28 Battery Charging

2.8.1 Battery chargers

The issue of charging batteries is of the utmost importance for maintaining batteries in good order and preventing premature failure. We have already seen, for example, how leaving a lead acid battery in a low state of charge can cause permanent damage through the process of sulphation. However, charging them improperly can also very easily damage batteries.

Charging a modern vehicle battery is not a simple matter of providing a constant voltage or current through the battery, but requires very careful control of current and voltage. The best approach for the designer is to buy commercial charging equipment from the battery manufacturer or another reputed battery charger manufacturer. When the vehicle is to be charged in different places where correct charging equipment is not available, the option of a modern light onboard charger should be considered.

Except in the case of photoelectric panels, the energy for recharging a battery will nearly always come from an alternating current (AC) source such as the mains. This will need to be rectified to direct current (DC) for charging the battery. The rectified DC must have very little ripple, it must be very well 'smoothed'. This is because at the times when the variation of the DC voltage goes below the battery voltage, no charging will take place, and at the 'high point' of the ripple it is possible that the voltage could be high enough to damage the battery. The higher the DC current, the harder it is for rectifiers to produce a smooth DC output, which means that the rectifying and smoothing circuits of battery chargers are often quite expensive, especially for high current chargers. For example, the battery charger for the important development vehicle, the General Motors EV1, cost about $2000 in 1996 (Shnayerson 1996).

One important issue relating to battery chargers is the provision of facilities for charging vehicles in public places such as car parks. Some cities in Europe, especially (for example) La Rochelle in France, and several in California in the USA, provide such units. A major problem is that of standardisation, making sure that all electric vehicles can safely connect to all such units. Recently the Californian Air Resources Board, which regulates such matters there, has produced guidelines, which are described elsewhere (Sweigert et al. 2001). This paper also gives a good outline of the different ways in which these car-to-charger connections can be made.

However, the great majority of electric vehicles, such as bicycles, mobility aids, delivery vehicles and the like, will always use one charger, which will be designed specifically for the battery on that vehicle. On hybrid electric vehicles too, the charger is the alternator on the engine, and the charging will be controlled by the vehicle's energy management system. However, whatever charging method is used, with whatever type of battery, the importance of 'charge equalisation' in batteries must be understood. This is explained in the following section.

2.8.2 Charge equalisation

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.

A problem with all batteries is that when current is drawn not all the individual cells in the battery lose the same amount of charge. Since a battery is a collection of cells connected in series, this may at first seem wrong; after all, exactly the same current flows through them all. However, it does not occur because of different currents (the electric current is indeed the same) it occurs because the self-discharge effects we have noted (e.g. equations (2.4) and (2.5) in the case of lead acid batteries) take place at different rates in different cells. This is because of manufacturing variations, and also because of changes in temperature; the cells in a battery will not all be at exactly the same temperature.

The result is that if nominally 50% of the charge is taken from a battery, then some cells will have lost only a little more than this, say 52%, while some may have lost considerably more, say 60%. If the battery is recharged with enough for the good cell, then the cells more prone to self-discharge will not be fully re-charged. The effect of doing this repeatedly is shown in Table 2.9.

Cell A cycles between about 20% and 80% charged, which is perfectly satisfactory. However, Cell B sinks lower and lower, and eventually fails after a fairly small number

Table 2.9 Showing the state of charge of two different cells in a battery. Cell A is a good quality cell, with low self-discharge. Cell B has a higher self-discharge, perhaps because of slight manufacturing faults, perhaps because it is warmer. The cells are discharged and charged a number of times

State of charge of cell A State of charge of cell B Event

100% 100% Fully charged

48% 40% 50% discharge

98% 90% 50% charge replaced

35% 19% 60% discharge

85% 69% 50% partial recharge

33% 9% 50% discharge

83% 59% 50% partial recharge 18% Cannot supply it, battery flat! 60% discharge required to get home of cycles.4 If one cell in a battery goes completely flat like this, the battery voltage will fall sharply, because the cell is just a resistance lowering the voltage. If current is still drawn from the battery, that cell is almost certain to be severely damaged, as the effect of driving current through it when flat is to try and charge it the 'wrong way'. Because a battery is a series circuit, one damaged cell ruins the whole battery. This effect is probably the major cause of premature battery failure.

The way to prevent this is to fully charge the battery till each and every cell is fully charged (a process known as charge equalisation) at regular intervals. This will inevitably mean that some of the cells will run for perhaps several hours being overcharged. Once the majority of the cells have been charged up, current must continue to be put into the battery so that those cells that are more prone to self-discharge get fully charged up.

This is why it is important that a cell can cope with being overcharged. However, as we have seen in Sections 2.3 and 2.4, only a limited current is possible at overcharge, typically about C/10. For this reason the final process of bringing all the cells up to fully charged cannot be done quickly. This explains why it takes so much longer to fully charge a battery than to take it to nearly full. The last bit has to be done slowly. It also explains something of the complexity of a good battery charger, and why the battery charging process is usually considerably less than 100% charge efficient. Figure 2.12 shows this process, using an example not quite so extreme as the data in Table 2.9. Unlike in Table 2.9, the battery in Figure 2.12 is 'saved' by ensuring that charge equalisation takes place before any cells become completely exhausted of charge.

So far we have taken to process of charge equalisation to be equalising all the calls to full. However, in theory it is possible to equalise the charge in all cells of the battery at any point in the process, by moving charge from one cell to the other, from the more charged to the less charged. This is practical in the case of the 'super-capacitors' considered in the next chapter, however it is not usually practical with batteries. The main reason is the difficulty of sensing the state of charge of a cell, whereas for a capacitor it is much easier, as the voltage is directly proportional to charge. However, in the case of lithium-based batteries charge equalisation by adding circuits to the battery system is more practical, and is used. Chou et al. (2001) give a good description of such a battery management system.

This issue of some cells slowly becoming more deeply discharged than others is very important in battery care. There are two particular cases where it is especially important.

Opportunistic charging: some users are able to put a small amount of charge back into a battery, for example when parked in a location by a charger for a short time. This is helpful, but the user MUST make certain that fairly frequently a full long charge is given to the battery to bring all cells up to 100% charged.

Hybrid electric vehicles: in these it is desirable to have the battery NOT fully charged normally, so that the battery can always absorb energy from regenerative braking. However, this must be done with caution, and the battery management system must periodically run the battery to fully charged to equalise all the cells to 100% charged.

4 The very large difference in self-discharge of this example is somewhat unlikely. Nevertheless, the example illustrates what happens, though usually more slowly than the four cycles of Table 2.9.

Electrical Battery Handbook

Figure 2.12 Diagram showing the need for periodic charge equalisation in a battery. The upper line (A) shows the state of charge of a normal cell working satisfactorily. The lower line (B) is for a cell more prone to self-discharge. Charge equalisation involves overcharging some of the cells while the others are brought up to full charge. This is occurring in the final 12 time units

Figure 2.12 Diagram showing the need for periodic charge equalisation in a battery. The upper line (A) shows the state of charge of a normal cell working satisfactorily. The lower line (B) is for a cell more prone to self-discharge. Charge equalisation involves overcharging some of the cells while the others are brought up to full charge. This is occurring in the final 12 time units

The issues of battery charging mentioned here apply to all battery types. However, they are more important for cells with higher self discharge rates, such as the lead acid. The only batteries for which this is not of the utmost importance are the small single cells used in electronic products; however, they are not relevant here.

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