Electric vehicle
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| 43 Table 3.2 Nominal battery parameters for nickel cadmium batteries Specific energy Wh kg −1 depending on current Energy density Wh l −1 depending on current Specific power W kg −1 before becoming very inefficient Nominal cell voltage V Amphour efficiency Good Internal resistance Very low, ∼0.06 per cell fora Ah cell Commercially available Good in smaller sizes, difficult for larger batteries Operating temperature to +80 ◦ C Self-discharge 0.5% per day, very low Number of life cycles to 80% capacity Recharge time h, rapid charge to 60% capacity 20 min can be normally be replaced within 1 hour, but as explained in Section 3.8, the cell must be run at a fairly low current, with most of the cells being overcharged, fora longer time. Alternatively the battery can be recharged at a lower, constant current – this is a simpler system, but takes longer. A clever feature of the NiCad battery is the way that it copes with overcharging. The cell is made so that there is a surplus of cadmium hydroxide in the negative electrode. This means that the positive electrode will always be fully charged first. A continuation of the charging current results in the generation of oxygen at the positive electrode via the reaction OH 2 HO+ O+ 4 e − (3.6) The resulting free oxygen diffuses to the negative electrode, where it reacts with the cadmium, producing cadmium hydroxide, using the water produced by reaction O+ 2 Cd + 2 HO 2 Cd(OH) 2 (3.7) As well as this reaction, the normal charging reaction will betaking place at this electrode, using the electrodes produced by reaction 3.4: 2 Cd(OH ) 2 + 4 e 2 Cd + 4 OH − (3.8) Comparing reactions 3.7 and 3.8 we see that the rate of production of cadmium hydroxide is exactly equal to its rate of conversion back to cadmium. We thus have a perfectly sustainable system, with no net use of any material from the battery. The sum total of the reactions 3.6–3.8 is no effect . This overcharging situation can thus continue indefinitely. For most NiCad batteries their size and design allow this to continue forever at the C 10 rate, that is at 10 A fora Ah battery. Of course this overcharging current represents a waste of energy, but it is not doing any harm to the battery, and is necessary in some cells while charging the battery in the final phase to equalise all the cells to fully charged. It should be noted that although the internal resistance of the NiCad battery is very low, it is not as low as for the lead acid battery. This results in a somewhat lower 44 Electric Vehicle Technology Explained, Second Edition maximum economic specific power. The empirical, good ‘first-approximation’ formula 2 for the internal resistance of a NiCad battery is R = no. of cells × 0 .06 C 3 (3.9) Comparing this with Equation (3.3) for the lead acid cell, it can be seen that there is a higher number (0.06 instead of 0.022). Also the number of cells will be greater, as has already been explained. 3.4.3 Nickel Metal Hydride Batteries The NiMH battery was introduced commercially in the last decade of the twentieth century. It has a similar performance to the NiCad battery, the main difference being that in the NiMH battery the negative electrode uses hydrogen, absorbed in a metal hydride, which makes it free from cadmium, a considerable advantage. An interesting feature of this battery type is that the negative electrode behaves exactly like a fuel cell, an energy source we consider more fully in the next chapter. The reaction at the positive electrode is the same as for the NiCad cell the nickel oxy- hydroxide becomes nickel hydroxide during discharge. At the negative electrode hydrogen is released from the metal to which it was temporarily attached, and reacts, producing water and electrons. The reactions at each electrode are shown in Figure The metals that are used to hold the hydrogen are alloys, whose formulation is usually proprietary. The principle of their operation is exactly the same as in the metal hydride Electrons flow round the external circuit Download 3.49 Mb. Share with your friends:
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