Batteries are made up of a number of cells, typically around 50.
Cells are normally Lithium Ion '18650s' which refers to their physical dimensions and shape. (See picture)
The orientation and connection of the cells determines the overall voltage of the battery.
The amount of power (Watt Hours) is denoted by the type and number of cells. Volts x ampHours = Watts Hours. A 100 Watt Hour battery can produce 100 Watts for 1 hour (at a given current output).
Watt hours is the only true measure of energy stored. Sometimes batteries are classed in amps, or amp hours. This is not the whole picture, a high amp battery with low voltage will hold less than a medium amp battery with high voltage.
All decent batteries have a BMS or Battery Management System. This includes temperature sensors to ensure the battery never overcharges, overheats, or is charged in too cold a temperature leading to damage.
One more thing - we still see some eBikes using Lead Acid batteries (like your car). They're not adequate for the task - avoid.
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