Store your solar energy in solar batteries!
Every energy revolution contributed to mankind’s development, from the use of wood in the Stone Age, to charcoal and oil in the 19th century.
We are now leaving the era of electricity.
Electricity is defined as a flow of charged particles: the electrons.
Unlike other fuels naturally available on Earth, electricity has to be produced from primary resources (charcoal, oil, wind, solar). Currently, the production of electricity originates massively from non-renewable sources (64%):
- Coal (38%)
- Gas (23%)
- Oil (3%)
Other production sources are from:
- Nuclear (10%)
- Hydro (16.2%)
- Wind (4.8%)
- Solar (2.1%)
- Biofuels (2.4%)
- Other renewable sources (0.5%)
Since the 70’s global electricity production has been multiplied by 5, and is still growing at a rate of 4% per year.
During the last 20 years, wind and solar technologies showed the fastest growth in terms of electricity production.
Energy storage is essential for on-demand use, or for mobile application. Storing natural sources of energy like charcoal, oil or gas in a tank is easy.
On the contrary, storing a secondary source of energy like electricity (electrons) is a technical challenge. We cannot put electrons in a container obviously!
For medium to long term storage, scientists and researchers came out with a chemical storage of the electrons called the battery.
Many different types of batteries were developed for all type of use. You will find one for the smallest electronic gadget (such as those stored in solar power banks) to the heavy-duty electricity storage.
In this article we will give an in-depth look at the different types of batteries available to store solar energy: the solar battery.
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 principle 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 time thanks to an exchange of ions in the electrolyte.
During charging the opposite reaction occurs. Oxidation at the cathode and in 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 our great 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 use electricity at all times, so you need to store your production. The most versatile and reliable way to store solar energy is through a battery.
This is the solar battery.
Why you should couple batteries to your solar panels
In real life condition our energy consumption is predictable. Consumption peaks occur in the morning around 7am when we wake up and in the evening around 7pm.
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.
Batteries are now widely used.
Coupled to 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 usefull battery terms
If you look at the technical terms of a commercial battery you might find yourself completely lost! Don’t worry, below is my top 10 list of the most relevant ones.
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 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.
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)
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 discharge.
You will also have instant voltage fluctuation if you connect a heavy load to your battery.
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.
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:
- 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 term 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 during 20 hours: 60 W*20 hours = 1.2 kWh.
In the end, if you excess on average this value of 60 W over the course of 20 hours, first you will discharge your battery quicker and then you will reduce your total capacity.
Batteries work only with Direct Current (DC) for charging and discharging.
All our daily appliances use alternative current (AC).
DOD (Depth of discharge)
To improve the battery life duration, manufacturers do not recommend to discharge the battery fully. The advised DOD might vary depending on the type of battery, for lead-acid: 50% and for lithium: 80%.
The cycle life is the number of time you could fully discharge and charge your battery untill 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 have in common the same working principle. However, they differ in their active material.
One of the most important features you’d like to look at is the 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 capacites. 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 would be the best for a solar battery.
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 heavy weight of lead electrodes.
Below a quick look at the main components of a lead acid battery:
|Anode (-)||Lead (Pb)|
|Cathode (+)||Lead (Pb)|
|Electrolyte||Sulfuric acid (H2SO4)|
|Membrane/Separator||Depends on the technology (fiber glass 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 two 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 slightly longer useful life compared to VRLA batteries. However, we would not recommend to purchase this type of batteries.
Wet batteries are not user-friendly.
They require safety precaution during maintenance and operation and have 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.
Absorbent Glass Mat (AGM) is the material that compose the battery separator of an AGM battery. In this type of lead acid battery the electrolyte is absorbed on a fiber glass 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 constructor recommend 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 the car batteries.
You might want to take a look at our comparative chart to choose the best technology that will fit your needs and your budget.
In this battery, silica is added to the sulfuric acid electrolyte to make it a gel.
Gel batteries are lead acid battery with a semi-solid (gel) electrolyte.
Are Gel batteries safe?
They are user friendly consequently safe to operate.
They are 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 similar life duration compared to AGM batteries.
Their total life cycle is greatly influenced by the depth of discharge. Constructor 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 buget technology but still efficient?
Gel batteries are a good choice. However, be aware that they will perform well during 2 years, then their performances will be greatly reduced.
Have a look at our comparative 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 80’s and made available in the early 90’s. 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 a quick look at the nature of the main components of a lithium battery
|Cathode (+)||Metal oxide|
|Membrane||Porous to Lithium Ion|
Currently there are 6 types of Lithium-ion batteries. However, we will only focus 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 compose 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 time. It also prevents overcharging and discharging.
How long do lithium iron phosphate batteries last?
LFP batteries have a rated life time 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.
Imagine that the lithium battery is only 20 years old, compared to 150 years old for lead acid!
However, their market share is rapidly extending (thanks to electric vehicle). Consequently their price is dropping years after years.
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 request a higher investment, but in the end they will show robustness and reliability toward variable solar charging.
Furthermore, you will find them cheaper to use on 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 to discharge them at 50% of their maximum capacity and for lithium battery 80%.
On average for the same available energy, a lithium battery is 2.5 time more expensive than a deep cycle lead acid battery.
The good news is that the life time of a lithium battery is 3 times the one of a deep cycle.
In the end, over their life time (2000 cycles), your lithium battery will cost less than a deep cycle battery.
|Cost per usable capacity||Overall life time 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 much longer charging time, up to 16 hours. This is most of the time not achievable with solar energy.
For example, lots of tropical countries have only 12 hours of sun per day.
On the contrary, 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 life time, they will be cost effective, at least 20% cheaper than the lead acid types.
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|
|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 ouput varies during the day and in case of climatic events.
The peak production of the sun is at 12 noon, however our peak electricity consumption occurs in the morning around 7am and in the evening at 8pm.
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 monocrystaline 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 1’700 USD. Above all it comes with a 10 years 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. Consequently, 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 current 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.
Five of the best solar batteries on the market
When looking for a solar battery you might want to focus on 4 main parameters:
- Usable battery capacity or depth of discharge
- Durability (the number of cycles)
- Adaptability to solar energy charging
We have selected for you the 5 best solar batteries on the market:
Best solar battery for heavy domestic use
- Huge 13.5 kWh storage capacity
- Plug-and-play design with a built-in inverter to deliver Alternative Current (AC)
- Enough energy for 3 days of total autonomy
- Longest warranty: 10 years at 70% initial capacity
- Lowest cost per kWh over its lifetime
Best GEL battery for in-board and domestic use
Renogy Gel battery Deep cycle, 12 V, 100 Ah
- 0.6 kWh of usable capacity
- Maintenance free
- Can supply high discharge current
- Has the lowest self discharging rate (3% month)
- Can operate at temperature below zero degrees
Best AGM battery for in-board and domestic use
Renogy AGM battery Deep cycle, 12 V, 100 Ah
- 0.6 kWh of usable capacity
- Maintenance free
- Extended life duration: 750 cycles at 50% DOD
- Lowest self discharging rate (3% months)
- Safe to operate
Best Lithium Iron Phosphate Battery for domestic use
Renogy Smart Lithium Iron Phosphate Battery, 48V, 50Ah
- 2 kWh of usable energy
- Extreme life duration: 4’500 cycles or 10 years
- Support fast charging/discharging up to 50A
- High tolerance to intermittent solar charging
- Associate multiple batteries to increase the capacity of your system
Best Lithium Iron Phosphate Battery for in-board use
Renogy Lithium Iron Phosphate Battery, 12 V, 50 Ah
- 0.8 kWh of usable energy
- Long life duration: 2’000 cycles
- Support fast charging/discharging up to 50 A
- High tolerance to intermittent solar charging
- Associate multiple batteries to increase the capacity of your system
- Light weight for in-board use
There is a solar battery for all needs and every budget. The AGM and Gel lead acid battery are well adapted to low budget and small capacity systems.
On the other hand, lithium iron phosphate batteries are the best when it comes to storing solar energy. They are perfect for domestic and in-board use (boat, RV…).