What Is A Solar Battery?

A solar battery is a battery that is designed to store electricity produced by solar panels.  

The battery is an essential component of your solar system, let’s see why.

Basic Working Prancing Of A Rechargeable Battery

Every battery is made of the four main components below:

  • Two electrodes: Anode (-) and Cathode (+)
  • A porous membrane that separates the electrodes
  • An electrolyte

The nature of those components will vary depending on the battery technology.

Anodes and cathodes are usually made of metal. They are connected by a wire and immersed in the electrolyte.

An electrolyte is a liquid that contains charged particles called ions, the membrane is porous to ions.

Let’s remember our chemistry lessons, with oxidation and reduction reactions:

During discharge, an oxidation reaction occurs at the anode that generates electrons.

Meanwhile, at the other electrode (cathode), a reduction reaction using the electrons is happening. A flow of electrons (electricity) is therefore created between the two electrodes. This is the output of the battery.

The solar battery maintains its electrical neutrality at all times thanks to an exchange of ions in the electrolyte.

 

During charging the opposite reaction occurs. Oxidation at the cathode and at the same time reduction at the anode. The flow of electrons is in the opposite direction compared to discharge mode.

Solar Energy – A Variable Source Of Energy

The photoelectric effect described by A. Einstein in 1905 is the birth date of our modern solar panels.

A solar panel converts visible light (photons) into electricity (electrons). Thanks to scientists, we now have highly efficient and reliable solar panels.

Our sun is a natural source of energy; therefore it is intermittent. You will see its intensity fluctuating during the day. Consequently, your solar electricity production will always change.

You will certainly prefer to always use electricity, so you need to store your production. The most versatile and reliable way to store solar energy is through a battery.

Why You Should Couple Batteries To Your Solar Panels

 

In real-life conditions our energy consumption is predictable. Consumption peaks occur in the morning around 7 am when we wake up and in the evening around 7 pm.

 

Unfortunately, those peak consumptions are far from the peak production of your solar panels. The biggest production is when the sun shines bright at noon.

This has resulted in the wide usage of batteries.

Coupled with solar panels they allow us to use solar energy at any time and in all conditions.

Your battery capacity should be sized according to your needs. It should also match the production of your solar panels.

Let’s see what the different battery technologies available on the market are.

10 Of The Most Useful Battery Terms

If you look at the technical terms of a commercial battery you might find yourself completely lost! Don’t worry, below you’ll find a top 10 list of the most relevant ones.

Energy

Rechargeable batteries store energy in the form of chemical energy.

Energy is defined as a certain power supplied during a certain time.

Energy is measured in kWh. For example, 1 kWh of energy is equal to 1 kW of power supplied all the time during 1hour.

Energy = Power * time

On your electricity bill, the energy company charges the kWh consumed monthly. For example, 0.20 USD per kWh in California.

Power

The battery supplies electrical power. Power is measured in Watt (W). Every load connected to your battery will request a certain power to operate at its nominal capacity.

Power is defined as Current (A) * Voltage (V)

Voltage

The voltage of your battery will fluctuate from its nominal value depending on the state of charge of your battery, the instant load, the temperature. The fluctuation is generally around 20% from the nominal value.

During charge, for example, a 12 V battery will reach 14.4 V, down to 10 V when fully discharged.

You will also have instant voltage fluctuation if you connect a heavy load to your battery. 

Current

Electric current is measured in Ampere. This is the flow of electrons (electricity) that your battery can release (discharge) or recover (storage). This value is closely related to the power of your battery.

Charging and Discharging

Charging is the storage of electricity in the battery.

Discharging is the supply of electricity by the battery.

Capacity

Your battery stores a limited amount of energy defined as its capacity.

The battery capacity is expressed in kWh, but you can also find the capacity in Ah (Ampere hour) or Amps.

The capacity of your battery is influenced by several factors:

  • Temperature
  • State of charge
  • Electrical load connected

 

A high discharge rate will drastically reduce your battery capacity.

Let’s look at the example below:

You might see this type of specification for your battery capacity:

12 V nominal voltage

100 Ah during 20h at 25°C

 

It means that your 12 V battery can supply a total of 100 Ah over 20 hours.

In terms of energy, it is equivalent to 1.2 kWh over 20 hours: 100 Ah*12 V= 1200 Wh

The important figure here is 20 hours. It means that the 1.2 kWh is supplied straight during 20 hours (no peak load, no cut off).

So that would be equivalent to 60 W of load at any time for 20 hours: 60 W*20 hours = 1.2 kWh.

In the end, if you exceed on average this value of 60 W over the course of 20 hours, you will discharge your battery quicker and you will reduce your total capacity.

DC: Batteries work only with Direct Current (DC) for charging and discharging.

AC: All our daily appliances use alternative current (AC).

 

DOD (Depth of discharge)

To improve the battery life duration, manufacturers do not recommend discharging the battery fully. The advised DOD might vary depending on the type of battery, for lead-acid: 50% and for lithium: 80%.

Cycle Life

The cycle life is the number of times you could fully discharge and charge your battery until its rated capacity falls below 70%.

What Are The Different Kinds Of Solar Batteries?

The 2 main battery technologies available as solar batteries are:

  • The lead acid battery
  • The lithium-ion battery

They both share the same working principle. However, they differ in their active material.

One of the most important features you’d want to look at is energy density. This is the energy your battery can store per kilo (kWh/kg). The higher this value the better.

Both technologies are available in different energy storage capacities. The nominal voltage of a lead-acid battery is typically 12 V. On the other hand, for a lithium-ion battery, it can be 12 V, 24 V, or 48 V.

Let’s take a closer look at the different technologies and see what the best for a solar battery would be.

Lead Acid Battery

Our oldest rechargeable battery technology was developed more than 150 years ago by French scientist Gaston Planté. Do you know that the first car to reach 100 km/h was an electric car with lead-acid batteries built in 1899?

 

The energy density of a lead-acid battery is low, on average 35 Wh/Kg. This is mainly due to the heavyweight of lead electrodes.

Below is a quick look at the main components of a lead-acid battery:

Component

Material

Anode (-)

Lead (Pb)

Cathode (+)

Lead (Pb)

Electrolyte

Sulfuric acid (H2SO4)

Membrane/Separator

Depends on the technology (fiberglass mat, rubber…)

Deep Cycle Battery

Lead-acid batteries were primarily used to crank cars. They produce high current pulses and will not undergo deep discharge.

On the contrary, a solar battery requests a steady discharge current flow over time with a deeper depth of discharge (DOD).

To increase the depth of discharge deep cycle batteries use thicker lead electrodes and separators.

Deep cycle lead-acid batteries are divided into 2 groups:

  • Wet or flooded battery (refillable)
  • Valve regulated lead acid battery (VRLA)

Wet/Flooded Lead Acid Battery

Wet batteries have a liquid electrolyte that you need to monitor to ensure that the electrodes are fully immersed.

 

They need to be refilled with distilled water and well ventilated as they could release hydrogen if overcharged.

Properly maintained wet batteries exhibit a slightly longer useful life compared to VRLA batteries. However, it is not recommended to purchase this type of battery.

Wet batteries are not user-friendly.

They require safety precautions during maintenance and operation and have a longer recharge time.

On the market, wet batteries are being replaced by the VRLA type.

Valve Regulated Lead Acid Battery (VRLA)

VRLA batteries are sealed and maintenance-free batteries.

They are available under the name of Gel and AGM batteries that highlights some of their technological features. They are available in two forms – AGM and Gel. Let’s take a look at some of their features:

AGM battery

Absorbent Glass Mat (AGM) is the material that composes the battery separator of an AGM battery. In this type of lead-acid battery, the electrolyte is absorbed on a fiberglass mat. Therefore, they are sealed and maintenance-free batteries.

 

Are AGM Batteries Safe?

This type of battery is safe to operate and maintenance-free as they don’t need to be refilled. They are sealed to prevent any emission of gas.

How Long Do AGM Batteries Last?

The total life cycle of AGM batteries is strongly dependent on the depth of discharge (DOD). The AGM battery life could vary between 200 and 1’000 cycles.  Usually, the constructor recommends a 50% DOD, allowing a total of 500 to 700 cycles.

Are AGM Batteries A Good Choice As Solar Batteries?

AGM batteries are a good choice as solar batteries for low-budget systems. They could undergo deep discharge and are more durable than car batteries.

You might want to take a look at our comparison chart to choose the best technology that will fit your needs and your budget.

Gel Batteries

In this battery, silica is added to the sulfuric acid electrolyte to make it a gel.

Gel batteries are lead-acid batteries with a semi-solid (gel) electrolyte.

Are Gel Batteries Safe?

They are user-friendly and therefore safe to operate.

They have sealed batteries with no liquid electrolyte. Gel batteries are leakproof and will not emit toxic fumes while charging.

How Long Do Gel Batteries Last?

Gel batteries have a similar lifespan to AGM batteries.

Their total life cycle is greatly influenced by the depth of discharge. Constructors always recommend a 50% DOD to reach 700 cycles of service max.

Are Gel Batteries A Good Choice As Solar Batteries?

Looking for a low-budget technology but one that is still efficient?

Gel batteries are a good choice. However, be aware that they will perform well for 2 years, then drop greatly in performance thereafter.

Have a look at our comparison chart to see what technology is the most robust and cost-effective.

The Lithium Battery

Lithium-ion batteries are rechargeable. They were developed in the ’80s and made available in the early ’90s. Therefore, this green technology is still quite new and benefits from constant improvement.

The energy density of a commercial lithium-ion battery is on average 120 Wh/kg.

Below is a quick look at the nature of the main components of a lithium battery

Component

Material

Anode (-)

Carbon

Cathode (+)

Metal oxide

Electrolyte

Lithium salt

Membrane

Porous to Lithium-Ion

 

Currently, there are 6 types of Lithium-ion batteries. However, we will focus only on the Lithium Iron Phosphate battery (LFP). This is the most widely available on the market.

 

Let’s have a look at the LFP batteries as solar batteries:

Lithium Iron Phosphate batteries are rechargeable lithium-ion type. Unlike lead-acid batteries which are one unique system, lithium batteries are composed of an assembly of smaller-sized cells of cylindrical shape.

The industrial standard is the 18650 lithium cell.

 

The number of cells can be adjusted to reach the required voltage and energy capacity of the battery pack.

Are Lithium Iron Phosphate Batteries Safe?

Thanks to multiple safety components Lithium Iron Phosphate batteries are safe to use.

Lithium metal in its pure form is highly flammable when in contact with air or water.

Therefore, lithium batteries have multiple safety components. Each unique battery cell is carefully sealed in an air-tight cylindrical format.

The cell assembly that composes the lithium-ion battery pack is monitored by a Battery Management System (BMS).

The BMS ensures a balanced state of charge in every battery cell at all times. It also prevents overcharging and discharging.

How Long Do Lithium Iron Phosphate Batteries Last?

LFP batteries have a rated lifetime of 2,000 to 4,500 cycles at 80% depth of discharge.

Why Are Lithium Iron Phosphate Batteries So Expensive?

This type of battery is more expensive at first than deep cycle lead-acid batteries because their technology is more recent.

The lithium battery is only 20 years old, compared to 150 years old for lead-acid batteries.

However, their market share is growing rapidly thanks to electric vehicles. Consequently, their price is dropping year after year.

Is There A Better Solar Battery Than Lithium?

Lithium batteries like the LFP (Lithium Iron Phosphate) are currently the best solar batteries available on the market.

I listed below some of their main advantages:

  • Light weight
  • High storage capacity
  • Tolerant to rapid and intermittent charging
  • Can supply high current load
  • Low self-discharging rates
  • Full discharge is possible

 

They still have a higher initial cost, but in the end, they demonstrate robustness and reliability in the context of variable solar charging.

Furthermore, you will find them cheaper to use in the long term compared to lead-acid types.

How Much Do Solar Batteries Cost?

The cost of a battery varies depending on its capacity and technology. Therefore, we can give a price estimation per usable kWh:

We are taking into account their usable capacity.

For lead-acid batteries, manufacturers recommend discharging them at 50% of their maximum capacity and for lithium batteries 80%.

 

On average for the same available energy, a lithium battery is 2.5 times more expensive than a deep cycle lead-acid battery.

The good news is that the lifetime of a lithium battery is 3 times the one of a deep cycle.

In the end, over its lifetime (2000 cycles), your lithium battery will cost less than a deep cycle battery.

 

Cost per usable capacity

Overall lifetime cost (2000 cycles)

Lithium Iron Phosphate battery (80% DOD)

899 USD/kWh

899 USD/kWh

Deep cycle battery (Gel, AGM) (50% DOD)

367 USD/kWh

1065 USD/kWh

 

What Kind Of Battery Is Best For A Solar System?

Lithium iron phosphate batteries are far more fitted for solar charging than deep cycle lead-acid batteries (Flooded/Wet type, AGM, Gel).

The main reason is that lead-acid batteries require a much longer charging time, up to 16 hours. Most of the time, this is not achievable with solar energy.

For example, lots of tropical countries have only 12 hours of sun per day.

Conversely, Lithium batteries are tolerant to fast and variable charging. For example, at noon when the solar panels are at their peak production, your lithium battery will easily absorb high current in a fast-charging mode. Whereas the deep cycle lead-acid battery will be limited by its technology.

In terms of pricing, you will find lithium batteries more expensive at first. But look over their whole lifetime, they will be cost-effective, at least 20% cheaper than the lead-acid types.

You can take a look at our comparative chart below that summarizes the main features of each technology:

Lithium iron phosphate

Wet type

AGM

Gel

Energy density (Wh/Kg)

120 Wh/Kg

45 Wh/Kg

35 Wh/Kg

35 Wh/Kg

Average life cycle 80% DOD

1000-2000

250-350

200-300

200-300

Cost per usable kWh

900 USD

300 USD

370 USD

370 USD

Self-discharging

5%

10%

5%

5%

Rapid charging

1 h

8-16h

8-16h

8-16h

Note that if your battery has reached its rated life cycle, you can still use it, but its capacity will decline further down to zero.

It will take more time for a lithium battery to reach zero capacity compared to lead-acid batteries.

 

Is It Worth Getting Solar Batteries?

Get a solar battery to store your solar panel production during the day and use it any time!

Our sun is a natural source of energy; therefore, it is variable. Its production output varies during the day and in the case of climatic events.

The peak production of the sun is at 12 noon; however, our peak electricity consumption occurs in the morning around 7 am and in the evening at 8 pm.

Our consumption is therefore disconnected from the production time. You now see the necessity to store solar energy to fully exploit the potential of your system.

If you travel in a camper van equipped with solar panels, you will certainly need electricity for the light in the evening. Consequently, you will store the electricity produced by your monocrystalline solar panel during the day to release it at night.

 

How Much Is A Solar Battery For A House?

 

A 2 kWh battery with a 2kW power peak for domestic use costs 1,000 USD. Above all, it comes with a 10-year warranty (or 4,500 cycles) at 70% of the rated capacity.

 

This storage capacity will be enough to run the whole domestic appliance for 1 day. You can associate multiple batteries to increase the overall capacity of your system.

You will need to purchase an inverter to convert DC current into AC used by your appliance.

Over the last ten years, the cost of lithium batteries for domestic use dramatically dropped, meaning that the technology is now accessible to all and is replacing the fuel generator in off-grid systems.

 

Can You Use Car Batteries For Solar Power?

 

If you are on a limited budget, you can use your car batteries as solar batteries. Even second-hand car batteries will still operate under low power load (lights).

Car batteries are 12V lead-acid batteries that can supply high currents for a limited time.

Coupling solar panels and car batteries will work. However, your system will not be optimized as car batteries are not designed for deep discharge or intermittent charging. In addition, the life duration of your battery will be low.

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