What Is A Solar Battery?

A solar battery is a rechargeable battery designed to store energy generated by solar panels during the day and release it for use later. It is an energy storage system for homes, businesses, and other applications that use solar energy.

Selecting the correct solar battery system for your panels is the key to ensuring the battery will last as long as you need it to.

In my work as a solar installer, I’ve encountered my fair share of damaged batteries installed in incompatible systems resulting in money being wasted unintentionally. 

Batteries suited to use in solar panel systems are widely available; however, it’s easy to choose the wrong type of solar battery.

Read on as I answer commonly asked questions about battery technology and how solar batteries work.  

🔋 Types Of Solar Batteries

It’s a general term for batteries suitable for solar-powered systems. Batteries are used in conjunction with panels and a solar inverter or charge controller to store excess energy generated by your solar panels to power your home.

Solar batteries are primarily used in domestic solar power systems and are Flooded or VRLA lead-acid, Gel, AGM, or Lithium-Ion batteries.

What makes a battery suitable for generating power is that it must be a deep-cycle battery. Deep cycle batteries can charge and deliver power for long periods without sustaining damage. 

Various types and sized batteries are available to cater to virtually all domestic solar system requirements. The following four solar power battery types are the most popular and are used extensively in domestic solar panel system applications.

Deep Cycle Lead Acid Batteries

Most of us know lead acid batteries as the batteries used in cars. Lead acid batteries are the most used for solar energy storage worldwide, primarily due to their widespread availability and affordability compared to other battery options. 

Lead acid batteries are available as flooded or sealed VRLA (Valve-regulated lead acid) options.

Flooded Lead Acid Batteries

Standard flooded lead acid batteries are fitted with removable caps that enable the user to monitor the battery cell’s water levels and perform maintenance as needed. The downside of these batteries is that they need the care to ensure they last a long time.

The chemical reaction while charging the battery causes the distilled water in each cell to evaporate over time which can lead to the cell drying up and being damaged, rendering the battery unusable.

Flooded batteries can withstand extensive overcharging without any ill effects. Discharging the battery totally damages flooded batteries and can permanently damage the battery. Recommended depth of discharge is fifty percent to extend the battery life.

Valve Regulated Lead Acid Batteries (VRLA)

VLRA batteries are sealed and don’t require any maintenance, which makes them a popular option. The design of the battery prevents the electrolyte from escaping during the charging process, which makes it unnecessary to replace any lost fluids.

Both Flooded and VLRA lead acid batteries used an electrolyte of sulfuric acid and distilled water solution. The expected battery life is between eight hundred to one thousand five hundred cycles, giving them an average lifespan of two to four years or longer using one cycle per day.

The recommended DOD for VRLA batteries is fifty percent the same as flooded batteries. 

Deep Cycle Gel Batteries

Gel batteries were developed over 30 years ago to prevent electrolytes from spilling out of batteries in unstable environments such as marine and RV applications.

Since then, they’ve been helpful in many applications due to their inherent safety and increased efficiency, including solar applications.

Gel batteries utilize the same chemical reaction as a flooded or VRLA battery to absorb and release energy. The primary difference s that the electrolyte is a gel called fumed silica, not liquid. Gel batteries are sealed, so they don’t require any maintenance.

Overcharging of gel batteries should be avoided as two known problems can occur. Gel batteries can be very resilient to over-discharging without damaging the battery.

The gel electrolyte is very dense and can develop pockets or cracks within the gel that forms a void, preventing adequate acid flow and a loss of battery capacity. 

Secondly, the gel can liquify under certain conditions, a “thixotropic” condition. Acid can leak out if the battery is moved during its liquid state. The gel, fortunately, solidifies over a reasonably short period.

Gel batteries recommended DOD should not exceed eighty percent, and charging requires a gel-specific charger built into most modern inverters. Gels tend to cost more than other lead acid-type batteries and some AGMs. 

AGM Batteries (VRSLAB)

Absorbed Glass Mat, valve-regulated sealed lead acid batteries are the most advanced lead-acid design battery available. The acid is infused into fine micro-fiber glass mats between the lead electrodes within the battery.

The AGM battery offers higher efficiency than the gel or flooded batteries and is capable of higher charge and discharge amperage. The AGM batteries are sealed and therefore don’t require any maintenance. 

AGM batteries charge faster than lead acid counterparts with lower internal resistance. 

AGM batteries can be mounted at any angle, even on their sides, as they don’t contain any liquids. 

📖 Related reading:
AGM vs Gel batteries
AGM vs Lithium batteries

Lithium-Ion Batteries

Lithium-Ion batteries have been a game changer in the Solar fraternity. Lithium-Ion Phosphate (LiFeP04 or LFP) is the most used type and offers high energy density, no maintenance, fast charging, and a ten-year plus lifespan.

Lithium batteries offer significant advantages over traditional solar batteries for residential solar storage. Lithium batteries are deep cycle batteries that provide 80 to 100% depth of discharge without damaging the battery.

Residential use lithium batteries are generally 48-volt, 100 Amp hour batteries that can connect in series depending on the size of battery bank you require.

Lithium batteries are more compact and thus take up less space than lead-acid batteries to achieve the same battery storage capacity.

Lithium batteries consist of a series of cells that, in turn, contain multiple small LiFeP04 batteries. A 48-volt battery generally has 16 cells of batteries, each delivering 3.2 volts. 

Solar lithium batteries come with a built-in battery management system contained within the battery casing.

The BMS manages each battery cell’s charge and discharge rate and protects against over or under-charging. The BMS also communicates with the inverter and supplies system performance data to the end users’ mobile or PC solar applications.

One downside to using lithium batteries is the potential risk of thermal runaway. Thermal runaway occurs when the battery starts heating up uncontrollably, which can lead to the battery catching fire. Proper installation of the battery reduces this risk to virtually zero.  

Lithium batteries have long lifespans, with most warranted for a minimum of ten years. Like lead-acid batteries, lithium batteries’ usable life is measured in cycles (charge and discharge equal a cycle).

Lithium batteries offer anywhere from 3,000 to 10,000 cycles, many times that of lead-acid batteries.      

Lithium batteries are more costly than standard lead-acid type batteries; however, their long functional life, no maintenance, and greater DOD justify the additional initial expense.  

Also, remember that for a 48 Volt system, at least four 200 Ah lead-acid batteries are needed. The cost of which would come very close to what a 48-volt lithium battery would cost.

Lithium batteries contain no liquids, so they can be installed vertically or horizontally. Most are also wall-mount compatible, which reduces the space required to install lithium batteries.

Solar Battery Type: Pros And Cons

Below is a summary of battery types’ most relevant pros and cons.

Flooded Lead-AcidAffordable
Withstands overcharging well
Requires regular maintenance
Contains hazardous spillable electrolyte
Must be well vented
Recommended DOD 50%
Can be damaged when fully discharged
Fewer battery cycles
Maintenance Free
Recommended DOD 50%
Can be damaged when fully discharged
Fewer battery cycles
Gel Maintenance Free
More resilient to total discharge damage
Good for extreme temperatures
More costly
Recommended DOD 50%
It can develop pockets in the gel when overcharged
Gel can liquify under certain conditions.
Fewer battery cycles
Gel requires a specific battery charger
AGMMaintenance Free
Can be mounted at any angle
More costly
Fewer battery cycles
Lithium-IonHigh-density cells
High charge and discharge rate
Fast charging
Long-lasting (10 years plus)
Many cycles 3000 plus
Contain a BMS with monitoring software.
Rack or wall mountable
Maintenance Free
Can be linked to other batteries
Compact design
Most costly option
Risk of thermal runaway

Deep Cycle Versus Shallow Cycle Battery

A deep cycle is a term often used in the solar industry. Deep cycle batteries are well suited to continuous use. Either by charging for a long time or slowly discharging as the stored energy is used to power a home. 

Shallow cycle batteries are used in applications where a high burst of energy is required for a relatively short period.

Typical shallow-cycle batteries are used in cars to start the engine. Once the engine runs, the battery is no longer needed and settles back into charging mode.

Interestingly, to achieve the higher burst of energy needed by the shallow cycle batteries, they contain many thin lead electrodes giving a larger surface area from which electrons are released, creating electricity. 

Deep cycle batteries contain thicker but fewer lead electrodes or plates. Enabling them to provide power for much longer and have more battery cycles.

Deep-cycle lead-acid batteries are physically larger and heavier than shallow-cycle batteries. 

♻️ The Battery Cycle

The longevity of a battery, regardless of its type, is measured in cycles. A cycle consists of one discharge from full to its maximum DOD and the subsequent recharge till the battery is charged fully.

Measuring the cycles on a lead-acid battery is tricky. Still, the industry average is one cycle per day, so if your battery has an 800-cycle battery, it should last just over two years.

Through their BMS (battery management system), lithium batteries record the number of cycles the battery has done. This information is extremely relevant regarding warranty claims for both the supplier and the customer.   

🚙 Batteries For RVs and Mobile Solar Systems  

RVs and mobile solar systems come in a variety of different types. 

RVs differ in their power needs, so they will have specifically sized solar cell systems and batteries to suit their power requirements. 

RVs are mobile units that vibrate and shake while being driven. Gel, AGM, or Lithium-Ion batteries are the best choice as they don’t contain liquids. 

Portable or mobile power units tend to be smaller in size as they’re often used to supply a light or two and to run Wi-Fi and a laptop.

As space is at a premium, these systems use lithium-ion batteries as they are lighter, more compact, and last longer than lead-acid batteries.

❔ Other FAQs

What Is The Difference Between Solar Battery And A Normal Battery?

Solar batteries used in domestic applications are deep-cycle batteries made to supply current over an extended period. 

Shallow cycle or standard batteries supply high amperage for a short time, making them unsuitable for solar applications.

Solar batteries used in domestic applications, like supplying a home with electricity, are generally larger, with the standard being 200Ah batteries for lead-acid types, while 100Ah lithium batteries are commonly used.   

Is It Worth Adding Batteries To Solar Panels?

Solar cell systems fitted with batteries are extensively used in off-grid solar applications. In hybrid systems, grid power becomes the backup power source.

In hybrid systems, the solar panels charge the batteries and supply the home with power. Once the batteries are fully charged, the excess power generated is fed into the grid.

When the sun goes down, the batteries supply power to the home until a preset DOD is reached and grid power takes over.

How Long Can A Solar Battery Hold A Charge?

The battery type, size, and application for which the battery is used will influence the rate at which the useable battery cycles are used. Battery cycles vary extensively and are determined by the type and brand.

On average, lead-acid batteries are warranted at anything between 1 and 5 years, while the standard for Lithium-ion batteries has become 10 years or longer.

📖 Read next: How long so solar batteries last?

🔑 Key Takeaways

Five types of batteries are commonly used in domestic solar systems:
Flooded lead acid
2️⃣ VRLA
3️⃣ Gel
4️⃣ AGM
5️⃣ Lithium-ion

A prerequisite of a Solar battery is that it must be a deep cycle. Deep cycle batteries work in solar systems as they can be charged ad discharged for long periods.  

Now that you know what a solar battery is, check out our hub on solar batteries for other relevant articles.

And if you are ready for the next step, contact a solar installer who will advise you on the best battery type and the number of batteries you’ll need for your specific solar panel system to generate and store renewable energy.   

I’m the website operator and editor here at ALTA Devices. The solar revolution is the most exciting thing to happen in a generation! I’ve written extensively on solar, electric vehicles, and the electrification of the marine industry. You can find out more on LinkedIn below: