Types of Solar Batteries and How To Choose The Right One

A solar battery is a storage unit that holds excess, unutilised electrical power generated by solar panels. More than one unit – a battery bank – is often required to function effectively as solar storage.

Solar batteries are usually used for off-grid solar installations and hybrid solar systems. However, they are also used in some grid-tied solar power systems.

The market is now flooded with a wide variety of solar batteries, which can make it a little challenging to choose the right one for specific needs. When selecting the best solar battery, you need to keep in mind three things: the type of solar battery, what you want to get out of the battery and the type of solar power system.

To do so, you need to understand the technical terms used to describe its characteristics. That is why we have put together this article to help you.

Understanding solar batteries

Photo by Kumpan Electric on Unsplash

Different types of solar batteries have varying capacities, depths of discharge (DoD), round-trip efficiencies, lifespans, warranties and maintenance needs. Here are some of the terms explained:

Capacity/Power

This is the total amount of electricity that a solar battery can store. It is measured in kilowatt-hours (kWh). A typical solar battery has a capacity of around 10kWh. The higher the capacity, the more solar power your solar system can store.

Depth of Discharge (DoD)

DoD indicates the percentage of power from a battery’s capacity that can be used before shortening its lifespan. The higher the DoD, the more power you can extract safely from the battery.

For instance, if a solar battery’s DoD is 80%, you should not use up more than 80% of its maximum stored power. Thus, if you have a 10kWh battery with a DoD of 80%, you can use 8kWh. Using more than 8kWh consistently or totally draining out the battery may damage it.

Round-trip efficiency

Round-trip efficiency is the difference between the amount of electricity sent to the battery and the subsequent available usable amount of energy.

Let’s say your solar panels sent a total of 10kWh to the battery. However, only 7kWh was actually stored. The battery consumed 3kWh of the 10kWh for its operating system. This makes its round-trip efficiency rating 70%.

The higher the round-trip efficiency, the more economical a solar battery is. Thus, you should look for a round-trip efficiency rating of around 80%. 

Battery life and warranty

A solar battery’s performance declines over time, and it has a lifespan like any other battery. Its lifespan ranges from 5 to 15 years, depending on the type and brand. Sometimes the warranty period can give you an idea of how long a battery will last.

Look out for these aspects of the warranty:

  • How many years it typically lasts
  • Number of cycles
  • Throughput

Since battery performance declines over time, a warranty that guarantees a number of cycles or years of good performance is essential. It should also guarantee the throughput, or the specific capacity that the battery can maintain.

Take for example a battery with a 10-year lifespan. After the 10th year, it will operate at a reduced capacity of maybe 70%.

However, if a battery goes through many cycles within a year, its performance will drop much faster. ‘Cycle’ refers to each time a solar battery is fully charged and discharged. Some solar batteries have about 10,000 cycles. The heavier the usage, the faster the battery will reach the end of its life.

Throughput refers to how much power (in megawatt-hours or MWh) a solar battery can release before its capacity drops to a certain percentage. This figure gives you an estimate of how much electricity you can use from the battery.

Types of solar batteries

When selecting the best battery for your solar system, it is best to understand the difference between the following types of batteries before making your selection.

Lead-acid batteries

These are the oldest and cheapest solar battery options. However, their storage capacities are lower than other batteries. This means you’ll need more lead-acid batteries to do the work of, say, a lithium-ion battery.

The DoD of lead-acid batteries is also low, at about 50% only, and their lifespan ranges from 5 to 10 years. Although they are the cheapest option, they are not popular for powering home on a daily basis.

They are best for:

  • DIY solar projects
  • Emergency back-up storage during power outage
  • Off-grid solar systems with infrequent rates of usage, such as at an off-grid vacation cabin

There are two types of lead-acid batteries: 

1. Flooded lead-acid (FLA) battery

Also known as a wet-cycle battery, it contains a liquid and disperses hydrogen gas. Thus, it requires more care and has to be kept upright and well-ventilated. Regular maintenance every 1 to 3 months is needed for it to operate properly.

2. Sealed lead-acid (SLA) battery

This type is maintenance-free and leak-proof compared with the FLA. There are two varieties of SLA: AGM (absorbent glass mat) and gel.

Lithium battery

Also called lithium-ion battery, it was previously used in laptops and cellphones only. Over the years it has become more popular in solar applications. Lithium-ion batteries are among the most expensive solar batteries for many good reasons:

  • High DoD
  • Longer lifespan
  • No maintenance
  • Lifespan of at least 10 years
  • More efficient power usage
  • More usable storage capacity
  • No off-gassing/ventilation needed
  • Smaller and lighter than lead-acid batteries

The biggest disadvantage to lithium-ion solar batteries is that they are prone to ‘thermal runaway’. This means they can overheat and catch fire if they are not installed properly.

They are best for:

  • Residential solar installations with limited space.

Lithium-polymer battery

This type of battery is similar to the lithium-ion battery but it is more resistant to overcharging or electrolyte leakage. It is also more expensive although it has less cycles than a lithium-ion battery. 

Nickel-cadmium battery

Called Ni-Cad or Ni-Cd in short, it is a rugged battery with a long lifespan if properly maintained. It requires a full discharge occasionally to maintain its longevity. Apart from that, it is maintenance-free and can operate at extreme temperatures.

However, cadmium is a toxic element so this battery cannot be discarded in landfills and is banned in some countries. Ni-Cad batteries are also prone to a reduced ability to hold a charge. For these reasons, they are not as widely used as lead-acid or lithium batteries.

They are best for:

  • Large scale applications, like utility solar energy storage

Saltwater battery

This is the most environmentally friendly solar battery because its electrolyte is literally saltwater, and it doesn’t use heavy metals. As it is made from salt water, it is non-toxic and non-flammable. Saltwater batteries are maintenance-free and have a longer lifespan than lithium batteries. However, they are bulkier and more expensive.

They are best for:

  • Home or commercial use where space is limited and there are cost restrictions

Solar gel battery

Similar to the lead-acid battery, this gel battery or gel cell also uses a gel electrolyte. However, you don’t need to keep it upright as it handles movement very well. 

They are best for:

  • Mobile homes, like RVs, campers, vans and boats

Flow battery

A flow battery contains a water-based electrolyte liquid. It is becoming more popular because it is fire-retardant, low-maintenance, has 100% DoD and a 30-year lifespan.

Its larger size, however, makes it more expensive than other battery types, thus less suitable for residential use. In addition, it has a very low charge and discharge rates, and a relatively low storage capacity.

They are best for: 

  • Large-scale installations, not residential

Deep-cycle battery

‘Deep cycle’ refers to a battery’s ability to complete a discharge-recharge cycle several times. Examples of solar batteries under this category are lithium-ion, flooded and gel batteries.

How much does a solar battery cost?

A solar battery can cost anywhere between $200 and $15,000 depending on what type of battery it is. Lithium-ion batteries, the priciest, average about $7,000 to $14,000 each. 

Which solar battery lasts the longest?

The most commonly used types of solar batteries are lead-acid, lithium-ion and saltwater. Of these three, lithium-ion batteries last the longest.

What is the best solar battery on the market?

Generally, for home use, lithium batteries are the best. These have the highest capacities, better performance, take up the least amount of space and don’t require maintenance.

Are solar batteries worth it?

It depends on what angle you are looking at. Yes, they are worth it for:

  • Those who want to save on utilities
  • Those who want to reduce their carbon footprint
  • Those who practice off-grid living and the mobile lifestyle
  • Situations where the power grid is unstable or unreliable

Conclusion

Each type of solar battery has its pros and cons. Currently, lithium-ion batteries are favoured for their qualities but lead-acid batteries are favoured for their price.

Saltwater batteries are the most environmentally friendly and the longest lifespan but it is the most expensive option and it is bulky. The other types fare somewhere in the middle in terms of performance while some are only suitable for large commercial applications, such as the flow and Ni-Cad batteries.

As a general rule of thumb, choose a solar battery with a DoD of at least 40% and a round-trip efficiency of around 80%. Other criteria to look out for are the battery’s lifespan, maintenance, warranty, capacity and size.

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