Electricity in motor vehicles

School Science Lessons
(UNPh32.5)
2025-08-16

Electricity in motor vehicles
Contents
32.5.1 Battery care
32.5.2 Lead-acid battery secondary cell
32.5.3 Discharging and charging a battery
32.5.4 Cell connections of a lead cell battery
32.5.5 State of charge of a motor vehicle battery
32.5.6 Prepare lead-acid battery electrolyte
32.5.7 Battery capacity, Ampere-hour (Ah)
32.5.8 Battery storage capacity
32.5.9 Deep-cycle battery
32.5.10 VRLA sealed battery
32.5.11 Starting the engine with jumper leads
32.5.12 Electric Bell
32.5.13 Spark plugs
32.5.14 Spark plug operating temperature
32.5.15 Spark plug gap
32.5.16 Lead accumulator cell

32.5.1 Battery care
1. Be careful! Never short-circuit an automotive battery.
Excessive current demand may cause battery acid to boil out.
1.Keep the battery case clean.
Use hot soapy water then make it perfectly dry.
Clean the battery terminals at vehicle service.
2. Remove filler caps and check the electrolyte level each month.
Fill the battery with deionized water or distilled water to just above the battery plates.
Do not overfill.
Mop and dry any spillage of water.
3.The battery must be secure in its cradle.
Vibrations of the battery can damage the battery plates.
Loose or corrodes battery terminals may cause breakdowns.
4.The battery will run down without regular use.
It runs down quicker if use is infrequent or used only for short runs.
To avoid problems make sure that the battery is regularly recharged and tested.
5. If battery is drained, because of leaving lights on revive it with a slow charge.
Under-charging and over-charging will shorten battery life.
6. Poor engine condition will shorten the life of a battery.
7. When working on a battery, do not smoke or have a naked flame nearby, because batteries produce hydrogen gas.
8. Before trying to jump start by pushing the car or using jump start cables, make sure that you understand the electrical design of that car.
Do not try to jump start a car fitted with electronic systems, but get expert assistance.
9. Battery patrol check list:
* No load battery voltage
* Discharge test voltage
* Discharge test gassing
* Physical damage evident
* Electrolyte level
* Terminal security and cleanliness
* Discharge to earth
* Alternator drive tension
* Alternator output
* Age of battery.

32.5.2 Lead-acid battery secondary cell
See diagram 32.5.2: Lead-acid battery secondary cell.
1. The lead acid battery is a group of two or more electric cells connected in series.
A 12 volt battery has six 2 volts cells.
A 6 volt battery has three 2 volt cells.
Secondary cells, which can be recharged many times, are also called storage cells or accumulators.
The chemical action can be reversed by passing a current through the cell in the direction opposite to the discharge current until the chemicals have been changed back to their original form, the cell is charged again.
2. The 6 volt battery has three identical lead-acid cells connected in series, so their voltage is added.
The current is the same in all cells.
Charging or discharging current must pass through all the cells in the battery.
The positive and negative plates correspond to the positive and negative electrodes of a primary cell.
3.These thin plates expose a large surface to the action of the electrolyte.
The plates are constructed from a lead-antimony grid filled with lead (IV) oxide, PbO, in the positive plate and a spongy form of pure grey lead (Pb) in the negative plate.
Separators are insulators to prevent contact between the plates and allow circulation of the electrolyte, a dilute solution of sulfuric acid in water.
The order of plates within the cell is negative plate, separator, positive plate, separator with the final plate in the series a negative plate.
4. The voltage (EMF) of a lead-acid cell varies from 2.3 volts when the cell is fully charged to 1.9 volts, when the cell is fully discharged.
A voltmeter connected to the battery terminals of a discarded battery containing some liquid will still show some reading.
When a lead cell accumulator is fully charged, the concentration of sulfuric acid is at maximum.
5. When the accumulator is fully discharged, "flat battery", the concentration of sulfuric acid is at a minimum.
A battery hydrometer is used to read the relative density (specific gravity) of the sulfuric acid in the electrolyte and check the charge of the battery.
The density varies from about 1.28 in a fully charged battery to 1.15 in a discharged battery.
The density of sulfuric acid purchased for use in accumulators is about 1.25 at 20oC.

32.5.3 Discharging and charging a battery
Discharging --->
PbO + 2H2SO4 + Pb ---> 2PbSO4 + 2H2O
<--- Charging
Lead (IV) oxide + sulfuric acid + lead ---> lead sulfate + water
When the battery is discharging, lead (IV) oxide in the positive plate combines with sulfuric acid (sulfuric acid) in the electrolyte to form lead sulfate (lead sulfate) and water.
Lead in the negative plate combines with sulfuric acid to form lead sulfate and water.
How much lead sulfate is produced is directly proportional to how much current flows.
The electrolyte not only takes part in the conversion of the stored chemical energy to an electric current, but also provides a low resistance path for the current through the cell.
The battery is discharged when not enough sulfuric acid is left in the electrolyte for effective chemical action and most of the active materials, lead (IV) oxide and lead, in both sets of plates have been converted into lead sulfate.
During discharge the electrolyte becomes weaker as the acid combines with the plates.
The lead sulfate that has formed fills the pores of the plates so circulation of the electrolyte decreases and the voltage of the battery drops.
The battery is fully discharged when the electrolyte cannot reach the remaining active material of the plates and react with it fast enough to maintain the working voltage and produce an effective current.
When the battery is charging, current passes through the battery in the reverse direction to the flow on discharge to reverse the chemical reactions and reform the dark brown lead (IV) oxide on the positive plates and spongy lead on the negative plates.
When the battery is fully charged, the specific gravity of the electrolyte is restored to its original value.
If electricity is passed through the battery after it is fully charged, more electrical energy is being supplied to the battery than can be converted to chemical energy.
The excess electricity decomposes the water in the electrolyte to form hydrogen gas and oxygen.
This is called gassing.

32.5.4 Cell connections of a lead cell battery
See diagram 32.5.4 Cell connections of 6 volt batteries
The lead-acid cell has a voltage of two volts when in average working condition.
Each cell has two terminals, the positive terminal and the negative terminal.
Inside the cell, the positive terminal is connected to the positive plates and the negative terminal to the negative plates.
The direction of conventional current flow is from the positive terminal to the negative terminal in the circuit outside the cell.
The movement of current carrying particles, i.e. electrons, is in the opposite direction.
In most of the electrical devices on motor vehicles it does not matter which way the current flows.
The negative terminal of the battery is usually connected to earth, so the current flows from the insulated live side of the circuit down through the appliance to earth.
However, in motor vehicles using alternators and rectifiers a wrong connection can cause damage.
A 6 volt battery has three 2 volt cells connected in series.
To simplify connection, the centre cell is placed in the container the opposite way round to the end cells.
Two complete 6 volt batteries form a 12 volt battery.
Two batteries may be connected in parallel for heavy duty operation.
Two identical batteries connected in parallel give the voltage of only one battery, but their capacity is double the capacity of each.
An extra battery may be connected in parallel with the battery in a vehicle to start the engine.
If a battery is charged with negative terminal connected to positive terminal of the charger, the battery may be damaged by reversal of the polarity of the plates.
If the name plate is facing the observer, the positive terminal will be at the front right hand corner.
Reverse assembly, the positive terminal will be at the front left hand corner.
The polarity of each individual cell can be found with a moving coil permanent magnet voltmeter with its terminals marked.
The positive post may be painted red, or becomes a dark chocolate colour after a very short period of use.
The negative terminal always looks cleaner and remains light grey in colour.
Each inter cell connector becomes discoloured in the same way, one end dark brown and the other end light grey.
The dark brown end is connected to the positive terminal of a cell, and the light grey end is connected to the negative terminal of the adjacent cell.
A lead-acid battery may self-discharge at the rate of 1% of its capacity per day.

32.5.5 State of charge of a motor vehicle battery
See diagram 32.5.5: Battery hydrometer.
There is no direct method to measure a battery's state of charge.
1. Measure its static voltage and compare it to a standardized chart.
This is the least accurate method, but it only involves an inexpensive digital meter.
2. Measure with an amp-hour meter to monitor all power moving in and out of the battery by time, and state of charge is determined by comparing flow rates.
3. Measure the density, specific gravity, of the sulfuric acid electrolyte with a battery hydrometer.
This is the most accurate test, but yet it is only applicable to flooded lead-acid batteries.
Electrolyte density is lower when the battery is discharged and higher as the cells are charged.
The battery's chemical reactions affect the density of the electrolyte at a constant rate that is predictable enough to get a good indication of the cell's state of charge.
4. Maintenance-free batteries supplied to some modern motor vehicles under normal operation conditions will not need water addition within its average battery ife.
The vent caps remain sealed and the electrolyte cannot be tested as explained below.
However, the vent caps are accessible for inspection of fluid level in the battery if required when using the battery under abnormal operating conditions.
5. The specific gravity, SG, of the electrolyte in each cell of a battery shows the battery's state of charge:
Fully charged, SG = 1.280, Three quarter charged, SG = 1.240, Half charged, SG = 1.200, Quarter charged, SG = 1.160, Discharged, SG = 1.120.
6. For battery testing, the hydrometer floats in a syringe.
The electrolyte is drawn into a glass tube so that level can be observed in relation to the scale marked on the narrow stem of the hydrometer.
Always take readings at eye level, i.e. your line of sight must be horizontal.
The battery hydrometer indicates the state of charge of each cell.
The electrolyte expands when heated and contracts when cooled so a reference temperature standard is needed.
7. The state of charge of a battery is not a true indication of its internal condition if the battery has any bulging, cracking, leakage of electrolyte, or lifting of the cell covers at the positive post end.
Check that you have enough electrolyte above the plates to make a hydrometer test, then read the specific gravity of the electrolyte in each cell.
If any reading is below 1.225, the battery has been recharged following the manufacturer's instructions.
A low reading in one or more cells after the battery has been recharged indicates internal trouble.
Test the ability of the battery to do the work required of it by drawing the normal starter motor current and measuring the voltage of the individual cells while they are discharging at a high rate with an accurate voltmeter.
No cell should read less than 1.5. volts.
As the cells of a battery are connected in series, each receives the same charging current and the same amount of current also flows through each on discharge.
If the cell voltages do not fall below 1.5. volts while the starter is being operated and the specific gravity readings are at least 1.250, the battery is in good ondition.

32.5.6 Prepare lead-acid battery electrolyte
This solution is sometimes called "accumulator electrolyte".
1. Be careful!
Use safety glasses and nitrile chemical-resistant gloves!
Wear protective clothing!
Follow the recommendations of the manufacturers for filling and initial charging that is usually printed on the battery.
The relative density of sulfuric acid is: fully charged 1.28, half charged, 1.21, discharged 1.15.
Slowly add concentrated sulfuric acid, with stirring, to a strong beaker two thirds full of deionized water or distilled water, until the solution almost boils.
Leave to cool and add more acid until the solution almost boils.
Leave to cool to room temperature.
Adjust the relative density by adding more acid or more water, according to the hydrometer reading.
When the cell is not in use, use a jar with a cover to prevent drying by evaporation.
Use safety glasses and nitrile chemical-resistant gloves!
Slowly add concentrated sulfuric acid, with stirring, to a beaker two thirds full of distilled water, until the solution is almost boils.
Leave to cool and add more acid until the solution almost boils.
After cooling to room temperature, adjust the relative density with more acid or more water, according to a hydrometer reading.
Sulfuric acid is highly corrosive and if spilt on the skin must be washed off immediately with plenty of water from a running tap.
Apply baking soda solution to the affected area then wash off with water.
Keep a labelled jar of baking soda solution near by in case of spills on the skin.
2. The electrolyte used in lead-acid batteries is a solution of chemically pure sulfuric acid in deionized water.
The efficiency and ultimate life of a battery depends on the purity of the electrolyte.
Always use deionized water or, demineralized water or rainwater to maintain the level of electrolyte in the cells, topping up.
Do not add tap water or water that has been in contact with metals, especially iron, because impurities cause secondary chemical reactions and the battery can self-discharge.
Do not fill above an electrolyte marker or the top of the splash guards, just fill to cover the separators that extend upwards above the top edges of the plates.
The electrolyte should be able to expand and rise when warmed without spilling out through the vent holes in the filler plugs.
BE CAREFUL! REMEMBER: ACID TO WATER, NEVER WATER TO ACID!
3. Charge the battery at the normal rate until the specific gravity stops rising and the cells are gassing.
If the SG is too high, draw off some electrolyte with the battery hydrometer syringe and replace it with deionized water or demineralized water or rain water.
If the SG is too low, draw off some electrolyte and replace it with 1.300 specific gravity acid.
Charge the battery for a further two hours after any replacement to mix the electrolyte thoroughly before taking another reading.
Use a clean glass container for mixing the electrolyte.
Put the water required into the container first then pour the acid slowly into the water while stirring.
Let the electrolyte cool to room temperature before taking the final specific gravity reading.
Keep the battery and its surrounding parts clean and dry to avoid corrosion of metal parts.
Overcharging may cause loosening and shedding of the active material from the plate grids.
The hot electrolyte may attack the separators and the life of the battery will be very short.
4. Sulfation
A battery is left for a long time in a partially discharged state is harder to recharge to its original capacity, because the fine crystalline lead sulfate may harden and become more dense, the battery becomes "sulfated".
To avoid sulfation, the generator must run long enough to restore the chemical energy used to start the engine and operate the lights and accessories while the engine is idle.
The internal resistance of the battery is higher when cold, and a short run may not raise the electrolyte temperature sufficiently to lower its resistance, so the voltage regulator operates earlier in the charging cycle, and at a lower specific gravity than it would do normally.
Do not allow the electrolyte level to become too low to expose the plates to the air, because oxygen in the air will combine with the spongy lead of the negative plate to form a layer of lead (IV) oxide.

32.5.7 Battery capacity, Ampere-hour (Ah)
1. Battery capacity
Capacity of a battery refers to the quantity of electricity it can deliver.
The ampere-hour (Ah), is the quantity of electricity equivalent to a current of one ampere flowing for one hour.
The milliampere-hour is one-thousandth of an ampere-hour.
(1 Ah = 3 600 coulombs of charge, 1 amp = 1 coulomb per second.)
Capacity in ampere-hours = current in amperes × time in hours.
A battery with a capacity of 50 ampere-hours can deliver a current of 5 amperes for 10 hours, or 10 amperes for five hours, after being fully charged at the beginning and completely discharging at the end.
However, the capacity of a battery is not the same for different rates of discharge if the battery has already discharged within the rated time.
2. Ampere-hour (Ah)
Amp-hour capacity for a battery, it is specified at either a given current, given time, or assumed to be rated for a time period, e.g. 8 hours or 10 hours.
Automobile batteries can be rated by measuring the ability to supply sufficient current to operate the starter, or operate the lighting system.
The approximate amp-hour capacities of some common batteries: :
* Typical automotive battery: 70 amp-hours at 3.5. A (secondary cell)
* D-size carbon-zinc battery: 4.5. amp-hours at 100 mA (primary cell)
* Nine volt carbon-zinc battery: 400 milliamp-hours at 8 mA (primary cell).
3. The 10-hour rating. For automobile batteries, a common standard rating is the 10-hour rating.
It is the steady discharge current, in amperes, to reduce the cell voltage to 1.8 in 10 hours at 15oC, with specific gravity of the electrolyte 1.120 when fully discharged.
The full theoretical capacity of a battery cannot be used, because the discharge current that must diffuse into the plates fast enough to replace the electrolyte used in the production of an electric current.
Lead sulfate clogs the pores of the plates and restricts the movement of the electrolyte so that chemical action is reduced, the voltage generated by the cell is lowered, and the useful current is reduced.
Also, the resistance of the materials on the plates increases as the cells are discharged and the resistance of the electrolyte increases as the battery discharges and the acid content of the electrolyte falls.
Both a 6 volt 50 ampere-hour battery and a 12 volt 50 ampere-hour battery will deliver a current of 5 amperes for 10 hours, so the stated capacity of 5 × 10 = 50 ampere-hours.
The advantage of the 12 volt automobile battery is that it has twice the watt-hour capacity of the 6 volt battery, i.e. it stores twice the amount of electrical energy.
The 6 volt battery delivers its 5 amperes at 6 volts, but the 12 volt battery delivers the same current for the same length of time at 12 volts.
4. Watt hour capacity
Power from 6 volt battery, W = V × I = 6 × 5 = 30 watts.
Energy capacity of 6 volt battery = power × time = watts × hours = 30 × 10 = 300 watt hours.
Power from 12 volt battery, W = V × I = 12 × 5 = 60 watts.
Energy capacity of 12 volt battery = power × time = watts × hours = 60 × 10 = 600 watt hours.
So watt hours = ampere-hours × voltage.
Watt hour capacity = ampere-hour capacity × battery voltage.

32.5.8 Battery storage capacity
The storage capacity is the amount of energy a battery can store.
The storage capacity of a battery is the current that can be delivered × time, so the storage capacity of a battery in watt-hours = ampere-hour capacity × battery voltage.
The storage capacity of a battery will have a maximum value as "maximum deliverable current" or a minimum as "shelf life" in a supermarket.
Battery life / battery longevity may be expressed as Reserve Capacity, (RC), and ampere-hours, Amp Hours (Ah or AH).
Amp hours is the most common unit for battery capacity.
Amp hours = current X time.
Batteries used in photovoltaic systems are rated in Ampere Hours (AH).
So a 100 AH battery can supply 1 amp for 100 hours, or 100 amps for one hour.
Small cells have storage capacity up to 200 milliamperes, nAH.
Large lead-acid batteries have storage capacity more than 100 AH.
If a battery is rated 10 AH at 12 volts DC, Power, P in watts = VI, volts × amps, multiply both sides by t, Watt-hours, Pt = VIt = 12 × 10 = 120 watt-hours.
The Ampere hour rating, Ah, is the current available when discharged evenly over a 20-hour period, the standard time length for rating batteries.
Reserve capacity is the number of minutes that a battery can supply a useful voltage (10.5. volts or more), under a 25 amp discharge rate.
Reserve capacity is used for batteries that run heavy loads.
A battery specification "RC@ 25A = 160 minutes" means that at 80oF (about 27oC), the battery can supply 25 amps of current at a usable voltage for 160 minutes.
Reserve capacity is often a truer test of battery life than amp hours, depending on how the battery is used.
The "shelf life" is the length of time a battery can remain in storage, (not connected to a load), without losing its energy capacity.
However, the metal plates eventually leak and react with each other, even though the battery is not in use.
Maximum deliverable current is the largest current a battery can push through a load without drop in its output voltage.

32.5.9 Deep-cycle battery
A deep-cycle battery should discharge at least 50% of its capacity, for the best life span versus cost.
It may be capable of withstanding repeated substantial discharges of up to 80% capacity.
The depth of discharge of the battery is related to the battery life, i.e. the number of charge and discharge cycles it can perform.
Deep-cycle batteries are used in sweepers, scrubbers, jacks, lifts, electric golf carts, boats, cathodic protection, uninterruptible power supplies for computers, and off-grid solar and wind power systems.
All deep-cycle batteries are classified and rated in amp-hours.
Amp-hours is the term used to describe a standardized rate of discharge measuring current relative to time.
It is calculated by multiplying amps and hours.
The generally accepted rating time period for most manufacturers is 20 hours.
Heavy duty deep-cycle batteries may rate up to 260 AH.
Deep-cycle batteries are generally rated in amp-hours at a C/20 discharge rate, which means 1/20 of the battery capacity for 20 hours until the battery reaches 1.75 volts per cell.
A battery rated at 100 amp-hours will maintain a 5 amp load continuously for 20 hours.
Actual capacity will vary with temperature, the size of the load and the rate of discharge.
Deep-cycle batteries used in UPS and telecommunication applications are rated in reserve capacity, which is the number of minutes the battery will maintain a constant 25A load at 80oF until voltage drops to 1.75 volts per cell.
To provide an approximate conversion to amp-hours, multiply reserve capacity by 0.6.

32.5.10 VRLA sealed battery
A sealed VRLA battery (Valve Regulated Lead Acid), is maintenance free, produces negligible gas when charging, easier to transport, because unlike wet "flooded"' batteries, they are not classified as "hazardous cargo".
The three sub-types are, wet, AGM, Gel.
The sealed wet type is primarily designed for the leisure and marine markets, some of which claim to be able to complete around 400 to 500 discharge cycles (to 80% depth of discharge).

32.5.11 Starting the engine with jumper leads
A starting battery used in most motor cars provides short, high current bursts for cranking the engine.
So it frequently discharges only a small percentage of its capacity.
Engine starting batteries are rated in cold cranking amps, CCA.
Engine starting batteries are designed to provide a heavy surge current of as much as 200 amps for a period of 5-10 seconds.
There is no direct correlation between CCA and amp-hours, because starting batteries are not designed for slow periods of deep discharge.
Automobile Jumper Leads
Precautions before jump starting the engine
Ensure that the ignition switch is in the OFF position.
Turn off the lights and power draining functions to help ease the strain off the donor battery.
Ensure that the donor battery is the same voltage as the flat battery.
Ensure that the two vehicles are not touching in anyway
Make sure that the vehicles are in either Neutral (Manual) or Park (Auto)
Remove the vent caps from non maintenance free batteries.
Be careful! Battery acid is corrosive!
Procedure for jump starting the engine
1. Connect the red coloured jumper lead to the (+) positive terminal of the booster battery and the other end of the red jumper lead to the (+) positive terminal of the flat battery.
2. Connect the black jumper lead to the (-) negative terminal of the booster battery and the other end of the black jumper lead to a good earth point on the disabled vehicle.
The engine block is typically the best place for a good earth point.
3. Start the engine of the disabled vehicle.
4. With the engine speed at idle, disconnect the jumper leads in the reverse order of connection.
The car has now been started and after any caps and covers have been replaced it is ready to drive.
The cause of the flat battery should be identified and rectified as soon as possible.
5. Late model cars are generally more dependent on complex electronics to function and any voltage spike can damage the delicate electronic circuitry.
To avoid this problem, use jumper leads fitted with a "spike guards", (surge protectors).

32.5.12 Electric Bell
See diagram 32.5..4.4: Electric bell, electric buzzer.
In the diagram a piece of soft iron A projects above the wooden base of the bell.
Rods of soft iron B and C are screwed into A.
Coils of insulated wire wound on wooden bobbins are placed over these rods and connected so that the polarity of one rod is opposite to that of the other.
E is another projection raised above the base.
H is a flat steel spring.
D is a flat bar of soft iron fixed to H and carrying the hammer that strikes the bell.
F is a brass post insulated from the base and carrying a contact screw with a tungsten point on the end of it at K.
A corresponding tungsten contact point is carried on an extension of the spring H.
J is a brass bar insulated from the base.
Connections to the battery are made at J and E through a bell push.
When the bell push is pressed, the circuit is completed and current flows through the two coils to F, through the contact points,
and back to the battery through H and E.
The current magnetizes A, B, C, and D so that B and C attract D, which moves towards B and C causing the hammer to strike the gong.
Just when the contact carried by D is pulled away from the fixed contact carried by F, and the circuit is broken.
The current stops, so that A, B, C, and D are demagnetized and the spring pulls D back.
The contacts touch again and the whole process repeats itself rapidly, causing repeated ringing of the bell and sparking between the contacts.
If the adjusting screw at F is too far in, the bell will not ring, because the contacts will be unable to part and current will flow without a break.
If the adjusting screw at F is too far out, the contacts will not touch in the rest position, so that when the bell push is pressed, no current will flow
and as there is no attraction of D, the bell will not ring.

32.5.13 Spark plugs
Spark plugs, operating temperature, pre ignition, spark plug gap
See diagram 32.5.5.5d: Spark plugs.
The spark plug ignites the air and fuel mixture in the combustion chamber by using a spark plug gap across which a spark passes to ignite the mixture.
Spark plugs have a corrosion resisting steel alloy body around an insulated central electrode.
The electrode has a terminal cap at the top to connect to a high tension lead from the distributor.
A second electrode that forms part of the earthed steel body of the spark plug has a small gap between it and the lower end of the central electrode.
The body of the spark plug is screwed into the cylinder head.
The spark jumps across the gap between the two electrodes.

32.5.14 Spark plug operating temperature
Pre-ignition can occur if the combustion chamber reaches a temperature of 800oC near the end of the compression stroke and before the spark occurs.
Impure oil and carbon are good conductors of electricity under the high voltage generated in the ignition system.
They short circuit insulator by oil, carbon or products of combustion can occur if the temperature of the insulator falls below 300oC.
The optimum temperature range is 500oC to 600oC.
The temperatures are high enough for any deposits on the spark plug to be burnt off, but not so high as to cause pre ignition.

32.5.15 Spark plug gap
If a spark plug is not gas tight, leakage of gas past a spark plug causes lowered gas pressure in the cylinder or damage to insulation from rising hot gases.
The end of the spark gap is flush with the wall of the combustion chamber, depending on the reach of the plug.
The reach of the plug is the length of the threaded part to the end of the body.
If the reach is too short, the spark will be pocketed to cause difficult starting and misfiring at low speeds.
If the reach is too long, the plug tip and the electrodes will project into the combustion chamber and become overheated to cause pinking, knocking, and, pre ignition, resulting in loss of power.
Protruding tip spark plugs run cooler at high engine speeds and run hotter at low engine speeds, because the insulator receives more heat, but is cooled more by the incoming fuel.
Too large a spark plug gap can cause the sparking voltage of the plug to be too high overloading the secondary winding of the coil or preventing a spark from occurring.
Too small a spark plug gap causes incomplete burning of the mixture and the gap may become fouled.
The wear, corrosion, and erosion of the plug electrodes cause gap growth.
Rapid gap growth is caused by operation at too high a temperature that accelerates the burning of electrode material.
Also, fuel containing tetra ethyl lead deposit lead oxides and sulfates to cause pitting of the central electrode.
Corrosion deposits on the insulator may short circuit the spark plug so that no spark occurs at the spark gap.
Deposits of oil on the insulator and electrodes are caused by oil when the cylinder bores and piston rings are worn.
Spark plugs should be cleaned to produce a smooth, polished insulator surface each side of the gap.
After cleaning the spark plug, the spark gap must be checked with a feeler gauge.

32.5.16 Lead accumulator cell
The Lead cell accumulator has EMF about 2 volts and very low internal resistance.
It is a secondary cell.
The terminals are usually marked + (red) and - (black).
Since the internal resistance is very low, great care must be taken to avoid "short circuiting " the cell, i.e. there must always be a resistance of at least 1 ohm in the external circuit connecting the terminals.
1. Use a 250 mL beaker or jar with a cover to prevent drying by evaporation when the cell is not in use.
You need 2 sheets of 40 × 10 cm thin lead foil and 2 lead strips 2 × 14 cm as terminals.
These lead pieces require thorough cleaning by means of wire wool.
Fold the long sheets of lead tightly to the shorter strips so that they make good electrical contact.
The projecting ends will serve as terminals.
A blotting paper B lead c terminals A sandwich is made of alternating strips of lead foil and blotting paper.
When the sandwich is ready it is rolled up quite tightly, secured round the outside with one or two elastic bands, and placed with terminals at the top, in the cup or jar.
Mark one terminal positive, and the other negative.
The roll is covered with a solution of sodium sulfate made by dissolving 40 g of anhydrous sodium sulfate crystals in 200 mL water.
The cell is now ready to charge with electricity.
This can be done with a 6 volt battery charger, or with any low voltage direct current supply giving up to 10 amps.
Connect positive on the charger to positive on the cell.
After only a few minutes charging, the cell will light a 1.5. volt bulb.
Provided that the cell is always connected to the charger in the same way, as described above, the more times it is charged and discharged, the more efficient it becomes.
There will be enough current to make a small 1 volt electric motor spin round.
The cell will remain serviceable for several months if the cover is put on when not in use.
2. Charge a simple lead-acid battery with two electrodes, lead plates, in a sulfuric acid solution for a short time and then discharge through a doorbell.
Charge two lead plates in 30% sulfuric acid and discharge through a flashlight bulb.
3. Internal resistance of batteries, weak and good battery.
Measure similar no load voltage on identical looking batteries and then apply a load to each and show the difference in voltage between a good and weak battery.