23 Lead Acid Batteries

2.3.1 Lead acid battery basics

The best known and most widely used battery for electric vehicles is the lead acid battery. Lead acid batteries are widely used in IC engine vehicles and as such are well known. However for electric vehicles, more robust lead acid batteries that withstand deep cycling and use a gel rather than a liquid electrolyte are used. These batteries are more expensive to produce.

In the lead acid cells the negative plates have a spongy lead as their active material, whilst the positive plates have an active material of lead dioxide. The plates are immersed in an electrolyte of dilute sulphuric acid. The sulphuric acid combines with the lead and the lead oxide to produce lead sulphate and water, electrical energy being released during the process. The overall reaction is:

The reactions on each electrode of the battery are shown in Figure 2.4. In the upper part of the diagram the battery is discharging. Both electrode reactions result in the formation of lead sulphate. The electrolyte gradually loses the sulphuric acid, and becomes more dilute.

When being charged, as in the lower half of Figure 2.4, the electrodes revert to lead and lead dioxide. The electrolyte also recovers its sulphuric acid, and the concentration rises.

The lead acid battery is the most commonly used rechargeable battery in anything but the smallest of systems. The main reasons for this are that the main constituents (lead, sulphuric acid, a plastic container) are not expensive, that it performs reliably, and that it has a comparatively high voltage of about 2 V per cell. The overall characteristics of the battery are given in Table 2.1.

One of the most notable features of the lead acid battery is its extremely low internal resistance (see Section 2.2.1 and Figure 2.1). This means that the fall in voltage as current is drawn is remarkably small, probably smaller than for any of the candidate vehicle batteries. The figure given in Table 2.1 below is for a single cell, of nominal capacity 1.0 amphours. The capacity of a cell is approximately proportional to the area of the plates, and the internal resistance is approximately inversely proportional to the plate area. The result is that the internal resistance is, to a good approximation, inversely proportional to the capacity. The figure given in Table 2.1 of 0.022 Q per cell is a rule of thumb figure taken from a range of good quality traction batteries. A good estimate of the internal resistance of a lead acid battery is thus:

Positive electrode changes from lead to lead sulphate

Positive electrode changes from lead to lead sulphate

Negative electrode changes from lead peroxide to lead sulphate Electrons flow round the external circuit

Negative electrode changes from lead peroxide to lead sulphate Electrons flow round the external circuit

Reactions during the discharge of the lead acid battery. Note that the electrolyte loses suphuric acid and gains water.

Positive electrode changes back from lead sulphate to lead.

Positive electrode changes back from lead sulphate to lead.

Figure 2.4 The reactions during the charge and discharge of the lead acid battery

Reaction during the charging of the lead acid battery. Note that the electrolyte suphuric acid concentration increases.

Figure 2.4 The reactions during the charge and discharge of the lead acid battery

Table 2.1 Nominal battery parameters for lead acid batteries

Specific energy

20-35 Wh.kg 1 depending on usage

Energy density

54-95 Wh.L-1

Specific power

~250W.kg-1 before efficiency falls very greatly

Nominal cell voltage

2 V

Amphour efficiency

~80%, varies with rate of discharge & temp.

Internal resistance

Extremely low, ~0.022 ^ per cell for 1 Amphour cell

Commercially available

Readily available from several manufacturers

Operating temperature

Ambient, poor performance in extreme cold

Self-discharge

~2% per day, but see text below

Number of life cycles

Up to 800 to 80% capacity

Recharge time

8 h (but 90% recharge in 1 h possible)

The number of cells is the nominal battery voltage divided by 2.0, six in the case of a 12 V battery. C10 is the Amphour capacity at the 10 hour rate.

2.3.2 Special characteristics of lead acid batteries

Unfortunately the lead acid battery reactions shown in Figure 2.4 are not the only ones that occur. The lead and lead dioxide are not stable in sulphuric acid, and decompose, albeit very slowly, with the reactions:

At the positive electrode 2PbO2 + 2H2SO4-> 2PbSO4 + 2H2O + O2 (2.4)

and at the negative Pb + H2SO4-> PbSO4 + H2 (2.5)

This results in the self-discharge of the battery. The rate at which these reactions occur depends on the temperature of the cell: faster if hotter. It also depends on other factors, such as the purity of the components (hence quality) and the precise alloys used to make up the electrode supports.

These unwanted reactions, that also produce hydrogen and oxygen gas, also occur while the battery is discharging. In fact they occur faster if the battery is discharged faster, due to lower voltage, higher temperature, and higher electrode activity. This results in the 'lost charge' effect that occurs when a battery is discharged more quickly, and which was noted in Section 2.2.2 above. It is a further unfortunate fact that these discharge reactions will not occur at exactly the same rate in all the cells, and thus some cells will become more discharged than others. This has very important consequences for the way batteries are charged, as explained below. But, in brief, it means that some cells will have to tolerate being 'over-charged' to make sure all the cells become charged.

The reactions that occur in the lead acid battery when it is being over-charged are shown in Figure 2.5. These gassing reactions occur when there is no more lead sulphate on the electrodes to give up or accept the electrons. They thus occur when the battery is fully or nearly fully charged.

We have noted that the charging and discharging reactions (as in Figure 2.4) involve changing the concentration of the electrolyte of the cells. The change in concentration of the reactants means that there is a small change in the voltage produced by the cell as it discharges. This decline in voltage is illustrated in Figure 2.6. For the modern sealed batteries the change is linear to quite a good approximation. It should be noted that this battery voltage cannot normally be used to give an indication of the state of charge of the battery. Is not normally possible to measure this open circuit voltage when the battery is in use, and in any case it is also greatly affected by temperature, and so a chance measurement of the battery voltage is likely to be strongly affected by other factors.

A notable feature of the overcharge reactions of Figure 2.5 and the self-discharge reactions of equations (2.4) and (2.5), is that water is lost and turned into hydrogen and oxygen. In older battery designs this gas was vented out and lost, and the electrolyte had to be topped up from time to time with water. In modern sealed batteries this is not necessary or even possible. The gases are trapped in the battery, and allowed to recombine (which happens at a reasonable rate spontaneously) to reform as water. Clearly there is

Hydrogen formed at positive electrode

Hydrogen formed at positive electrode

Oxygen formed at negative electrode

Figure 2.5 The gassing reactions that occur when the lead acid battery is fully charged

Oxygen formed at negative electrode

Figure 2.5 The gassing reactions that occur when the lead acid battery is fully charged

0.00

1.00

Depth of discharge

Figure 2.6 Graph showing how the open circuit voltage of a sealed lead acid battery changes with state of charge

0.00

1.00

Depth of discharge

Figure 2.6 Graph showing how the open circuit voltage of a sealed lead acid battery changes with state of charge a limit to the rate at which this can happen, and steps must be taken to make sure gas is not produced too rapidly. This is dealt with in the sections that follow.

Manufacturers of lead acid batteries can supply them in a wide range of heights, widths and lengths, so that for a given required volume they can be fairly accommodating. However, a problem with the wide use of lead acid batteries is that different designs are made for different applications, and it is essential to use the correct type. The type of battery used for the conventional car, the so-called starting, lighting and ignition (SLI)

battery, is totally unsuitable for electric vehicle applications. Other lead acid batteries are designed for occasional use in emergency lighting and alarms; these are also totally unsuitable. The difference in manufacture is dealt with by authors such as Vincent and Scrosati (1998). It is only batteries of the 'traction' or 'deep cycling' type that are suitable here. This is the most expensive type of lead acid battery.

2.3.3 Battery life and maintenance

We have seen that gassing reactions occur within the lead acid battery, leading to loss of electrolyte. Traditional acid batteries require topping up with distilled water from time to time, but modern vehicle lead acid batteries are sealed to prevent electrode loss. In addition the electrolyte is a gel, rather than liquid. This means that maintenance of the electrolyte is no longer needed. However, the sealing of the battery is not total; there is a valve which releases gas at a certain pressure, and if this happens the water loss will be permanent and irreplaceable. This feature is a safety requirement, and leads to the name valve regulated sealed lead acid battery (VRLA) for this modern type of battery. Such a build up of gas will result if the reactions of Figure 2.5, which occur on overcharge, proceed too fast. This will happen if the charging voltage is too high. Clearly this must not be allowed to happen, or the battery will be damaged. On the positive side, it means that such batteries are essentially maintenance-free.

However, this does not mean that the batteries will last forever. Even if there is no water loss, lead acid batteries are subject to many effects that shorten their life. One of the most well known is the process called sulphation. This occurs if the battery is left for a long period (i.e. two weeks or more) in a discharged state. The lead sulphate (see Figure 2.4) on the electrodes forms into larger crystals, which are harder to convert back into lead or lead dioxide, and which form an insulating layer over the surface of the electrodes. By slowly recharging the battery this can sometimes be partially reversed, but often it cannot.

Making sure the battery is always kept in a good state of charge can prevent the problem of sulphation, and this is explained in Section 2.8. Section 2.8 also explains the very important issue of charge equalisation - getting this wrong is a major cause of battery failure. Other problems cannot be prevented however much care is taken. Within the electrodes of the battery corrosion reactions take place, which increase the electrical resistance of the contacts between the active materials that the electrode supports. The active materials will gradually form into larger and larger crystals, which will reduce the surface area, reducing both the capacity of the battery and slowing down the rate of reaction. The effects of vibration and the continual change of size of the active materials during the charge/discharge cycles (see Figure 2.4) will gradually dislodge them. As a result they will not make such good electrical contact with their support, and some will even fall off and become completely detached.

All these problems mean that the life of the lead acid battery is limited to around 700 cycles, though this strongly depends on the depth of the cycles. Experience with industrial trucks (fork-lifts, luggage carriers at railway stations, etc.) suggests that service lives of 1200-1500 cycles are possible, over 7-8 years. Fleet experience with electric cars indicates a life of about 5 years or 700 cycles. The shorter life of the road vehicles is the result of much greater battery load, the battery typically being discharged in about two hours, as opposed to the 7-8 hours for industrial trucks (Bosch 2000).

2.3.4 Battery charging

Charging a lead acid battery is a complex procedure and, as with any battery, if carried out incorrectly it will quickly ruin the battery and decrease its life. As we have seen, the charging must not be carried out at too high a voltage, or water loss results.

There are differing views on the best way of charging lead acid batteries and it is essential that, once a battery is chosen, the manufacturer's advice is sought.

The most commonly used technique for lead acid batteries is called multiple step charging. In this method the battery is charged until the cell voltage is raised to a predetermined level. The current is then switched off and the cell voltage is allowed to decay to another predetermined level and the current is then switched on again. A problem is that the predetermined voltages vary depending on the battery type, but also on the temperature. However, the lead acid battery is used in so many applications, that suitable good quality chargers are available from a wide range of suppliers.

An important point that applies to all battery types relates to the process of charge equalisation that must be done in all batteries at regular intervals if serious damage is not to result. It is especially important for lead acid batteries. This is fully explained below in Section 2.8, after all the main battery types have been described.

2.3.5 Summary of lead acid batteries

Lead acid batteries are well established commercially with good backup from industry. They are the cheapest rechargeable batteries per kilowatt-hour of charge, and will remain so for the foreseeable future. However, they have low specific energy and it is hard to see how a long-range vehicle can be designed using a lead acid battery. Lead acid will undoubtedly continue for some considerable time to be widely used for short-range vehicles. Lead acid batteries have a greater range of efficient specific powers than many other types (see Figure 2.3) and so they are very much in contention in hybrid electric vehicles, where only a limited amount of energy is stored, but it should be taken in and given out quickly.

Hybrid Cars The Whole Truth Revealed

Hybrid Cars The Whole Truth Revealed

Hybrid Cars! Man! Is that a HOT topic right now! There are some good reasons why hybrids are so hot. If you’ve pulled your present car or SUV or truck up next to a gas pumpand inserted the nozzle, you know exactly what I mean! I written this book to give you some basic information on some things<br />you may have been wondering about.

Get My Free Ebook


Post a comment