instantaneous surge power makes them well-suited as automotive batteries for starting engines in cars, trucks, and other vehicles.

However, FLA batteries are inefficient at delivering a continuous supply of electricity and have an extremely shallow depth of discharge (DoD).

FLA batteries can be used in a small solar panel setup that doesn’t require much power.

However, they’re rarely the best choice.

FLA solar batteries may be cheap, but they’ll cost you more in the long run.  

Pros

• Lowest upfront price
• Well-suited for starting motors and engines
• May be suitable for low-current solar applications

Cons

• Short cycle life (300 – 1000 charges/discharges at no lower than 50% State of Charge)
• Shallow discharge
• High susceptibility to extreme temperature — particularly freezing
• Requires significant routine maintenance and refilling with distilled or deionised water
• Must be installed or stored upright to avoid hazardous leaks and spills
• Offgassing of toxic fumes requires operating in a well-vented space 

Sealed Lead Acid Batteries (SLA/VRLA)

Sealed lead acid (SLA) and valve-regulated lead acid (VRLA) are interchangeable terms for a newer, less volatile variation on traditional FLA battery technology.

By “starving” or immobilising the liquid electrolyte solution used in FLA batteries, VRLA batteries minimise the inconveniences of wet cells and provide numerous performance improvements.  

SLA batteries are often marketed using the following terms:

• Dry batteries
• Deep-cycle batteries
• Acid-starved batteries
• Maintenance-free batteries

There are two types of VRLA solar batteries: Absorbent Glass Mat (AGM) and Gel Cell.

Because the performance and marketing of both SLA (dry) cell types are similar, many consumers don’t make a distinction.

AGM and Gel Cells differ significantly in construction and materials but offer many of the same advantages (and disadvantages) over “wet” FLA solar batteries.

Pros

• Faster charging
• Increased cycle life
• Lower self-discharge rate
• Depth of discharge up to 80% — But be sure to read the fine print. Many SLA manufacturers recommend maintaining a 50% State of Charge (SoC), not frequent discharges to 80% DoD
• No need to install upright
• Operate in an unvented space
• Less hazardous if the case is damaged
• No refilling or watering
• Less sensitive to extreme heat or cold
• Resistant to vibration
• No spills or leaks

Cons

• Higher price
• Less surge power (starting watts)
• Less electrolyte by volume
• Care must be taken not to overcharge or charge with current/voltage outside recommended parameters 

AGM and Gel Cell solar batteries beat FLA hands down for performance in high-wattage solar applications like whole home generators. 

But how do they compare to each other?

Let’s take a look.  

Absorbent Glass Mat (AGM) Batteries

AGM battery cells are constructed using fibreglass mats to soak up (starve) the liquid electrolyte solution. 

Because AGM batteries are sealed, they don’t require watering and need little maintenance other than keeping the battery contacts clean.

(Source: ResearchGate)

The main components of an AGM battery are:   

• Cathode (positive electrode): Lead-dioxide plate
• Anode (negative electrode): Metallic lead plate
• Electrolyte: Sulfuric acid diluted with distilled water (H2SO4 + H2O ) 
• Absorbent glass mat (AGM)

Besides adding absorbent glass mats between the positive and negative lead plates, the raw materials used in AGM battery cells are the same as FLA.

The mats absorb the liquid electrolyte of sulfuric acid and immobilise it.

The AGMs remain wet, but there is no free-flowing acid within the cell.

For this reason, AGM batteries are sometimes referred to as “acid-starved.”

Trapping the electrolyte in fibreglass mats doesn’t prevent the electrolysis process for charging and discharging the battery from occurring. 

On the contrary, by most critical metrics, it enhances performance.  

One historic advantage of FLA batteries over SLA batteries is their ability to produce more specific (or instantaneous) power.  

That’s one reason why FLA has traditionally been used for automotive batteries.

AGM batteries have long been used in motorcycles and vehicles like golf carts.

Thanks to improving technology, AGM batteries are increasingly replacing FLA batteries in cars and trucks. They’re less susceptible to extreme cold and last longer.

But what about the advantages and disadvantages of AGM over Gel Cell batteries?

Pros

• Rapid charging
• More forgiving charging parameters
• Less susceptible to overcharging
• High energy density
• More surge power output
• Better for starting mechanical motors
• Less sensitive to extreme heat
• Wider range of applications

Cons

• Shallower depth of discharge
• More susceptible to undercharging
• Less efficient at lower charging rates
• Slightly shorter average lifespan due to dendrites and stratification

Gel Cell Batteries

You may notice at this point that FLA and AGM batteries use primary materials in common.

Gel Cell also uses lead plates as electrodes and diluted sulfuric acid as the electrolyte.

The main difference between AGM and Gel cell is the method and materials used to reduce the volatility and improve the performance of the liquid electrolyte used in all lead-acid batteries. 

(Source: ResearchGate)

The primary components of a Gel Cell battery are:  

• Cathode (positive electrode): Lead-dioxide plate
• Anode (negative electrode): Metallic lead plate
• Electrolyte: Sulfuric acid diluted with distilled water (H2SO4 + H2O ) 
• Silica dust is added to immobilise the liquid electrolyte and give it a gel-like consistency 

The differences between Gel Cell and AGM battery cells are relatively minimal when it comes to performance.

Here are the most critical advantages and disadvantages. 

Pros

• Deeper cycles 
• Better suited for slow and steady charging/discharging 
• Less sensitive to vibrations

Cons

• May be marginally more expensive on average
• More susceptible to overcharging
• Greater sensitivity to variations in current and voltage
• Slightly more temperature sensitive — particularly in extreme heat

The verdict?

Gel Cell batteries may perform slightly better than AGM batteries in most photovoltaic applications.

Less susceptibility to damage from low current and undercharging may provide minimally more protection against solar power’s inherent intermittency.

On the other hand, gel cell solar batteries tend to be slightly higher in price.

Neither type of SLA battery comes close to lithium-ion solar batteries. 

Both AGM and Gel Cell SLA batteries are left in the dust behind lithium-ion and LiFePO4 solar batteries when it comes to performance. 

If you have your heart set on a VRLA solar battery, the most significant determining factor will likely be price.

Nickel Cadmium (Ni-Cd) Batteries

Until the late twentieth century, nickel-cadmium (Ni-Cd/Ni-Cad) rechargeable batteries were the only commercially available alternative to lead-acid.

Ni-Cd solar batteries offer numerous advantages over lead-acid batteries, such as a longer cycle life and greater resistance to cold and heat.

Like FLA, Ni-Cd are wet cell batteries and require occasional filling with distilled or de-ionized water to top up the electrolyte solution.

The essential components of a nickel-cadmium battery cell are:

• Cathode (positive electrode): Nickel Hydroxide (Ni-OH)  
• Anode (negative electrode): Cadmium (Cd)  
• Electrolyte: Potassium Hydroxide (KOH)

Once a popular alternative to sealed lead acid (SLA) batteries, new Ni-Cd solar batteries are increasingly hard to come by.

Cadmium is a toxic heavy metal that’s difficult to recycle or dispose of.

It’s extremely harmful to the environment and human life.

As a result, Ni-Cd batteries are largely outlawed in Europe.

Aside from environmental considerations, lithium-ion solar batteries far outstrip Ni-Cd in performance.

Here is a summary of the benefits and disadvantages of NiCad vs. flooded lead-acid solar batteries.

Pros

• Higher energy density
• Greater depth of discharge
• Faster charge/discharge rate
• Less sensitive to extreme temperatures
• Longer cycle life

Cons

• No longer widely available
• Expensive
• Additional maintenance requirements due to the “memory effect” or crystalline formation
• High self-discharge rate
• Cadmium is carcinogenic and highly toxic
• Improper disposal or recycling threatens water supplies as well as human and aquatic life and 
• Largely banned in the EU for environmental reasons

Traditional Lithium-Ion Batteries (Li-ion/LCO)

All variations of lithium-ion battery cells contain the following components:

• Anode (graphite)
• Cathode* 
• Electrolyte (Lithium salt solution such as lithium hexafluorophosphate (LiPF6)
• Separators (polyethylene (PE)
• Positive and negative current collectors (aluminium and copper foil)

*Cathode materials vary by application.

It’s the material used in the cathode — such as (nickel-magnesium-cobalt (NMC) or lithium iron phosphate (LiFePO4/LFP) — that determines the battery type.

Many consumer electronics, including smartphones, laptops, and digital cameras, use lithium cobalt oxide (LCO/LiCoO2) cathodes. 

Because of LCO’s high energy density and specific energy output, it excels in low-load applications where minimal size and weight are a priority.

Like in an iPad or iPhone.  

Because LCO batteries were first to market, they’re often referred to simply as lithium-ion batteries. 

However, both NCM and LFP are also subsets of Li-ion batteries.  

LCOs were also the first Li-ion solar batteries and are still in widespread use — especially in older systems.

However, NCM and LFP cathodes have unique properties that make them the material of choice in high-load solar and whole-home backup batteries.  

When it comes to efficiency and performance, even the best Sealed Lead Acid (SLA) batteries pale in comparison to LCO solar batteries — let alone NCM and LFP.

Here are the advantages and disadvantages of traditional Li-ion (LCO) solar batteries over VRLA batteries.

After that, we’ll drill down on the best-performing types of Li-ion batteries. 

Pros

• Cycle Life: 500-1000 full charges/discharges — higher than FLA and most SLA solar batteries 
• 4x faster charging
• Less energy lost when charging (higher efficiency)
• Higher energy density 
• Lower recurring costs: Li-ion batteries will need to be replaced less frequently
• Greater depth of discharge: Can operate continuously at 80 – ~100% DoD instead of the 50% – 80% State of Charge typically recommended by SLA manufacturers)
• Higher capacity
• Lower self-discharge rate
• Wide range of operating temperatures
• Ease of installation
• No routine maintenance

Cons

• Price
• Thermal runaway 

Nickel Manganese Cobalt (NMC) Batteries

Along with lithium iron phosphate (LFP), nickel manganese cobalt (NMC) is a newer subset that uses a different cathode material than traditional lithium-ion solar and home backup batteries.

NMC batteries deliver a performance boost over Li-ion for photovoltaic applications but not as significant an improvement as LFP (LiFePO4). 

NMC batteries are most commonly used in EVs, but LFP is seeing increasing adoption because of its excellent safety profile and humanitarian reasons.

Cobalt is an essential material in NMC batteries and most other Li-ion types, including LCO. 

Over 70% of the world’s cobalt is mined in the Democratic Republic of Congo (DRC) under terrible conditions, often by children and the elderly.

As you can see in the video above, labour practices at cobalt mines in DRC are so horrendous that the metal is often referred to as blood cobalt.

The cathode in LiFePO4 solar batteries is composed of lithium iron phosphate — no cobalt is required.

Here are the benefits and disadvantages of NCM solar batteries over traditional Li-ion.

Then, we’ll move on to LFP.

   Pros

• Higher-rated power output
• Higher energy density
• Improved cycle performance
• Longer cycle life (1000-2000)
• Less susceptible to thermal runaway
• Less prone to mechanical abuse (Nail test)
• Requires less cobalt (typical cathode composition: 80% nickel, 10% manganese, 10% cobalt)
• Less  cycle life impact from the State of Charge (SoC) 
• Deeper recommended DoD (80%-90% operating DoD)

  Cons

• More expensive
• Slightly lower energy density
• Lower specific energy
• Shallower actual DoD.  (LCO batteries continue working until 95-100% discharged, but full discharges adversely affect cycle life 

Lithium Iron Phosphate (LFP/LiFePO4) Batteries

Lithium iron phosphate (LFP/LiFePO4) batteries deliver the advantages of LCO and NCM batteries over lead-acid batteries, plus unique characteristics that make them ideal for photovoltaic applications.

The primary components are similar to other Li-ion batteries, but the cathode is made from lithium iron phosphate (LiFePO4/LFP).

• Anode (graphite)
• Cathode (lithium iron phosphate) 
• Electrolyte (Lithium salt solution such as lithium hexafluorophosphate (LiPF6)
• Separators (polyethylene (PE)
• Positive and negative current collectors (aluminium and copper foil)

Unlike LCO batteries, there is almost no danger of thermal runaway in LFP batteries.

Mitigating the risk — however remote — of fire or explosion caused by thermal runaway is of enormous benefit in high-capacity photovoltaic applications.

Thanks to its excellent safety profile, LFP is being rapidly adopted by EV manufacturers like Tesla, often replacing NMC.

In addition to safety, there are other reasons why EV and solar battery manufacturers are increasingly switching from NMC to LiFePO4 batteries.

• Decobaltization: As discussed above, cobalt is a conflict mineral. Brands are under increasing pressure to stop using it in their products.
• Falling Prices: NMC is a more established technology. Initially, LFP batteries were more expensive because manufacturers had to invest in new equipment and processes. However, the price of LFP and NMC solar batteries is now at about par. LiFePO4 batteries are expected to keep dropping in price. Nickel and cobalt NMC batteries are considerably rarer and more expensive minerals than iron and phosphate. As demand for LFP increases, prices should continue to fall. Meanwhile, NMC prices are expected to remain stable or go up.

 Pros

• Safety: LiFePO4 is the safest lithium-ion battery chemistry for high-load applications like residential solar power and EVs. Strong covalent bonds in LFP cathodes virtually eliminate the risk of thermal runaway. 
• Wider operating temperature range: LFP batteries operate between -4°F (-20°C) to 140°F (60°C). Li-ion batteries operate best at 32°F (0°C) to 113°F (45°C)
• Cycle Life: LFP batteries like EcoFlow’s DELTA Pro Ultra   

 Cons

• Slightly lower energy density
• The average voltage is lower than that of LCO and NCM (3.2V vs. 3.8V). If voltage shortfalls occur, solar panels can be connected in series instead of parallel. 

Learn more about the pros and cons of LiFePO4 solar batteries vs. traditional lithium-ion (LCO)

Emerging Solar Battery Technologies

Manufacturers, governments, and academics worldwide are always in hot pursuit of the next big breakthrough for renewable energy storage.

Here is a list of emerging solar battery technologies that have shown some promise but are likely years away from commercial availability for residential use.

• Flow Batteries
• Iron Air Batteries
• Solid State Batteries
• Sodium-ion Batteries

The Importance of High-Quality Solar Batteries

Residential solar power is a long-term investment.

High-efficiency rigid solar panels typically last over 25 years before significantly declining in performance.

SLA “deep-cycle” solar batteries have a much shorter cycle life than LiFePO4 batteries and often need to be replaced after only a few years of regular use.

On the other hand, EcoFlow’s whole-home generator solutions can provide over 10 years of daily use.

The key to saving and making money from your PV investment is to purchase high-quality solar batteries and other components.

It might lengthen your solar payback period,  but you’ll get better long-term ROI.

Government solar incentives can also significantly decrease your upfront investment.

Frequently Asked Questions

Final Thoughts

If you’re considering switching to solar to power your home, a solar + storage solution offers many advantages.

A Grid-tied system can help you save money on electricity bills, but it won’t eliminate them completely.

And you won’t have power during a blackout.

Combining PV modules with the high-quality LFP solar batteries in EcoFlow’s whole-home generator solutions allows you to enjoy energy security and maximise the return on your solar investment.

EcoFlow offers a wide range of portable and hybrid off-grid power solutions.