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Solar batteries for your solar system: Basics to make the right choice

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Solar batteries for your solar system: Basics to make the right choice

25 Nov 2020
11 min
Intermediate

Tatiana IvanovaAuthor

Electricity comes and goes. It does its job perfectly, powering your household appliances, lightening the rooms and charging your electric car, but then it goes away. But what if you could make it stay for a while? Wouldn't it be nice to have a strategic stockpile, just in case? Solar batteries are at your service.

The history of batteries

In the beginning was the word, and the word was 'battery' coined by Benjamin Franklin in 1749. He used it to describe an array of linked glass capacitors, which, in fact, looked nothing like the batteries we are used to.

The first true battery – the Voltaic Pile – was invented by the Italian physicist Alessandro Volta in 1800. Basically, it was a pile of stacked discs of copper (Cu) and zinc (Zn), separated by cloth soaked in salty water. Contact between the two metals created a difference in potential (i.e. 'voltage'), which in a closed circuit produced a continuous stable current. However, it didn't last long.

In 1836, John Frederic Daniell invented the Daniell Cell that lasted longer than the Voltaic Pile. His battery could produce about 1.1 volts and was used to power objects such as telegraphs, telephones, and doorbells.

The turning point in the battery's history was 1859, when the French inventor Gaston Plante created the first lead-acid battery that could be recharged. The technology behind that battery is still used today to start most internal combustion engine cars.

Today batteries come in various modifications and sizes and suit various purposes, being able to store the power from solar farms, among others.

Battery is a container consisting of cells, in which chemical energy is converted into electricity and used as a source of power.

Lead-acid batteries will serve you faithfully until you let them run down

Lead-acid batteries have already turned 150, but are still widely applied in different industries. The materials used – lead and lead dioxide – are cheap and high in conductivity, which makes it extremely difficult for current technologies to outperform good old lead-acid batteries.

Lead-acid batteries have four main components:

Positive plate covered with a paste of lead dioxide
Negative plate made of sponge lead
Separator – an insulating material between the two plates, needed to enable conduction without the two plates touching
Electrolyte consisting of water and sulphuric acid, which helps conduct electricity

These constituents are enclosed in a plastic cell which keeps the electrolyte in. The voltage difference between the two plates is approximately 2 volts. That's why a 12 V battery has six single cells linked in series.

When the battery discharges, lead and lead dioxide react with sulfuric acid in the electrolyte to form lead sulfate. It cakes the space between the plates. If not much builds up (with a 20% discharge, for example), the battery can recharge easily – lead sulfate reverts back to lead, lead dioxide, and sulfuric acid. However, if there's too much lead sulfate, you'll never be able to recharge the battery. It can happen if you run a conventional lead-acid battery to zero charge a few times, for example.

This is when deep cycle batteries come in handy. Their capacity can be largely used and then recharged easily, hence the name. A deep cycle battery differs from a conventional one in three ways:

Plates are thicker, which allows increasing energy density space;
Plates are put further apart so that lead sulfate debris can fall off of them;
There's some room below the plates to accommodate the debris.

Advanced lithium-ion batteries are better all round except for price

Lithium-ion batteries are literally everywhere – from laptops and cell phones to hybrids and electric cars. This technology owes its popularity to the light weight, high energy density, and ability to recharge.

Lithium-ion batteries have four main components:

Anode (positive cobalt-oxide electrode) and cathode (negative graphite electrode) which store the lithium
Separator which blocks the flow of electrons inside the battery
Electrolyte carrying positively charged lithium ions from the anode to the cathode and vice versa through the separator
Two current collectors (positive and negative).

While the battery is discharging, the anode releases lithium ions to the cathode, generating a flow of electrons from one side to the other. The battery is fully charged when no more ions flow. When the battery is getting charged, lithium ions are released by the cathode and received by the anode. If all the ions have moved back, it means that the battery is fully discharged.

All lithium-ion batteries are deep cycle, meaning they have the ability to be fully charged and discharged, and thus can be used for solar energy storage. In 2015, Tesla announced their new lithium-ion solar battery, Powerwall 1, which became an important milestone in the development of cost-effective solar batteries.

Before the Tesla Powerwall, most solar storage systems were composed of lead-acid battery banks. Today the solar market offers a range of lithium-ion solar batteries, which, however, have the only drawback – much higher prices compared to those of lead-acid batteries.

How do solar batteries work?

Step 1: Solar panels generate electricity

Sunlight hits the solar panels, which turn the visible light into DC electricity. It can then either be converted to AC power or kept as DC power, depending on the type of battery (AC or DC) the system uses.

Step 2: Battery gets charged

But only after the home is fed up. First, the electricity produced by the panels will go to power the household appliances. If there is some extra electricity (which happens quite often), instead of going to the grid, it will flow into the battery and charge it up.

Step 3: Battery waits for its moment of glory

Now that the battery has been charged up with surplus solar power, that stored energy can be used later, when your solar panels aren't producing electricity or when there's a power outage in your area.

Step 4: Battery gets discharged

Once you're cut off the electricity for any reason, the battery storage system activates to power your household appliances until the electricity supply is restored (see step 1). 50-60% is the deepest discharge most manufacturers find safe for batteries.

Three ways you can add batteries to you solar system

There are three ways batteries can be integrated into a solar PV system: using DC coupling, AC coupling or both.

DC coupled battery system

Type of battery used: DC battery
Installed: between solar panels and inverter

DC batteries do not have a built-in inverter and are charged directly from DC power. That's why a specialized inverter is required to pass DC power produced by the solar panels directly into the battery without inverting it to AC. When the battery powers the home, the DC power from the battery is fed into the external solar inverter, where it's converted into AC used by most household appliances.

The greatest advantage of DC batteries is that they have higher efficiency. When electricity is inverted from DC to AC (or vice versa), about 5% of the power is lost to heat. It means, the more you invert the power, the more total energy you lose. That is why DC batteries are more efficient than AC ones, since they're not inverting the power as many times. But these systems do not provide protection from grid outages. So, if the grid goes down, the system also stops working.

Homes run on AC (alternating current), while all batteries need DC (direct current) to charge

AC coupled battery system

Type of battery used: AC battery
Installed: after inverter

AC batteries can accept incoming AC power and use a built-in inverter to convert that to DC power, which then charges the battery. When the battery is needed to power the home, the inverter converts the DC power coming from the battery pack back into AC.

AC coupled battery systems are the most common type of storage. Although they aren't as efficient or as cost effective as DC batteries, AC batteries are very flexible and can be used with any solar system. Any solar inverter can be paired with them, including microinverters.

Battery system with a hybrid inverter

This type is a combination of the two above. It allows the power to be saved into batteries without unnecessary conversions like a DC coupled storage, while providing protection from grid outages. However, this all-inclusive option costs a lot.

Home solar battery is a small step to a greener future

Energy storage is expected to grow fast in the near future. The benefits solar batteries offer to homeowners, namely the ability to store energy for later use, can also be applied at a larger scale for the entire electricity grid.

For the electricity grid to work properly, the demand-supply balance should be maintained, which becomes especially challenging as more renewable resources, like solar and wind power, are added to the grid. Unlike traditional coal- or natural gas-fired power plants, the output of solar panels and wind turbines can't be quickly increased or reduced to meet the demand.

Installing more energy storage technologies like solar batteries can help electric utilities and grid operators to manage the flow of electricity from renewable resources more easily. In the long-term perspective this will make it possible to integrate more renewable energy resources into the national electricity grid, including home solar panel systems.

Remember, when installing a battery at your home, you contribute to the development of renewable energy at the national level.

Three reasons why solar batteries are must-have

Although solar batteries aren't among the compulsory components of a solar PV system, they are a real must-have if:

You go off-grid. In fact, there is no other way an off-grid solar system can work properly (even if you have a generator).
You want a power backup. This is what is called a hybrid solar PV system. It ensures a continuous electricity supply even if a power outage occurs.
You want to take advantage of time-of-use (TOU) rate plans from utility companies. The plans state, among others, that the extra energy produced by solar panels in the middle of the day is less valuable than the power drawn in from the grid at night. Thus, using a solar battery can help you save more money by avoiding the peak utility charges in the evening.

Find more about the difference between on-grid, off-grid and hybrid solar systems and which of them require batteries