Chapter 6

Introduction

So far, we've followed the Sun's energy from a photon leaving the surface of the Sun to electricity flowing through our homes. 

But there's one obvious problem.

Solar panels only generate electricity while the Sun is shining. As evening approaches, electricity production gradually falls to zero, yet our demand for energy often increases. We still want to cook dinner, switch on the lights, watch television and charge our phones long after sunset.

This creates one of the biggest challenges in renewable energy. The Sun decides when electricity is produced. We decide when we want to use it.

A battery bridges the gap between those two moments. Instead of using all the electricity immediately, it stores surplus energy during the day and releases it whenever it's needed later.

But how can a box sitting quietly on the wall store electricity for hours, or even days?

The answer lies in chemistry.

Why Do Batteries Exist?

Imagine collecting rainwater in a storage tank during a storm. You probably don't need all of that water immediately, but you'll be grateful to have it during a dry spell.

A battery performs exactly the same role for electricity.

During the middle of a sunny day, a solar system often produces far more electricity than a home is using. Rather than allowing all of that surplus energy to be exported to the electricity grid, a battery can store it for later.

As the Sun sets and solar generation begins to fall, the battery gradually releases its stored energy back into the home. This reduces the amount of electricity that needs to be imported from the grid and allows more of the energy generated during the day to be used after sunset.

A battery therefore doesn't create electricity. It simply changes when that electricity can be used.

What's Inside a Battery?

Although a home battery appears to be a single metal box from the outside, it actually contains many individual battery cells connected together. Each cell is a miniature energy-storage system capable of converting electrical energy into chemical energy and then reversing the process when that energy is needed.

Inside each cell are four essential components:

1. The anode - usually made from graphite, a form of carbon arranged in thin atomic layers. During charging, lithium ions move into spaces between these layers, allowing energy to be stored within the cell.

2. The cathode - made from a lithium-containing material. During charging, lithium ions leave the cathode and travel towards the anode. During discharge, they return.

3. The electrolyte - a substance positioned between the electrodes that allows electrically charged lithium ions (Li⁺) to move between the anode and cathode.

4. The separator - an extremely thin, porous material positioned between the electrodes. It prevents the anode and cathode from physically touching, which would cause an internal short circuit, while still allowing lithium ions to pass through.

The key to understanding how the battery works is that lithium ions and electrons travel along two completely different pathways.

Inside the battery, lithium moves primarily as positively charged lithium ions (Li⁺) through the electrolyte. Electrons cannot pass through this electrolyte, so they cannot simply travel directly between the two electrodes.

Instead, electrons must travel through an external electrical circuit connecting the anode and cathode. This separation is crucial because the movement of electrons through that external path is what allows electrical energy to be transferred into or out of the battery.

You can therefore imagine every battery cell as having two routes connecting the same destinations. There is an internal route through the electrolyte that only the lithium ions can use, and an external route through the electrical circuit that the electrons must use.

During charging, energy from the solar panels forces lithium ions through the internal route towards the anode while electrons travel towards the anode through the external route. During discharging, both journeys reverse: lithium ions travel back towards the cathode internally while electrons travel through the external circuit, making their energy available to the home.

The lithium itself therefore moves back and forth between the two electrodes rather than being consumed. What changes is where the lithium is stored and the chemical energy associated with that arrangement.

Every lithium-ion battery, from a mobile phone to a home solar battery, relies on this same fundamental principle: lithium ions move inside the cell while electrons travel through the external circuit.

Charging the Battery

When the solar panels generate more electricity than the home needs, the surplus energy is directed into the battery. At first glance, it might seem as though the battery is simply filling up with electrons.

In reality, something much more interesting is happening.

The electrical energy drives a series of reversible chemical reactions inside each battery cell. During charging, lithium stored within the cathode releases lithium ions and electrons. In simplified form:

Li → Li⁺ + e⁻

The positively charged lithium ions (Li⁺) travel through the electrolyte towards the anode. The electrons also need to reach the anode, but the electrolyte blocks them from taking the same route.

Instead, the electrons must travel through the external electrical circuit. The lithium ions and electrons therefore travel along separate paths before both arriving at the anode.

The anode is typically made from graphite, a form of carbon arranged in thin atomic layers. When the lithium ions reach it, they move into spaces between these layers, where they are stored alongside the incoming electrons. This process is known as intercalation.

In simplified form, the reaction at the graphite anode can be represented as:

C₆ + Li⁺ + e⁻ → LiC₆

As charging continues, more lithium moves from the cathode into the graphite anode.

This is how the battery stores energy. The electricity from the solar panels has moved the lithium into a higher-energy chemical arrangement, converting electrical energy into chemical potential energy.

Discharging the Battery

When the Sun begins to set and the home needs electricity, the battery's control system detects that stored energy is required. The battery does not simply begin discharging by itself. Its Battery Management System (BMS) and inverter control the process, electrically connecting the battery to the system and allowing current to flow.

Once this electrical pathway is available, the chemical reactions inside the battery can begin to reverse. The chemical potential energy stored during charging is now converted back into electrical energy.

During charging, lithium was stored between the layers of the graphite anode. During discharge, it leaves this structure, releasing a lithium ion and an electron:

LiC₆ → C₆ + Li⁺ + e⁻

The positively charged lithium ions (Li⁺) begin travelling through the electrolyte towards the cathode. The electrons also want to move towards the cathode, but the electrolyte blocks them from travelling through the inside of the battery.

Instead, the electrons are forced to take the long way around, through the external electrical circuit.

As the electrons flow through this external path, their electrical energy becomes available to the home. The battery inverter converts the battery's DC electricity into AC electricity, allowing it to power lights, televisions, refrigerators and other appliances.

Meanwhile, the lithium ions travel separately through the electrolyte. Eventually, the lithium ions and electrons both arrive at the cathode, where they are incorporated back into the cathode material. The lithium has now returned towards the lower-energy chemical state it occupied before the battery was charged.

The process is therefore the reverse of charging: lithium ions travel internally through the battery while electrons travel externally through the electrical circuit. Their movement continues until the battery stops supplying power or reaches its permitted minimum state of charge.

A useful way to picture this is the ball-on-a-hill analogy. Charging uses solar energy to push the ball uphill, placing the battery into a higher-energy chemical state. Turning on the battery and providing a path for current is like opening a gate that allows the ball to roll back downhill. As the lithium returns towards its lower-energy state, the stored chemical energy is released as electrical energy.

Importantly, the lithium itself is not consumed. The same lithium ions repeatedly shuttle between the anode and cathode as the battery charges and discharges, allowing the process to be repeated thousands of times.

Battery Capacity and Battery Management

Not all batteries are the same.

One important difference is capacity, which describes how much energy a battery can store. Capacity is measured in kilowatt-hours (kWh) and can be thought of as the size of a fuel tank. A larger battery can store more energy before it needs recharging.

Another important characteristic is power, measured in kilowatts (kW). While capacity tells us how much energy is available, power tells us how quickly that energy can be delivered. A battery with a high power rating can supply large amounts of electricity over a short period, making it capable of powering demanding appliances such as kettles or air conditioners.

Modern batteries also contain a sophisticated computer known as the Battery Management System (BMS). The BMS continuously monitors the voltage, current and temperature of every battery cell, ensuring they all charge and discharge safely. If abnormal conditions are detected, it can disconnect the battery almost instantly to prevent damage or unsafe operation.

Together, the battery cells and the Battery Management System work as a team, allowing modern lithium-ion batteries to operate safely, efficiently and reliably for many years.

Why Lithium-Ion?

Many different battery technologies exist, but almost all modern residential solar systems use lithium-ion batteries.

Compared with older technologies such as lead-acid batteries, lithium-ion batteries store more energy in a smaller space, charge more quickly, last for thousands of charge cycles and require very little maintenance.

These advantages have made lithium-ion batteries the preferred choice for home energy storage around the world, and ongoing improvements continue to make them cheaper, safer and more efficient every year.

Looking Ahead

We've now discovered how solar energy can be used immediately or stored inside a battery for later.

Whether that electricity powers your home, charges a battery or is eventually exported to the grid, one thing remains true: it must always be controlled safely.

Electricity is an incredibly useful form of energy, but if a cable becomes damaged, a component fails or a fault develops, it can quickly become dangerous.

So how do engineers prevent electrical faults from becoming fires, damaged equipment or electric shock?

In the next chapter, we'll explore the protection systems built into every modern solar installation and discover how multiple layers of safety work together to keep both people and equipment safe.