Vehicle Electrical Systems

Construction

A 12 V battery has six cells in series, each of about 2.1 V.

 Terminal(+)                  Terminal(-)
   |                              |
 [+plates PbO2 | separator | -plates Pb] x n  (cell 1)
 ----- cell connector ----- cell 2 ... cell 6
 Container (polypropylene) filled with dilute H2SO4

• Positive plates: lead dioxide ($PbO_2$) on a lead-antimony or lead-calcium grid.
• Negative plates: spongy lead ($Pb$).
• Separators: microporous plastic sheets that prevent short circuit.
• Electrolyte: dilute sulphuric acid, specific gravity about 1.26 to 1.28 when fully charged.
• Container, cell connectors, vent plugs (sealed maintenance-free types have none).

Chemical action

$$PbO_2 + Pb + 2H_2SO_4 \underset{\text{charge}}{\overset{\text{discharge}}{\rightleftharpoons}} 2PbSO_4 + 2H_2O$$

On discharge both plates change to lead sulphate and the acid becomes weaker (water forms). On charging, the plates return to $PbO_2$ and $Pb$ and the acid strengthens.

Ratings

• Ampere-hour capacity: the current a battery can supply for a given time to a specified cut-off voltage (10.5 V for 12 V) at 27 degrees C. Usually the 20-hour rate: a 60 Ah battery gives 3 A for 20 hours.
• Cold cranking amps (CCA): the current in amperes that a fully charged battery can supply for 30 seconds at -18 degrees C while keeping every cell above 1.2 V (7.2 V for 12 V). It shows starting ability.

State of charge

• Hydrometer: measures specific gravity of the electrolyte, with 1.28 = full charge, 1.20 = half, 1.12 = discharged.
• Open-circuit voltage: 12.7 V full charge, 12.2 V half.
• Load (high-rate discharge) test for a service condition.

(a) Alternator and charging system

The alternator supplies electrical loads and recharges the battery when the engine runs. It is preferred to a DC generator because it gives output even at idle, is light and has no commutator.

 Rotor (field winding, slip rings) in stator (3-phase winding)
 Stator -> 6-diode rectifier -> B+ (battery, loads)
 Voltage regulator -> controls field current

• Rotor: claw-pole electromagnet with field winding fed through slip rings and brushes; driven by the engine belt.
• Stator: a fixed three-phase winding in a laminated core, where AC is induced as the rotor field cuts the conductors.
• Rectifier: six silicon diodes change AC to DC.
• Voltage regulator: senses the system voltage and varies the field current (by switching) to keep the output at 13.8 to 14.4 V at any speed and load. The output rises with speed otherwise, since the induced emf depends on speed and field.
• A charge-warning lamp is on when the alternator does not charge.

(b) Starting system

 Battery -> Ignition switch -> Solenoid -> Starter motor
                                |-> shift lever -> pinion -> flywheel ring gear

• Starter motor: a series-wound DC motor, giving very high torque at low speed (up to 150 to 300 A).
• Solenoid (magnetic switch): when the key is turned, the solenoid pulls the plunger. This closes the heavy contacts to the motor and moves the shift fork that pushes the pinion (Bendix or pre-engaged drive) into the flywheel ring gear.
• Overrunning clutch: it lets the pinion transmit torque to the engine, but frees when the engine starts to prevent the armature from over-speeding.
• Reduction ratio of pinion to ring gear is about 1:10 to 1:15.

A relay and neutral safety switch may be in the circuit.

(a) Wiring harness, fuses, relays, instruments

• Wiring harness: a bundle of colour-coded, insulated copper wires of different gauges tied together with tape and sleeves and clipped on the body. It gives a neat and protected layout. Connectors are polarised and sealed. Wire size is chosen for current and voltage drop. Modern cars use a single-wire system with the body as the return (negative earth) and CAN bus to reduce the wire count.
• Fuses: a thin strip (blade fuse) that melts when current exceeds its rating (e.g. 10 A, 15 A), and protects wiring and devices from short circuits. Fusible links protect heavy circuits.
• Relays: an electromagnetic switch in which a small control current closes contacts to carry a heavy current (headlamps, horn, fuel pump), so that long, thick wires and dashboard switch contacts are avoided.
• Instrument cluster: speedometer, tachometer, fuel gauge, coolant temperature gauge, oil pressure and charge lamps. Sensors send signals; modern clusters are digital and fed by the CAN bus.

(b) Numerical

Total load

$$P = 2(55) + 2(5) + 10 + 60 = 190\ \text{W}, \qquad I = \frac{P}{V} = \frac{190}{12} = 15.83\ \text{A}$$

Time on battery alone

$$\text{usable capacity} = 0.8 \times 60 = 48\ \text{Ah}, \qquad t = \frac{48}{15.83} = 3.03\ \text{h}$$

Engine running

$$I_{charge} = 30 - 15.83 = 14.17\ \text{A}, \qquad t = \frac{20}{14.17} = 1.41\ \text{h}$$

(Charging efficiency is below 100 %, so the real time is somewhat longer.)

Answer: load $= 190$ W ($15.83$ A); battery lasts $3.03$ h; charging current $= 14.17$ A; $20$ Ah is restored in $\approx 1.41$ h.