Vehicle Electrical Systems Part 1

Forewords
Development of a rechargeable Battery in the ‘20s revolutionised the Electrical System of a Car thereafter. Today, particularly in the luxury sedans’ of the west, they can be as complex as in a long-haul passenger jet airliner – deploying some kilometers of specialised wiring, relays and switches.

Electrical problems like a flat battery or malfunction of the key parts are most frequent that plague an average motorist. So sooner one understands the basics, the better for him and his vehicle. Due to space constraints, this one is split into 2-parts – Part 1 covering the basics and Part 2 some Do’s and Don’ts.

Some Basics
The ONLY source of ‘power’ in a conventional vehicle is its engine. Therefore, its Alternator and in turn the Battery are ‘secondary’ sources, ‘powered’ by the engine. Today, all cars have 12V DC Systems whereas heavy commercial vehicles deploy 24V DC for understandable reasons.

Flow of electricity in a circuit is like flow of water in a stream, where ‘velocity’ of stream (kph) is equivalent to system ‘voltage’ (V) and rate of flow (cusecs) is eqvt to current in ‘Amperes’ (A). Further, an electrical ‘load’ is measured in ‘Watts’ (W) and in a DC system, it’s a product of VxA.

Battery
The fundamental requirement of a car battery is to provide reliable power to its ‘starter-motor’ and its Ignition System, to fire the engine even under most adverse conditions – such as sub zero temperatures, prolonged shut downs, poor state of engine health etc.

Therefore, a car battery is ‘sized’ accordingly i.e. higher the power required to ‘crank’ the engine, ‘larger’ (AH) the Battery. Typically, a battery type ‘NS40S’ has an AH capacity of 30@20 hrs. In other words, such a fully charged battery can deliver 1.5 Amps for 20-hrs.

It’s note worthy that ‘A’ and ‘H’ are not always inversely proportional i.e. if such a battery is discharged @ 5 Amps, it’ll last < 6 hrs!

Alternator
Having got a suitable Battery on-board, it’s logical that it’d need to be kept fully charged, as it gets substantially discharged every time the engine is cranked. Until mid-‘60s, this job was done by a ‘dynamo’ but as load demands increased, it was found wanting in more ways than one – such as inability to charge at low/idling engine speeds and relatively short/unreliable life due to the ‘commutator’ it had to have, to convert the basic ‘AC’ it generated into ‘DC’ for use on board - coz the Batteries are DC only.

This need paved way to development of ‘Alternators’ that overcome such deficiencies, where wear-prone ‘commutator’ got replaced with a solid-state ‘Rectifier Plate’ and its external electro-mechanical ‘Voltage Regulator/Cut-out’ got replaced with in-built (solid state) voltage regulator – to maintain the Alternator output within 12-15 Volts regardless of the engine RPM.

Today’s average sedans have Alternators with an output capacity of ~ 60 Amps/15 Volts a/a 20 Amps of Dynamo’s of yore and are so designed that they’re able to meet practically all the ‘designed’ electrical loads of a car while on the move, including while idling at traffic lights, and yet have required spare capacity to keep the battery fully charged at all times