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Storage for Electric Vehicles - Lesson Summary

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The battery in an electric vehicle is composed of cells that are used as a storage tank. In internal combustion engine vehicles, the storage tank is used to store petrol, and the tank itself does not cost very much. In EVs, the battery is a complex unit, it is a storage of electricity, and its costs are high. Also, the battery unit eventually has to be replaced, while a petrol tank will last the life of the vehicle, and the batter unit is much heavier than a petrol tank. The benefit of a battery unit is that it is 4 to 6 times more energy-efficient than petrol and the cost of petrol per km is much higher than the cost of electricity per km.


Consider a Battery of 48 Volts with a Capacity (C) of 15kWh
Battery Capacity can also be defined by Ampere-hour (Ah): Ah= 1500Wh/48V = 300 Ah
or Battery Capacity (C) Battery Voltage * Battery Ah

The State of Charge (SoC) of the battery is a measurement of the percentage of battery charged.

A C-rate is a measure of the rate at which a battery is discharged relative to its maximum capacity. A 1C rate means that the discharge current will discharge the entire battery or charge the battery in 1 hour.

A battery reaches the end of its life when it reaches between 70 and 80 percent of its initial Capacity (C) and it can no longer be used for the vehicle because the range of the vehicle decreases. But the battery can have a second life and be used in other applications.

The parameter that makes the biggest difference when selecting what type of battery to use is called Gravitational Energy Density or Specific Energy Density, and it refers to the amount of energy stored in a battery per unit of mass, expressed in kilowatts per kilogram. Lead-Acid batteries used to be very popular until Lithium-Ion Battery Cells emerged. Their increasing energy density and lower prices made it the dominant rechargeable battery of today. Even with emerging new technology, it is very likely that we will continue to have Lithium-Ion Batteries for at least 20 more years.

There are three kinds of containers for battery cells, they are cylindrical, pouch, and prismatic.

• Cylindrical cells have a higher energy density than pouch and prismatic cells but their number of cycles is usually less. The 18650 Model (18 mm diameter and 65 mm length) has a solid body without terminals with a capacity of 2.2/3.8 Ah at 3.7 volts. The 26650 Model has a larger diameter (26mm) and large threaded terminals.
• Pouch Cells have soft, flat bodies, 90 to 95 percent packing efficiency, and can be laser or ultrasonic welded. They are available at 24, 44, 50, or higher Ah. Used in larger vehicles and the cost to use is higher than cylindrical cells.
• Prismatic Cells have a semi-hard plastic case with large threaded terminals. There are cells from 7Ah, 15Ah, 24Ah, 30Ah, 40Ah, 44Ah, and 50Ah. Used in two and three-wheel vehicles. Can be spot-welded, laser welded, and ultrasonic welded.


The State of Charge (SoC) of a battery is the percentage at which the battery is charged. For example, an SoC of 70% means the battery is 70% charged and 30% discharged.

There are two methods we can use to measure the SoC of a battery.

• The first is the Voltage Method, where we obtain the Open Circuit Cell Voltage (OCV) vs the SoC in a lab at a very low charging rate (C/25 to C/100) for different temperatures. Using this method, the SoC is a non-linear function of open-circuit voltage.
• The other method is known as Coulomb Counting, which is very accurate dependent on accurate SoH (State of Health) and precision of current measurement. Measuring the current (total Coulombs) flowing in and out of the battery gives on a change is Soc if the SoH and the initial Capacity of the battery are known.

A number of cells assembled together make up a battery pack. If you take 10 cells each at 4.8V to make a pack, you have a 48V battery pack.

There are two ways to ways to build a battery pack, mSnP, and nPmS.

• In an mSnP battery pack, the cells are connected in series to form a String. Strings can then be connected in parallel to increase capacity.
• In an nPmS battery pack, the cells are connected in parallel to form Modules. These modules can then be connected in series to make a battery of higher voltage.