What are the different types of solar batteries?

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Many Australians are turning to solar batteries to store excess energy generated by their solar systems. However, making sure your home is outfitted with the right solar battery system can be intimidating if you’re not familiar with the different types available.

In this guide, we will explore all the different types of solar batteries currently on the market and their pros and cons.

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Types of solar batteries

In Australia, there are four options to choose from in an array of different sizes:

• Lead-acid batteries
• Lithium-ion batteries
• Lithium-ion phosphate (LiFePO4) batteries
• Flow batteries

To assess their pros and cons, this guide focuses on the following factors:

• Price: How much does it cost to purchase a unit.
• Lifespan: How long does a battery last before experiencing a significant decline in the volume of stored energy.
• Recommended depth of discharge (DoD): This refers to the capacity of the solar battery’s energy that can be discharged without damage to itself — the higher it is, the more usable capacity and the shorter the frequency of charges between uses.
• Charge Rate: This refers to how long it takes for each charge or discharge cycle.
• Temperature sensitivity: How sensitive each storage type is to temperature changes.
• Energy density: This refers to how much energy (in kWh) can be stored for every unit volume of battery.
• Size, weight and portability.

Lead-acid batteries

Lead-acid batteries, having first been invented in 1859, are one of the oldest and formerly most common solar storage solutions.

To store a charge, lead-acid batteries involve the flow of electrons and ions between the positive and negative terminals within an electrolyte solution.

Pros: 

• Lead-acid batteries are affordably priced.
• Tried-and-tested solution with a proven track record.
• More resilient towards higher temperatures, maintaining a stable performance.

Cons: 

• Short lifespan of three to five years or 1,500 to 3,000 charge cycles.
• Recommended DoD is low (typically 50%), resulting in a lower usable capacity, shorter discharge cycles and frequent recharges.
• Slower recharge/discharge cycles can affect how much you save on your power bill.
• Frequent replacements can be more costly in the long run.
• Low energy density, resulting in heavy and bulky batteries.

Lithium-ion batteries

Lithium-ion batteries are the most popular solar battery technology in Australia.

Lithium-ion batteries hold onto a charge through the movement of lithium ions between two electrodes, the cathode and the anode.

One such popular example is the Tesla Powerwall 2.

Pros: 

• Long lifespan of 5-10 years or 2,000 to 4,000 charge cycles.
• Recommended DoD is high (typically 80%), resulting in a larger usable capacity and deeper discharge before needing a refill.
• Fast discharge/recharge cycles.
• Higher energy density, resulting in a more space-friendly size and weight.
• Widely available in Australia.

Cons:

• Upfront cost is expensive, reaching as high as $15,000.
• More sensitive to extremely high or low temperatures, which can cause efficiency loss and overheating.

Lithium-ion phosphate (LiFePO4) batteries

LifeP04 is a rising new technology that is starting to gain traction in Australia. 

LiFePO4 technology is fairly similar to lithium-ion. However, LiFeP04 batteries contain Lithium-iron phosphate cathodes. 

Pros: 

• Very long lifespan of 10 years or more. 
• Recommended DoD is near 100% or a fully usable capacity. 
• Highly resilient to extreme hot or cold temperature stress, maintaining a stable performance. 
• Ultra-stable chemical composition that’s extremely resistant to overheating and battery fires. 
• Extremely slow battery degradation that requires thousands of charge/discharge cycles before seeing a drop in usable capacity. 

Cons:

• Upfront cost is similarly expensive to lithium-ion batteries. 
• Slower charge/discharge cycles compared to lithium-ion batteries. 
• Modestly lower energy density than lithium-ion batteries (this difference is negligible with off-grid sizes). 
• Modestly heavier weight and size than lithium-ion batteries. 
• Not as widely available as lithium-ion batteries.

Flow batteries

Flow batteries, powered by Vanadium technology, are a newer form of storage that uses two different electrolytes separated by a membrane to store a charge.

They are a state-of-the-art storage solution that has yet to be widely accepted across the country.

Note: The only residential flow battery that was available in Australia was the ZCell, designed by Redflow in Brisbane and manufactured overseas. However, Redflow is no longer in business.

Pros:

• Incredible lifespan of up to 30 years.
• 100% DoD with little loss in battery capacity over its lifespan.
• Extreme resistance to hot or cold.
• Modular design allows for efficient and cheaper repairs or replacements.
• Virtually no risk of explosion due to its modular construction.
•  Zinc bromine solution stored within is a natural flame retardant.

Cons: 

• Upfront cost is very expensive.
• Slow and low discharge and recharge rates.
• Flow batteries are much less power-dense compared to their lithium-ion counterparts.
• Flow battery storage tanks occupy a significant amount of space.

What type of battery is best for solar?

The lithium-ion battery has proven itself to be the most popular choice in Australia. However, the best type of solar battery depends on what you’re hoping to personally achieve.

If your primary goal is to reduce your energy bill and avoid bill shock,  a battery with a high recommended DoD and charge rate, such as a lithium-ion battery, will allow you to store more energy quickly — this is especially valuable on a time-of-use tariff with peak and off-peak period pricing or a controlled load tariff.

Alternatively, if you intend to go fully off-grid, a battery with a long lifespan and minimal degradation, such as a LiFePO4 battery, is crucial for keeping your home powered 24/7 without having to frequently replace it.

Depending on the brand, make and model, batteries with full or partial backup capabilities are also available for Aussies who have to constantly contend with power outages.

At the end of the day, the best battery for solar ultimately depends on the specific needs of your system.

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Things to consider before buying a solar battery

Before you hit the ‘buy’ button or sign up your credit card, there are a few important questions you should ask yourself.

How much are you willing to spend on a solar battery?

From the upfront cost to how long it’ll take for you to break even, your budget will limit the battery options available to you.

It’s worth noting that there are green financing options offering instalment plans (albeit with modest interest rates) and federal and state government rebates to help dampen your upfront spend.

How much energy is your solar system producing?

The size of your solar system determines the capacity of your battery. If your solar system is below 5kW, it is unlikely that you’re generating enough energy to warrant spending thousands on a battery.

For example, if all the excess energy produced by your system is used to power your home, you won’t have any energy to store for nighttime use.

How much electricity do you consume?

Firstly, consider how much energy you use and when you use it. This will help determine the size and type of battery you need.

There are a handful of ways for you to measure your electricity usage:

• If you have a smart meter, you may be able to download your energy retailer’s dedicated app to monitor your energy usage in real-time.
• Install an electricity usage monitor or solar power monitoring system for real-time monitoring.
• Read your energy bill’s ‘energy usage summary’ section — however, this section is a rough average and isn’t as accurate as a real-time assessment.

Do you have enough space to house a solar battery?

Different types of solar batteries have different energy densities, with some being larger and heavier than others — the amount of free space you have will determine the size and type of solar battery you choose.

Consider if you have enough space, either indoors or outdoors, to safely install a solar battery. More notably, there are strict installation guidelines that must be followed (AS/NZS 5139:2019).

For example, your solar battery cannot be situated within 600mm and 900mm under windows or ventilation openings into an occupied room.

Because only licensed solar installers are permitted to install them on your premises, consult them with your concerns about limited space before proceeding.

Is your current solar system compatible with a solar battery? 

If you’re installing a battery onto an existing solar power system, it’s worth finding out if your current setup is equipped to handle the additional load of the battery.

For instance, solar batteries can only store direct current (DC) power. Surplus AC power must be converted to DC power first by a compatible inverter in order to be stored.

As a result, your solar system must have a compatible inverter (i.e. hybrid inverter) to facilitate AC-to-DC conversions. If you don’t have one, these extra peripherals will add to your upfront spend.

Are battery storage solutions worth the extra investment?

The short answer is yes, but whether a solar battery is worth it or not depends on a range of factors that ultimately boil down to your circumstances.

With the increasing demand for renewable energy, it’s essential to understand the different types of batteries available to homeowners in Australia.

While there are many options on the market, these four – lead-acid, lithium-ion, flow, and lithium-ion phosphate batteries – are the most commonly used solar battery types.

Aside from your house and car, buying a solar battery (or several) can prove to be one of the biggest investments you make, which is why it’s best to speak to an expert before signing on the dotted line.

Once you choose the right one, you can enjoy uninterrupted power, save on energy bills and contribute to a cleaner environment.

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