CHARGING SYSTEM

The charging system performs two jobs: (1) it recharges the battery and (2) it generates current during operation. The two types of charging systems used on automotive and construction equipment are a dc charging system (fig. 4-2, view A) or an ac charging system (fig. 4-2, view B). Both systems generate an alternating current (at); however, the difference is the way they rectify the ac current to direct current (de) for charging the battery.

Dc Charging System

A dc charging system has a generator and a regulator. The generator supplies the electrical power and rectifies its current mechanically by using commutator bars and brushes. The regulator performs three jobs: (1) it opens and closes the charging circuit, (2) it prevents overcharging of the battery, and (3) it limits the output of the generator to safe rates.

Ac Charging System

An ac charging system has an alternator and a regulator. The alternator is really an ac generator. Like the generator, it produces an ac current but rectifies it electronically, using diodes. Most alternators are more compact than generators of equal output and supply a higher current output at low-engine speeds. The regulator in an ac charging system limits the alternator voltage to a safe, preset value.

Charging System

All charging systems operate in three stages: (1) during starting, the battery supplies all load current; (2)

Figure 4-2.-Charging systems.

during peak operation, the battery helps the generator supply current; and (3) during normal operation, the generator supplies all current and recharges the battery.

In both electrical systems, the battery starts the electrical circuit that supplies the spark to start the _engine. The engine then drives the generator or alternator that produces current to take over the operation of the ignition, lights, and accessory loads.

The battery also supports the generator or alternator during peak operation when the electrical loads are excessive. But once the engine is started the generator or alternator is the "work horse," providing current to the ignition and accessory circuits. The generator supplies current as long as the engine is at speed and running. When the engine slows down or stops, the battery takes over part or all of the load.

STARTING CIRCUITS

High voltage is often necessary to ensure sufficient starting power due to the high compression ratios of some diesel engines. Three systems are used to increase either the voltage or amperage to accomplish this task These are parallel, series, and series-parallel systems.

Parallel System

An example of a parallel system, as shown in figure 4-3, view A, is two 12-volt, 200-amp batteries are connected from the starter to the positive terminal of one battery to the positive terminal of the second battery. The negative side of the batteries are connected from the ground to the negative terminal from one battery to the negative terminal of second battery. This system provides 12 volts and 400 amps, providing more amperage for starting.

Series System

An example of a series system, as shown in figure 4-3, view B, is two 12 volt, 200-amp batteries are connected from the positive terminal of one battery to the negative terminal of the second battery. The remaining positive terminal is connected to the starter and the remaining negative temninal is connected to the ground. This system provides 24 volt and 200 amps, providing more volts for starting.

Series-Parallel System

A series-parallel system provides a series connection of the batteries for starting and a parallel

Figure 4-3.-Starting circuits.

connection for normal operation. An example of a series-parallel system is when two sets of parallel batteries, as shown in figure 4-3, view C, are connected in series and the negative terminal from one set of the batteries is connected to the positive terminal of the other set. This system provides 24 volts and 800 amps. This combination is used for cranking large construction equipment.

CAUTION

Use extreme care when jump starting is required. Hooking up jumper cables from a 24-volt system to a heavy-duty 12-volt system can cause severe battery damage, starter destruction, or even an explosion. If you are unsure of the starting circuit, get help from the mechanic field crew for assistance.