Batteries and fuel cells:Battery charging.

Battery charging

Small commercial batteries (up to 10 Ah)

The rechargeable batteries used in portable equipment are mainly lithium ion, nickel cadmium or nickel metal hydride. Recharging of these batteries can be carried out by the following methods:

● transformer-rectifier

● switched-mode power supply unit

● capacity charger

The transformer-rectifier circuit is used to reduce ac mains voltage to a lower dc volt- age. The charge delivered to the battery can be regulated using a constant current, a constant voltage, constant temperature or a combination of these. The transformer in this system provides the inherent advantage of isolation between the mains and low- voltage circuits.

In the switched-mode power supply unit, the mains voltage is rectified, switched at a high frequency (between 20 kHz and 1 mHz), converted to low voltage through a high-frequency transformer, rectified back to dc, which is then regulated to charge the battery in a controlled manner. Although more complex than the transformer-rectifier circuit, this technique results in a smaller and more efficient charging unit.

In a capacity charger the mains is rectified and then series coupled with a mains- voltage capacitor with current regulation directly to the battery packs. This makes for a small and cheap charging unit, but it has to be treated with care, since the capacitor is not isolated from the mains.

In addition to these three methods, several manufacturers have responded to the need for shorter recharge periods and better battery condition monitoring by designing specific integrated circuits that provide both the charging control and the monitoring.

Automotive batteries

The main types for recharging automotive batteries are:

● single battery, out of the vehicle

● multiple batteries, out of the vehicle

● starter charging, battery in vehicle

Transformer/reactance is predominantly used in each of these applications, with either a ballast in the form of resistance, or resistance to control the charging current. More sophisticated chargers have constant voltage and current-limiting facilities to suit the charging of ‘maintenance free’ batteries. A typical circuit is shown in Fig. 12.18.

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Vented automotive batteries are usually delivered to an agent in the dry condition. They have to be filled and charged by the agent. A multiple set of batteries connected in series is usually charged for a preset time in this case, and this requires the use of a bench-type charger. Bench chargers are rated from 2 V to 72 V, with a current capability ranging from 10–20 A dc (mean). Examples are illustrated in Fig. 12.19. The bench-type charger may alternatively be left on charge indefinitely using a constant- voltage, current-limited charger.

Starter charging is used to start a vehicle which has a discharged battery. A large current is delivered for a short period, and starter charges have usually a short- term rating. They can be rated at 6 V, 12 V or 24 V, delivering a short-circuit rated starting current from 150 A to 500 A dc (rms) and a steady-state output of 10 A to 100 A dc.

Output voltage is from 1.8 V to 3.0 V per cell. Voltage ripple may be typically up to 47 per cent with a simple single-phase transformer-rectifier, but the ripple will depend upon output voltage, battery capacity and the state of discharge. Voltage regulation is important in starter charging applications since high-voltage excursions can damage sensitive electrical equipment in the vehicle.

Since these chargers are usually short-time rated, some derating may be required if they have to operate at a high ambient temperature.

Safety features are built into automotive battery chargers in order to avoid the risk of damage to the battery, to vehicle wiring or to the operator. The normal safety features include:

● reverse polarity protection

● no battery – short-circuit protection

● thermal trip – abuse protection

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Motive power

Motive power or traction battery chargers are used in applications where the batteries provide the main propulsion for the vehicle. These applications include fork lift trucks, milk floats, electrical guided vehicles, wheelchairs and golf trolleys. The requirement is to recharge batteries which have been discharged to varying degrees, within a short period (7–14 hours). Both the battery and the charger may be subject to wide temper- ature variations. A well-designed charger will be simple to operate, will automatically compensate for fluctuations in main voltage and for differences between batteries arising from such factors as manufacture age and temperature, and will even tolerate connection to abused batteries which may have some cells short-circuited.

The most common type of charger is the modified constant potential or taper charger shown in Fig. 12.20. In all but the smallest chargers, the ballast resistor shown in this circuit is replaced by a reactance; this reactance may be in the form of a choke connected in series with the primary or secondary windings of the transformer, or more usually it is built into the transformer as leakage reactance.

While the battery is on charge, the voltage rises steadily from 2.1 V per cell to 2.35 V per cell, at which point the battery is approximately 80 per cent charged, and gassing begins. Gassing is the result of breakdown and dissipation of the water content in the electrolyte. Charging beyond this point is accompanied by a sharp rise in volt- age, and when the battery is fully charged the voltage settles to a constant voltage, the value of which depends upon a number of factors, including battery construction, age and temperature.

During the gassing phase (above 2.35 V per cell) the charging current must be limited in order to prevent excessive over heating and loss of electrolyte. The purpose of

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the ballast resistor in the charger circuit is to reduce the charging current as the battery voltage rises, hence the name taper charger. By convention, the current output from the charger is quoted at a voltage of 2.0 V per cell, and the proportion of this current which is delivered at 2.6 V per cell is defined as the taper. A typical current limit recommended by battery manufacturers is one-twelfth of the battery capacity (defined as the 5-hour rate in ampere-hours) at the mean gassing voltage of 2.5 V per cell. A dis- advantage of the high-reactance taper charging system is that the output may be very sensitive to changes in the input voltage; the charge termination method and the required recharge time must therefore be taken into account while sizing the charger.

Because of the inefficiencies of energy conversion, particularly due to the heating and electrolysis during the gassing phase, the energy delivered by the motive power charger during recharging is 12–15 per cent higher than the energy delivered by the battery during discharge.

To recharge a battery fully in less than 14 hours a high rate of charge is necessary and the termination of charging when the battery is fully charged must be controlled. The two types of device for termination of charge are voltage–time termination and rate-of-charge termination.

A voltage–time controller detects the point at which the battery voltage reaches V per cell, and then allows a fixed ‘gassing’ time for further charging, which is usually 4 hours. This method is not suitable for simple taper chargers with nominal recharge times of less than 10 hours because of the variation in charge returned during the time period as a result of mains supply fluctuations. If a short recharge time is required with a voltage–time controller then a two-step taper charger is used, in which a higher charging current is used for the first part of the recharge cycle. When the voltage reaches 2.35 V per cell, the timer is started as discussed here, and the current during the further charging period is reduced by introducing more ballast in the circuit.

The rate-of-charge method of termination has predominated in large chargers during the past decade offering benefits to both the manufacturer and the user. In the rate-of-charge system the battery voltage is continuously monitored by an electronic circuit. When the battery voltage exceeds 2.35 V per cell, the rate of rise of the battery voltage is calculated and charging is terminated when this rate of rise is zero, that is when the battery voltage is constant. This method can be used with single-rate taper chargers with recharge periods as short as 7 hours because of the higher precision of termination. In order for a rate-of-charge termination system to operate satisfactorily, there must be compensation for the effects of fluctuations in mains supply. A change in mains voltage results in a proportional change in the secondary output voltage from the charger transformer; if uncompensated this will cause a change in charging current and therefore in battery voltage. For a 6 per cent change in mains voltage, the charging current may change a much as 20 per cent and the battery voltage may change by 3 per cent.

Many batteries have the facility for freshening or equalizing.

Freshening charge is supplied to the battery after the termination of normal charge in order to compensate for the normal tendency of a battery to discharge itself. A freshening charge may be continuous low current, or trickle charging, or it may be a burst of higher current applied at regular intervals.

Equalizing charge is supplied to the battery in addition to the normal charge to ensure that those cells which have been more deeply discharged than others (due, for instance, to tapping off a low-voltage supply) are restored to a fully charged state.

A controlled charger is a programmable power supply based on either thyristor phase angle control or high-frequency switch mode techniques. The main part of the recharge cycle is usually at constant current and the power taken by the charger is there- fore constant until the battery voltage reaches 2.35 V per cell. Many options are avail- able for the current–voltage profile during the gassing part of the recharging cycle, but all of these profiles deliver a current which is lower than the first part of the cycle.

Voltage drop in the cable between the charger and the battery is important because the charge control and termination circuitry relies upon an accurate measurement of the battery voltage. It is not normally practical to measure the voltage at the battery terminals because the measuring leads would be either too costly or too susceptible to damage, and it is common practice to sense the voltage at the output of the charger and to make an allowance for the voltage drop in the cables. Alteration to the length or cross section of these cables will therefore cause errors, especially with low-voltage batteries.

Motive power chargers are typically available from 6 V (three cells) to 160 V (80 cells) with mean dc output currents from 10 A to 200 A. A typical circuit is shown in Fig. 12.21.

The output current is rated at 2.0 V per cell; the taper characteristics set by the transformer reactance then results in 25 per cent output current at typically 2.65 V per cell. Rating is not continuous, and derating to 80 per cent is typical to take into account the taper characteristic. Consideration must be given to this if a multiple shift working pattern is to be adopted.

Voltage ripple is typically 15–25 per cent for single-phase chargers and 5–15 per cent for three-phase chargers. The precise level of ripple will vary with time and it will depend upon mains voltage, battery capacity and the depth of the battery.

Standby power applications

Typical standby power applications include emergency lighting, switch tripping, switch closing and telecommunications. The main functional requirements for the battery charger in these cases are:

● to ensure that the state of charge of battery is maintained at an adequate level, without reducing battery life or necessitating undue maintenance

● to ensure that the output voltage and current of the complete system are compatible with the connected electrical load

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● to ensure after a discharge that the battery is sufficiently recharged within a specified time to perform the required discharge duty

● to provide adequate condition monitoring, to the appropriate standards

Assuming initially that the battery is fully charged, the simple option is to do nothing. A charged battery will discharge if left disconnected from a load and from charging equipment, but if the battery is kept clean and dry this discharge will be quite slow. For some applications, open-circuit storage is therefore acceptable.

For most applications, however, there is a need for battery charge to be maintained. The current–voltage characteristic is not linear, and a small increase in charging volt- age will result in a large-scale increase in current. Nevertheless, it is always possible to define a voltage which, when applied to a standby power battery, will maintain charge without excessive current, and the charging current flowing into the battery has only to replace the open-circuit losses in the battery, which are usually small.

Once these open-circuit losses have been made up, any additional current flowing is unnecessary for charging purposes and is normally undesirable. In vented cells it causes overheating and gassing and eventually, if not checked, damage and loss of capacity of the cell. In sealed cells there can be overheating, in extreme cases expulsion of gases through the pressure vent and ultimately, a loss of capacity.

On the other hand, if the battery voltage is allowed to fall too much, the open- circuit losses will not be replaced and the battery will slowly discharge.

The charging voltage has therefore to be controlled carefully for best battery maintenance. The usual limits are within ± 1 per cent of the ideal voltage, which is normally termed the float voltage. The float voltage has a negative temperature coefficient, which must be accounted for when batteries are to operate in exceptionally hot or cold environments. Float voltages for the major types of standby power cell are shown in Table 12.3.

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For vented cells there are, however, circumstances under which the float voltage should be exceeded. Batteries that are new or have suffered abuse will benefit from a vigorous gassing up to the boost voltage shown in Table 12.3. Batteries which have stood on float charge with no discharge–charge cycle for many months will benefit from a refresh charge with gassing, at the refresh voltages which are also shown in Table 12.3.

Few dc standby power systems can be designed without taking account of limits on the load voltage. For good battery operation it is necessary to charge at the float voltage (or sometimes a higher level), but it is also necessary to discharge the battery to a sufficiently low voltage if the full capacity is to be released from the cells. These considerations impose fundamental limits that define the maximum excursion of the system output voltage. Table 12.4 shows the minimum and maximum voltages which are reasonable in a 50 V standby power system for each for the three major battery types. Systems with other voltages will require excursions which are in direct proportion.

It can be seen from Table 12.4 that, allowing for the discrete steps in voltage when changing the number of cells, it is not possible to achieve a voltage excursion of less than about ±12 per cent under conditions of float alone; the excursion limits are larger in refresh operation, and still larger with boost.

Table 12.4 shows that the sealed lead acid cell offers minimum overall voltage variation because of the absence of the larger excursions due to refresh and boost charges. If it is not possible to use a sealed lead acid system, another alternative is to disconnect the load for the full boosting operation; this should normally be necessary only at the time of system installation in any case.

A voltage regulator should be included in the system if closer limits of voltage variation are required. Diode regulators are now reliable and widely used; they operate by

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switching banks of series-connected diodes in and out as the battery voltage slowly varies. It is important to ensure that switching in the regulator occurs only as a result of changes in battery voltage, and not as a result of load changes; if the regulator responds to changes which occur as a result of load change, excursions outside the specific voltage limits may occur because of the delays in the process of switching the diode bank. Ensuring that this distinction is made normally requires a computer simulation for all but the simplest regulators. Control of the switching of the diode bank is best achieved by a programmable controller, especially if additional complicated relay- type logic is required in the system. Diode regulators are large and their heat dissipation is substantial. Higher efficiency regulators using actively switched devices are becoming available, but they are at present limited to relatively low power applications.

Condition monitoring is now included in most dc systems in order to warn of excessive battery voltage excursions. The applications are diverse, but the main features are:

high voltage detection: this is necessary in order to prevent a fault on the supply system from damaging the battery or load circuit

low voltage detection: this warns of load failure due to insufficient voltage, and it is also needed to trigger the disconnection of sealed batteries which may be damaged by excessive discharging

charge failure: this is needed in order to stimulate action to restore the ac supply or to prepare for disconnection of the load

earth leakage: this is needed where an unearthed load system is used, for safety and for avoidance of double faults

Communication of a fault is through volt-free contact on a relay, which usually signals to the monitoring centre using a 110 V or other voltage supply.

It has been seen in Table 12.4 that there are restrictions on the choice of charging voltage. The preference for many applications is to limit the voltage to the float volt- age, and while it may be necessary to increase the charging voltage to speed up the recharging, the voltage should be returned to the float voltage as soon as possible. At this float voltage level, all the cell types discussed will be recharged to about 80 per cent of their nominal capacity. Assuming the charger current is at the adequate level shown in Table 12.5, recharge to 80 per cent capacity will be achieved in the times indicated in the table.

To recover the remaining 20 per cent of the charge is more difficult, and different techniques are necessary for the three cell types. A vented lead acid battery will be fully recharged at float voltage in about 72 hours, but if the charging voltage is boosted to 2.7 V per cell, a full recharge will take about 14 hours. For a vented nickel cadmium cell full charge will never be achieved without increasing the voltage above float level. Exact times may vary between cell types, but typically a refresh charge at 1.55 V per

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cell will give full charge in about 200 hours on the highest performance cells, and boosting to 1.7 V per cell will reduce this time to 9–10 hours. Sealed lead acid cells will reach full charge after about 72 hours at float voltage; an increase of charge volt- age to 2.4 V per cell will reduce charging time to about 48 hours, but there is no way in which this can be significantly reduced further.

If a fast recharge is essential, an alternative is to oversize the battery; for instance if 100 Ah capacity is required with a full recharge within 8 hours, then a battery with 125 Ah capacity could be installed.

Most cells will protect themselves from excess charging current, provided that the voltage is limited to the float voltage, but above this level excessive charging current can damage the cell. For vented lead acid cells the 7 per cent of capacity shown in Table 12.5 is recommended as an upper limit. For sealed lead acid cells, the recom- mended upper limit is 50 per cent of capacity. Nickel cadmium cells normally require a minimum charge current of 20 per cent of capacity, as indicated in Table 12.5, but a lower limit of 10 per cent of capacity is recommended when boosting in order to avoid excessive gassing and electrolyte spray.

Standby power chargers are available in a wide range of capacities to suit many applications. Typical dc outputs are 6, 12, 24, 30, 48, 60, 110, 220 and 240 V, with dc mean output current ranging from 1 A to 1000 A. DC output current is rated at 100 per cent of the output current at the full specified voltage. Regulation of the output is gen- erally within ±1 per cent for an input voltage change of ±10 per cent and a load cur- rent change of 0–100 per cent. A typical circuit is shown in Fig. 12.22. Although standby power systems are continuously rated, some derating may be necessary for operation in tropical climates if this was not originally specified.

Differing levels of output smoothing can be incorporated into the charging system, depending upon the application. General applications require a maximum of 5 per cent ripple, but for telecommunications supplies, specifications are based on CCITT telecommunications smoothing, which requires 2 mV phosphometrically weighted at 800 Hz. The key components of an installation are shown schematically in Fig 12.23.

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