Instyle Solar How to Understand Solar Battery Datasheets Image

Solar systems need batteries with high power and/or high capacity depending on how your system will run. In this article, we will go over what kinds of solar batteries are available and how to properly use the information from their datasheets.

Truth be told, you will not be a pro at the end of this, but you will know enough not to make the big mistakes when choosing batteries. We all have to start somewhere, and this is as good a place as any!

Solar Battery Companies

In Australia, there are a variety of companies to choose from who offer solar batteries primarily for the residential and small-business markets. Aquion, BYD, Fronius, Sunverge, Enphase, LG, Redflow, Samsung, Panasonic and Tesla are some of the better-known manufacturers.

Having many options is an advantage in that what you are looking for is definitely out there—but it can be a disadvantage when searching for a solution, so our advice is to breathe, take your time and get some quotes.

Solar Battery Characteristics

Power and Capacity

The power of a solar battery is the rate at which the battery is charged or discharged and is measured in Watts (W). The capacity of the solar battery (found by multiplying power and time) is how much energy can be stored in the solar battery measured in Watt-hours (Wh).

For a standalone solar system, you will want to have a high-capacity battery to avoid failing to meet your energy demand. For a grid-tied system, high power, which comes with the ability to charge and discharge quickly, may be more important, so choose wisely.

The capacity of a battery does not remain constant and like many things lessens with age. The graph below taken from www.mpoweruk.com/performance.htm shows an example of what happens to the capacity of a battery over time.

Instyle Solar How to Understand Solar Battery Datasheets Battery Cycle Life

As the battery goes through charging and discharging cycles, the overall capacity of the battery starts to decrease. The chemical reaction taking place in the battery becomes contaminated with external elements and that reduces the reaction’s effectiveness.

Steps are taken, through research and development, to slow this reduction in battery capacity but all batteries, every single one of them, goes through this process, so you need to prepare for it.

Depth of Discharge (DoD)
This is a measure of what percentage of the battery has been discharged at a given time. A Depth of Discharge (DoD) of 90% means that the battery is at 10% capacity. In order to get the best performance and batteries that are longer lasting, you should keep above the battery’s rated minimum DoD. If you go below the minimum DoD the chemical reaction of the battery may be irreversibly disturbed and result in poor battery performance.

Charge Cycle
A battery charge cycle is the discharging of the battery to a certain depth of discharge and then charging it again. The cycle count of a battery is referring to how much charging and discharging has taken place in the lifespan of this battery.

Lifespan and Warranty
The lifespan of a battery is measured in cycles where a cycle is a completing charging and discharging of the battery. When the warranty of a battery is given it will be given in terms of years and cycles an example of this being 6000 cycles/10 years. The warranty expires when the number of cycles has been run through or the warranty time has elapsed, whichever happens first.

Round Trip Efficiency
This is the percentage of energy put into the battery storage that can actually be retrieved. The round trip efficiency can be found by dividing the amount of energy stored in the battery by the amount of useful energy taken from the battery and is typically upwards of 80%.

Types of Solar Battery

All solar batteries operate with the same general principle of using a well-known chemical reactions to convert electrical energy into chemical energy. The energy will remain in the battery in the form of chemical energy until it is changed back to electrical energy to be used by the household appliances. The differences that we will see in the performance of the batteries is because of the materials used in the chemical reaction.

Lead-acid
This is a well-known chemical reaction that has been used in batteries for a very long time and is tried and trusted. The metal, lead, reacts with an acid in what is known as a reversible reaction. Lead-acid batteries are cheaper to manufacture but have lower DoD and a shorter lifespan.

Lithium-ion
A combination of the metal lithium and some charged molecules called ions makes for a lighter and more compact battery. The DoD and lifespan of these batteries are higher than for lead-acid batteries, but this comes at an increased cost.

Saltwater
This is very new technology that does not use metals in the battery’s chemical reaction to store energy. The electrolyte is the fluid in a battery where the chemical reaction takes place and these batteries use saltwater electrolytes to store and release energy. This is untested technology and is, in theory, cheaper than the alternatives, but has not yet earned the trust of the solar energy community.

Example of a Battery Datasheet

We will use the Fronius Solar Battery 7.5 as our example of a solar battery whose datasheet we will analyse. The more involved you will be in the design process, the more datasheet parameters you need to know and understand.

Electrical Parameter
Usable Capacity 6000Wh
Cycle Stability 8000
Voltage Range 200V – 290V
Nominal Charging Power 4000W
Nominal Discharge Power 4000W
Maximum Charging Current 16A
Maximum Discharge Current 16A

Electrical Parameter

Battery Technology

Lithium-Eisenphosphat Technologie

Height

955mm

Width

570mm

Weight

611mm

Degree of Protection

IP 20

Protection Class

1,0

Installation

Indoor installation

Ambient Temperature Range

5 - 35°C

Permitted Humidity

0 - 95%

DC Connection Technology

Screw terminals 2.5 - 16 mm²

Long Service Life

> 20 years

Usable Capacity
Capacity is a measure of how much energy the battery can store. In theory, all the energy that enters the battery can be put to good use, but practically this is not the case and some must remain unused to keep the battery stable.

At some level of discharge, there are no longer enough chemicals in the battery to keep the chemical reaction going stably and discharge of the battery stops. Usable capacity is a measure of this and is always lower than the overall capacity of the battery.

Cycle Stability
This is a measure of how many charge cycles the battery will go through before its capacity starts to degrade. The more often the battery is charged and discharged, the less time it will last so take this into consideration when doing maintenance.

Voltage Range
As a battery discharges, the voltage at its output decreases as shown in the discharge curve. In the context of solar energy, this decrease in voltage may result in a voltage too low to be used by the inverter connected to it.

A high battery voltage indicates that the battery is fully charged, and the charge controller will stop charging the battery. As a battery ages, the maximum amount of energy that it can hold decreases and so does its maximum voltage.

Nominal Charging Power
This is a measure of the rate at which energy is stored by the battery at standard testing conditions. This value may be lower in practice, so using this value in your designing of the solar system is a smart move because it caters for the worst-case scenario.

Nominal Discharge Power
This is a measure of the rate at which the battery releases stored energy at standard test conditions. This value may be higher in practice and can be used when designing the solar system.

Height, Width and Weight
The solar battery needs to be placed in a cool area with ventilation. Overheating will result in a decrease in efficiency and knowing its dimensions can help you set up the right environment.
The weight may be an issue in the event that the battery is suspended from the ceiling or the wall. Remember, this is a long-term arrangement, so whatever supporting structure is used, it has to be able to handle the weight for a long period of time.

Degree of Protection
The International Protection Marking, or IP rating, gives the degree of protection provided by the battery casing against intrusion, which is basically preventing any object or body part entering the battery.

The two numbers that follow the ‘IP’ indicate protection from solid objects and liquid objects. IP20 indicates protection against solid protection of 12mm (2) and no protection against liquid (0)—so no dipping this battery in water!

Protection Class
This is a rating not specific to batteries, but all electrical appliances have a protection class. In order to prevent flow of electricity into the metallic parts of an electrical appliance, the metallic parts may be connected to an earth or ground line that runs into the ground.

The earth protection required when connecting class 1 appliances like most solar cells is that the chassis of the device should be connected to electrical earth.

Ambient Temperature Range
When talking about electronics, the ambient temperature is the temperature outside the appliance. If the external temperature becomes too high or too low, the circuit may actually stop functioning correctly. Batteries are very sensitive to this because there is a chemical reaction going on inside the battery.

Permitted Humidity
If the surrounding air is too humid, the balance of chemical reactions in the battery may be tilted in the wrong direction. In extreme cases of humidity, the metals involved in the chemical reactions may start to rust, which causes irreversible damage to the battery.

Graphs

Discharge Curve
In general, when a battery is fully charged it is at its maximum voltage and when it is fully discharged it is at its lowest voltage. If we are looking at battery datasheets, we can get a more accurate reading than this using a discharge curve. The picture below is a good example.

Instyle Solar How to Understand Solar Battery Datasheets Battery Discharge Curve

Most batteries will have this same general shape and the flatter the curve is, the easier it is to control the voltage from the battery. The voltage values from this curve show when to start and stop discharging a battery, which is usually taken care of by a charge controller but can be taken care of by you depending on how involved you are in the system setup.

Self-Discharge
The capacity retention ratio is a good measure of how long a battery can hold charge without losing it. In a perfect world, batteries will store energy forever, but because of the internal resistance of the battery, small amounts of energy leak out from the system. The rate at which this happens increases when the temperature around the battery increases.

Instyle Solar How to Understand Solar Battery Datasheets Battery Storage Characteristics

Most solar systems are designed in such a way as to store small amounts of energy in the battery and use these amounts of energy up quickly. You should not be storing electrical energy for several days at a time—this is poor design.

Conclusion

Batteries tend to look the same, so when choosing batteries for your system, the differences are in the datasheets. The information in the datasheet is the result of testing done in standard test conditions and these are stated on the datasheet so you get an idea of how your battery will perform in your conditions.

Every component of your solar energy system requires you to sacrifice some good results in one area of the system to gain good performance in another. When choosing solar batteries, you have a major decision to make between high-capacity batteries and high-power batteries.

If you have a standalone solar system, a high-capacity battery with high depth of discharge will be appropriate to make sure that all your energy needs are met. For a grid-tied system, high charging and discharging speeds may be more important so high power is a major advantage.

When all is said and done, the battery is a necessary component of (almost) every solar system. There are ready-made solar systems that come with all the components you need, but even if that is the direction you want to take, it helps to know how these systems were put together so have a look at the datasheets, don’t be lazy now…

Next Step

If you want to see how much solar or battery storage could save you over the next 5 years, then take our solar saving calculator quiz below!

Or talk to an Instyle Solar expert about the best solutions for home energy storage or PV-panels.

Otherwise, head back to the solar blog to find even more great educational content.

Photo credit: Depositphotos, Electropaedia

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