Electric vehicle
Download 3.49 Mb. View original pdf
|
| Figure 3.2 Graph showing the change is amphour charge capacity of a nominally 42 Ah battery. This graph is based on measurements from a lead acid traction battery produced by Hawker Energy Products Inc. Example: Express the current 21 A from our example 42 Ah battery in ‘C ’ notation. As a ratio of 42 A, 21 is 1 / 2 or 0.5. Thus the current 21 A 0.5C 10 This way of expressing a battery current is very useful, as it relates the current to the size of the battery. It is almost universally used in the battery literature and specifications, though the subscript relating to the rated discharge time is often omitted. 3.2.3 Energy Stored The purpose of the battery is to store energy. The energy stored in a battery depends on its voltage and the charge stored. The SI unit is the joule, but this is an inconveniently small unit, and so we use the watthour instead. This is the energy equivalent of working at a power of 1 W for 1 hour. The watthour is equivalent to 3600 J. The watthour is compatible with our use of the amphour for charge, as it yields the simple formula Energy in watthours = voltage × amphours, or Energy = V × C (3.2) However, this equation must be used with great caution. We have noted that both the battery voltage V and even more so the ampere hour capacity C vary considerably depending on how the battery is used. Both are reduced if the current is increased and the battery is drained quickly. The stored energy is thus a rather variable quantity and reduces if the energy is released quickly. It is usually quoted inline with the amphour rating that Batteries, Flywheels and Supercapacitors 33 is, if the charge capacity is given fora hour discharge, then the energy should logically be given for this discharge rate. 3.2.4 Specific Energy Specific energy is the amount of electrical energy stored for every kilogram of battery mass. It has units of Wh kg. Once the energy capacity of the battery needed in a vehicle is known (watthours) it can be divided by the specific energy (Wh kg) to give a first approximation of the battery mass. Specific energies quoted can be no more than a guide, because, as we have seen, the energy stored in a battery varies considerably with factors such as temperature and discharge rate. We will see in Section 3.2.6, and in the Ragone plot of Figure 3.3, how much the specific energy of a battery can change. 3.2.5 Energy Density Energy density is the amount of electrical energy stored per cubic metre of battery volume. It normally has units of Wh m. It is also an important parameter as the energy capacity of the battery (Wh) can be divided by its energy density (Wh m) to show the volume of battery required. Alternatively, if a known volume is available for batteries, the volume (m) can be multiplied by the battery’s energy density (Wh m) to give a 1 100 10 1 10 2 200 20 Specific Energy/Wh.kg −1 Specific Power/W.kg − 1 Ragone plot for Lead Acid and Nickel Cadmium traction batteries lead acid nickel cadmium 50 Download 3.49 Mb. Share with your friends:
|