A battery pack and battery management system (BMS) is a set of cells grouped into modules inside a case, together with an electronic board that watches every cell's voltage and temperature, balances the cells and stops overheating. A single cell gives only a few volts. A pack joins many cells so that a vehicle gets enough voltage and energy. How this is done is explained in battery cells, modules and packs.
The BMS is needed because cells differ slightly. One cell may charge a little faster, or heat up more, than the others. If it is overcharged or overheated, it can fail or catch fire. So the BMS measures every group of cells, many times a second. It also evens out the charge of the cells, which is called balancing. Most packs do this by draining a little energy from the fuller cells as heat. Some packs move energy from the fuller cells to the emptier ones instead. It also tells the charger how much current is safe, and disconnects the pack if something goes wrong.
Say a scooter pack is being fast-charged on a hot afternoon in Ahmedabad. One group of battery cells reaches 43 °C while the others are at 38 °C. The BMS asks the charger to reduce the current from 40 amperes to 20 amperes. If the group reached 45 °C, the BMS would open its switches and stop the charge.
The BMS also answers the question every rider asks: "how far can I go?" It cannot measure the charge in a cell directly. It estimates it from the voltage, the current, the temperature and how the pack behaved earlier. This estimate is called the state of charge. A second estimate, the state of health, tells how much capacity the pack has lost with age. After a number of scooter fires, India made its safety rules for electric vehicle battery packs stricter, and the BMS is a main part of meeting them.
The BMS passes its readings to a telematics device, which sends them to the cloud, where battery analytics studies them. A pack built for quick charging is a fast-charging battery. Its switches use power semiconductors. The pack powers the electric two-wheeler through the powertrain, and the same design is used for swappable batteries and for large battery energy storage systems.