24 Nickelbased batteries

2.4.1 Introduction

A range of commercial batteries using nickel in the positive electrode have been developed since Edison's work in the late 19th century. These batteries include nickel iron, nickel zinc, nickel cadmium and nickel metal hydride batteries. Two of these batteries are discussed below, the nickel metal hydride battery showing the most promise. The nickel zinc battery has a reasonable performance but it has a very limited life of 300 deep cycles and is not discussed further. The nickel/iron battery is also rarely used.

2.4.2 Nickel cadmium

The nickel cadmium battery was considered to be one of the main competitors to the lead acid battery for use in electric vehicles and these batteries have nearly twice the specific energy of lead acid batteries.

The NiCad battery uses nickel oxyhydroxide for the positive electrode and metallic cadmium for the negative electrode. Electric energy is obtained from the following reaction.

The reactions at each electrode, which also help to explain 'where the electrons come from' and how the battery works, are shown in Figure 2.7. This battery makes an interesting comparison with the lead acid in that here the electrolyte becomes more concentrated as the cell discharges.

Nickel cadmium batteries have been widely used in many appliances, including use in electric vehicles. The NiCad battery has advantages of high specific power, a long life cycle (up to 2500 cycles), a wide range of operating temperatures from —40°C to +80°C, a low self-discharge and good long term storage. This is because the battery is a very stable system, with equivalent reactions to the self-discharge of the lead acid battery (equations (2.4) and (2.5)) only taking place very slowly. The NiCad batteries can be purchased in a range of sizes and shapes, though they are not easy to obtain in the larger sizes required for electric vehicles, their main market being portable tools and electronic equipment. They are also very robust both mechanically and electrically and can be recharged within an hour and up to 60% capacity in 20 minutes.

On the negative side, the operating voltage of each cell is only about 1.2 V, so 10 cells are needed in each nominally 12 V battery, compared to 6 cells for lead acid. This partly explains the higher cost of this type of battery. A further problem is that the cost of cadmium is several times that of lead, and that this is not likely to change. Cadmium is also environmentally harmful and carcinogenic.

Electrons flow round

Reactions during the discharge of the NiCd battery. the external circuit

Note that the electrolyte loses water, becoming more concentrated..

Figure 2.7 Reactions during the discharge of a nickel cadmium cell. The reactions are reversed during charge

Electrons flow round

Reactions during the discharge of the NiCd battery. the external circuit

Note that the electrolyte loses water, becoming more concentrated..

Figure 2.7 Reactions during the discharge of a nickel cadmium cell. The reactions are reversed during charge

Table 2.2 Nominal battery parameters for nickel cadmium batteries

Specific energy

40-55 Wh.kg—1 depending on current (see Figure 2.3)

Energy density

70-90 Wh.L—1 depending on current

Specific power

~125W.kg—1 before becoming very inefficient

Nominal cell voltage

1.2V

Amphour efficiency

Good

Internal resistance

Very low, ~0.06 ß per cell for a 1 Amphour cell

Commercially available

Good in smaller sizes, difficult for larger batteries

Operating temperature

—40°C to +80°C

Self-discharge

0.5% per day, very low

Number of life cycles

1200 to 80% capacity

Recharge time

1 h, rapid charge to 60% capacity 20 mins

The high cost of NiCad batteries, typically 3 times that of lead acid, is offset to an extent by its longer cycle life. Its charge efficiency decreases rapidly over 35°C but this is unlikely to affect its use in electric vehicles. It has been used successfully in cars such as electric versions of the Peugeot 106, the Citroen AX and the Renault Clio, as well as the Ford Think® car shown in Figure 1.6. The overall characteristics of the battery are given in Table 2.2

As with lead acid batteries, NiCad batteries need to be properly charged. The points made in Section 2.8 below apply to this type of battery as well. However, because NiCad cells are less prone to self-discharge, the problem raised there is not so great as with lead acid. Normally the battery is charged at a constant current until its cell voltages reach a predetermined level, at which point the current is switched off. At this point the cell voltages decay to a lower predetermined voltage and the current is switched back on. This process is continued until the battery is recharged. A good proportion of the charge can be normally be replaced within 1 hour, but as explained in Section 2.8 the cell must be run at a fairly low current, with most of the cells being overcharged, for a 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:

The resulting free oxygen diffuses to the negative electrode, where it reacts with the cadmium, producing cadmium hydroxide, using the water produced by reaction (2.6).

As well as this reaction, the normal charging reaction will be taking place at this electrode, using the electrodes produced by reaction (2.4).

Comparing reactions (2.7) and (2.8), 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 (2.6), (2.7) and (2.8) is no effect. This overcharging situation can thus continue indefinitely. For most NiCad batteries their size and design allows this to continue forever at the C/10 rate, i.e. at 10 A for a 100 Amphour 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 nickel cadmium battery is very low, it is not as low as for the lead acid battery. This results in a somewhat lower maximum economic specific power. The empirical, good 'first approximation' formula2 for the internal resistance of a nickel cadmium battery is:

0.06

Comparing this with equation (2.3) 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.

2.4.3 Nickel metal hydride batteries

The nickel metal hydride (NiMH) battery was introduced commercially in the last decade of the 20th 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 nickel cadmium cell; the nickel oxyhydroxide 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 2.8.

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 hydrogen stores used in conjunction with fuel cells, and described in more detail in Section 5.3.5. The basic principle is a reversible reaction in which hydrogen is bonded to the metal, and then released as free hydrogen when required. For this to work the cell must be sealed, as an important driver in the absorption/desorption process is the pressure of the hydrogen gas, which is maintained at a fairly constant value. A further important point about the sealing is that the hydrogen-absorbing alloys will be damaged if air is allowed into the cell. This is because they will react with the air, and other molecules will occupy the sites used to store the hydrogen.

2 The factor 0.06 in this formula is based on measurements from a small sample of good quality NiCad traction batteries.

Metal alloy "sponge" that absorbs, and then

Metal alloy "sponge" that absorbs, and then

the external circuit

Figure 2.8 The reactions during the discharge of the nickel metal hydride cell. When charged the reactions are reversed. Note that when both discharging and charging water is created at exactly the same rate at which it is used, and that therefore the electrolyte does not change with state of charge the external circuit

Figure 2.8 The reactions during the discharge of the nickel metal hydride cell. When charged the reactions are reversed. Note that when both discharging and charging water is created at exactly the same rate at which it is used, and that therefore the electrolyte does not change with state of charge

The overall chemical reaction for the NiMH battery is written as:

In terms of energy density and power density the metal hydride cell is somewhat better than the NiCad battery. Ni/MH batteries have a nominal specific energy of about 65 Wh.kg-1 and a nominal energy density of 150 Wh.L-1 and a maximum specific power of about 200 W.kg-1. Table 2.3 gives this and other information about this class of battery. In most respects its performance is similar to, or a little better than that for the nickel cadmium cell. The nominal cell voltage is 1.2 V.

One area where the NiMH is better than the NiCad is that it is possible to charge the battery somewhat faster. Indeed, it can be charged so fast that cooling becomes necessary.

Table 2.3 Nominal battery parameters for nickel metal hydride batteries

Specific energy

~65 Wh.kg 1 depending on power

Energy density

-150 Wh.L-1

Specific power

200 W.kg-1

Nominal cell voltage

1.2V

Amphour efficiency

Quite good

Internal resistance

Very low, ~0.06 ß per cell for a 1 Amphour cell

Commercially available

A good range of small cells, traction batteries

difficult to obtain

Operating temperature

Ambient

Self-discharge

Poor, up to 5% per day

Number of life cycles

-1000 to 80% discharge

Recharge time

1 h, rapid charge to 60% capacity 20 mins

As well as heat energy being created by the normal internal resistance of the battery, the reaction in which hydrogen is bonded to the metal adjacent to the negative electrode is quite strongly exothermic. Unless the vehicle is a cycle or scooter, with a small battery, a cooling system is an important feature of NiMH battery systems. They are available commercially in small sizes, but larger batteries suitable for electric vehicles are beginning to appear. An example of a commercial NiMH battery is shown in Figure 2.9. Notice that the battery has cooling fans fitted as an integral part of the battery casing, for the reason explained above.

The NiMH battery has slightly higher energy storage capacity than NiCad systems, and is also a little more costly. There is one area where its performance is notably worse than that for NiCad batteries, and that its self-discharge properties. Hydrogen molecules are very small, and they can reasonably easily diffuse through the electrolyte to the positive electrode, where it will react:

This effectively discharges the cell; hydrogen is lost from the negative and nickel hydroxide is formed at the positive. The result is that this battery is subject to quite rapid self-discharge.

An interesting feature of the cell, which can be seen by reference to Figure 2.8, is that the composition of the electrolyte does not change during charge or discharge; water and OH- ions are created and used at exactly the same rate. The result is that the internal resistance and open circuit voltage of the cell are much more constant during discharge than with either lead acid or NiCad batteries. Being backed by a metal layer, the internal resistance is also a little lower, but it is not greatly different.

The charging regime is similar to that of the NiCad battery, the current being switched on and off to keep the cell voltage between an upper and a lower limit. Like NiCad batteries the NiMH battery can be charged within 1 hour. Most cells can cope with an overcharge current of about 0.1 C, like the NiCad cell. As will be explained in Section 2.8, overcharging is necessary in a battery to make sure each and every cell is fully charged.

Of all the new battery systems NiMH is considered to be one of the most advanced and has been used in a range of vehicles including the Toyota Prius, which has been by far the most successful electric hybrid to date. The market volume of NiMH batteries is still small, but with quantity production the price will drop. This battery is considered to be one of the most promising for the future.

DIY Battery Repair

DIY Battery Repair

You can now recondition your old batteries at home and bring them back to 100 percent of their working condition. This guide will enable you to revive All NiCd batteries regardless of brand and battery volt. It will give you the required information on how to re-energize and revive your NiCd batteries through the RVD process, charging method and charging guidelines.

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