As we just made the preorders for the bigger battery version of the SIERRA, the SIERRA MAX available, today appeared as a great opportunity to thoroughly explain the fundamental principles and physics behind Lithium-ion batteries and why they currently are the state-of-the-art in electric bikes. When looking at batteries, you basically want to maximise stored energy while minimising volume, weight, and price. As a society, we have been primarily using fossil fuels for personal transportation. This is because this type of power source is really good at storing a lot of energy in a small volume and with low weight while remaining affordable. Now that these chemicals are slowly but surely running out, and we witness how impairing they are to our sustainable development, cleaner options such as rechargeable electric batteries are progressively becoming the norm.
There are many types of those: Nickel-Cadmium, Lead-acid, Silver-Zinc, Lithium-Ion, etc. The following graph produced by NASA shows that Lithium-Ion chemistry has the highest energy density and is consequently the best choice to store electrical energy in small volumes and low weights.
While still 60 times less energy-dense than gasoline, Lithium-ion cells are the best available alternative to store electric power safely and economically in personal vehicles.
The following diagram simplifies the discharge mechanism in a Lithium-ion battery, which is tantamount to the motor pulling power from it. Positive Lithium-ions travel across an electrolyte and a separator from an anode towards a cathode, which is two dissimilar conducting materials. The cathode then becomes more positively electrically charged than the anode. This creates a voltage between the anode and the cathode, which is essentially a driving force that pushes electrons between two points. The higher the voltage, the higher the force. This can be visualised as a waterfall. The taller the waterfall, the greater the force driving the water from its top to its bottom.
This voltage drives electrons across the electromechanical systems of your ebike. They leave the anode to go through the controller, followed by the motor and back to the cathode. As electrons move across the system, the voltage of the battery goes down. The charging process is the exact opposite. A voltage is applied across the cathode and the anode which forces electrons to move in the opposite direction and the Lithium-ions to go back towards the anode. This restores the driving voltage between the anode and the cathode for later discharge.
These chemical processes are contained within cells. Today most cells, whether in laptops, cars or ebikes, are cylindrical pieces of metal. This allows to contain pressure better and increase batteries’ safety and resilience to their environment. These cells usually provide a nominal voltage of 3.6V. This means that they reach 4.2V when fully charged and progressively go down to a safe minimum of 2.5V during discharge after which a battery management system shuts your battery down to preserve its capacity and health.
Lithium-ion cells have to be connected and assembled to build a battery with a total voltage that is enough to drive its target electro-mechanical systems; the controller, motor and screen in our case. Individual cells are packed and connected in Series to increase the total battery voltage to the required value (for example, 36V). Once this value is reached, these series of cells are packed in parallel to reach the total battery capacity, the value usually quoted in Ah. An average cell packs 3.6V and 2.5Ah, hence a battery producing 10Ah at 36V most likely consists of 4 groups of ten cells connected in series, which are then connected in parallel. I know that this is tough to get your head around but the following diagrams, courtesy of Cadex, should help you understand:
If you have any questions, do not hesitate to ask in the comments!