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Do electric cars stop charging when their batteries are full?
Nimit Arora
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Published on 7th Dec 21
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The most important part of an electric vehicle is the battery. Cost-wise, it constitutes almost 40% of the EV cost. A battery pack comprises of the lithium-ion cells that power the EV drivetrain & a ‘Battery Management System’.

Battery Management System:

Battery Management Systems (BMS) are used to control power storage systems, monitor the health of battery cells and deliver power to vehicle components. It manages stored power in an onboard battery and delivers power to the rest of the vehicle.

The main functions of a Battery Management System for electric vehicles are:

Cell Monitoring:

When charging or discharging, the battery cells need to be monitored at all times. Any malfunction must be identified & reported along with the triggering of the safety mechanism. At this stage, some algorithms run to calculate the state of charge (SOC) & state of health (SOH).

SOC is determined to ensure that the battery is never over or undercharged. SOC can also be seen as a fuel indicator of an electric vehicle as it indicates the energy remaining in the battery. SOH is an indicator of the overall health of the battery and gives an insight into the operating conditions of the battery.

Power Optimization:

The direct outcome of cell monitoring is the optimization of battery power. While the cell monitoring functionality determines SOC and SOH, the EV battery management system’s job is to keep the SOC and SOH parameters within the specified values.

Safety:

Safety is the primary objective of any BMS. An undetected thermal breakout can cause a major mishap. The BMS captures data such as voltage, temperature & current to optimize power flow. A similar set of data is also utilized for ensuring safety.Charging Optimization: The battery cells deteriorate over time. An intelligent EV BMS monitors this deterioration that results in changes in battery parameters such as voltage, current, etc. This reduces the stress on the overall battery pack and enhances its life.


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Overcharging a battery is the concept of leaving your electric car plugged in at 100% charge and then electrons continue to be pumped into the battery pack. This could lead to overheating and eventually damage. The worst-case is a battery fire. 

An intelligent battery management system effectively shuts down the charger once the battery is fully charged or has reached the desired level of charge. Some e-cars tell you that the battery is 100% charged and shuts charging when actually it's only 80%. This stops ‘dendrite’ growth on battery electrodes.

Once the BMS detects the battery is nearing a 100% charge, it slows down the charging process. When the battery reaches a 100% charge, it starts a process called ‘trickle charge’, which means the battery will be charged periodically at the same rate as its discharging. This maintains a full charge for the battery without overcharging. This is why it is completely safe to leave an electric car charging or plugged in overnight. Some high-end e-cars like TESLA Model 3 have a sentry mode, where motion sensors & cameras are always on and constantly drawing power. Here, the car continues to draw power even after its fully charged. This small loss of power is called phantom or vampire power. However, it is advised for all e-cars, to not let the battery sit at 100% charge level for more than 8 hours.

The battery management system maintains a 100% charge without overcharging the battery.

A BMS employs 2 important algorithms as well. These are explained below:

Cell Balancing Algorithm:

Different cells in a battery can develop different capacities with time. The cell balancing algorithm comes to the rescue in such circumstances. Two kinds of cell balancing techniques are deployed- active & passive cell balancing. In the active balancing technique, the stronger cells are used to charge the weaker cells so that their potential is equalized. Passive balancing works by discharging the excess voltage of the stronger cells so that charging can continue until all cells are fully charged.

Communication Algorithm:

Different units in an EV communicate with the battery in order to work efficiently. This communication is facilitated by the battery management system’s communication algorithms. The BMS communicates with an external charging device to pass on the required current and voltage information. It also controls when the charging starts and stops. If there is any technical issue, BMS cuts off the connection and stops the charging.

Conclusion:

An intelligent battery management system with innovative power-stage components is most likely to optimize the performance and lifespan of the vehicle’s battery while performing tasks to help validate the safety of the battery. Detailed and frequent monitoring of vital operating parameters, robust communications among all of the nodes within the system and fast decision making followed by effective control and protection mechanisms are essential in EV power systems.

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