Batteries work by storing energy and releasing it when an electrical circuit is completed. The energy can be harnessed and put to work to create light, heat or movement. This energy is more commonly called electricity.
When you press the ‘on’ switch of a cordless power tool you complete the electrical circuit which allows the power to flow from the battery into the tool and causes, for instance, the drill bit to turn.
The battery can only store a limited amount of power and when it runs out it needs to be recharged using a charger. The charger uses electricity from the mains to refill the battery with energy and it’s ready to go again.
If you are interested in the chemistry that makes all this work, read on!
A rechargeable battery designed for cordless power tools is made up of multiple battery ‘cells’ and is known as a battery pack. The more cells, the more work the battery can do before running down.
Inside each cell there is an anode, a cathode and an electrolyte. An anode and cathode, known collectively as the ‘electrodes’, are made from materials that react if they are put together. The electrolyte is a liquid or moist paste that separates the electrodes from each other.
Everything in the world is made up of tiny molecules which interact based on their electrical charge (positive, negative or neutral). In order to understand the battery, we need to look at how the molecules in the electrodes interact with each other.
A molecule is made up of one or more atoms which are the smallest building blocks.
Each atom has a ‘nucleus’ at its centre which contains neutrons and protons. Whizzing around the nucleus are electrons. Neutrons have a neutral charge, protons have a positive charge and electrons have a negative charge. The balance between the charges dictates the overall charge of the atom and the balance between the atoms in a molecule dictates the overall charge of the molecule.
Each molecule wants to become neutral. The only way they can do this is by losing or gaining electrons. If they have an overall positive charge they attract electrons, if they have an overall negative charge, they lose electrons.
The anode molecules have a neutral charge until they react with the electrolyte which causes electrons to be released (known as an ‘oxidation reaction’) and positive ions (charged molecules) to form.
These ‘free’ electrons build up in the anode, turning the anode negative.
The cathode molecules also have a neutral charge until they start reacting with the electrolyte which uses up free electrons and forms negative ions (known as a reduction reaction).
The consumption of free electrons causes the cathode to become increasingly positive until there are no electrons to use up.
The anode is now repelling electrons and the cathode is demanding them but, if the circuit is incomplete, the free electrons in the anode cannot cross over to the cathode because they cannot pass through the electrolyte.
When the circuit it completed, the free electrons can flow through the conductive wire away from the anode and towards the cathode. As they pass through the tool, the energy they are carrying can be used to perform ‘work’ such as turning a drill bit in a cordless power drill.
When they reach the cathode they supply the electrons to continue the reduction reaction, producing even more negative ions as electrons are added.
Meanwhile, in the anode, the loss of electrons is producing even more positive ions which are being attracted to the negative ions in the cathode, so the positive ions start moving through the electrolyte and mixing in with the negative ions in the cathode.
Once all of the positive ions have moved across to the cathode and there are no more free electrons left, the battery stops working properly and needs to be recharged.
Chargers pass a voltage that is higher than the battery voltage through the discharged battery. This causes the reactions in the battery to reverse.
The input of electricity from the charger forces the electrons in the cathode back along the circuit towards the anode. As the anode becomes increasingly negative due to all the electrons, the positive anode ions start leaving the cathode and moving through the electrolyte back to the anode where they join to the free electrons and become neutral again.