Starting and Charging Systems


  • Describe an engine starting and battery charging system
  • List the functions of a battery equalizer
  • Explain how an alternator works
  • Explain how a starter motor works

Starting and Charging Systems

The prime mover in most mobile equipment is generally a gasoline or diesel engine.

Most mobile systems have several major components whose purpose is to start and run the engine, and to provide electrical power to the control system.

  • Alternator (1) to generate electrical power.
  • Starter motor (2) to turn the engine.
  • Ignition key (3) to select Stop, Run, or Start mode.
  • A charge indicator light (4) or gauge.
  • A battery ignition switch (5).
  • Batteries to store electrical energy.
  • Grounding terminals (7) and fuses (8).


The most common batteries for mobile applications are lead-acid type rechargeable batteries. You can find them on every truck and they are designed to provide electrical power to the starter, ignition, lighting, electronic control module (ECM) and auxiliary equipment.

Lead-acid batteries are made of six cells; when connected in series and fully charged they can produce 12.6 volts. Voltage measured across a 12 volt battery while charging will be greater than 13 volts.

To select the right battery, it is not enough to know the voltage and size of the battery; you also need to know the capacity (C20) of the battery and the cold cranking amps (CCA).

  • C20 capacity is rated by the number of amperes that a battery can continuously supply for 20 hours at 80°F while maintaining a cell voltage above 1.75 volts. A 200 Ah battery will have a C20 rating of 10A (200 Ah / 20 h = 10 A).
  • CCA rating refers to the number of amperes that a battery can continuously supply for 30 seconds at 0°F while maintaining a cell voltage above 1.2 volts. A 500 CCA battery can supply 500 amperes for 30 seconds at 0°F.
Remember: You need to be very careful when handling lead-acid batteries to avoid a chemical burn.

Sometimes multiple 12 volt batteries are required in order to operate 12 volt and 24 volt loads. Batteries are connected in series to increase the voltage, or in parallel to provide more current capacity.

Battery B2 (shown in image) provides the required supply voltage for the 12 volt loads.

Battery B1 (shown in image) in series with B2 provides the required supply voltage for the 24 volt loads.

The batteries can be charged by various sources such as by 12 volt and 24 volt alternators, or a 12 volt tractor unit.

The isolator ensures the current will only flow from the charging device to the battery and not vice versa.

High current diodes are used in the isolator.

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