Fundamentals
Automotive electricity, from charge to voltage drop.
Chapter 01
Electrical charge
Everything electrical starts with charge — a property of electrons that makes them push and pull on each other.
Charge is measured in coulombs. One coulomb is roughly 6.24 × 10^18 electrons. Nothing in a circuit is created or consumed; charge simply moves.
On the vehicle
A battery does not store electricity. It stores the chemical ability to push charge around a circuit.
What this means when diagnosing
If charge is only ever moved, then a dead circuit means the path is broken, not that the electricity has run out.
Chapter 02
Voltage — the push
Voltage is the pressure that makes charge move. No pressure, no movement.
One volt is one joule of energy per coulomb of charge. Voltage is always a difference between two points, never an absolute value at one point.
On the vehicle
A 12 V battery gives every coulomb 12 joules of energy to spend on its way round the circuit.
What this means when diagnosing
This is why a meter needs two probes. 'Twelve volts at the connector' means nothing until you say what it was measured against.
Chapter 03
How a battery makes voltage
Chemistry inside each cell separates charge, creating a pressure difference between the plates.
A lead-acid cell produces about 2.1 V regardless of its size. Six cells in series give roughly 12.6 V.
On the vehicle
A big commercial battery and a small car battery both sit at about 12.6 V. The big one just delivers far more current for far longer.
What this means when diagnosing
Bigger plates mean more capacity and more current capability — not more voltage. Voltage tells you charge state, not health.
Chapter 04
Battery chemistries
Different chemistries give different cell voltages, lifespans and charging rules.
Lead-acid rests near 12.6 V, AGM slightly higher, lithium iron phosphate cells sit around 3.2 V nominal and are stacked in series to reach system voltage.
On the vehicle
An EV pack built from 3.7 V cells needs roughly 108 cells in series to reach 400 V, with parallel groups for capacity.
What this means when diagnosing
Never judge a battery against the wrong chemistry's numbers. High-voltage systems require specific training and equipment — do not probe orange cabling.
Chapter 05
Discharging and charging
Discharging converts chemical energy to electrical energy. Charging reverses it.
In a lead-acid cell, discharge converts plate material to lead sulphate and dilutes the electrolyte; charging reverses the reaction.
On the vehicle
A resting voltage of 12.2 V is about half charged, not slightly low.
What this means when diagnosing
Sulphation from long periods at low charge is permanent. A battery repeatedly run flat will never test the same again.
Chapter 06
Volts as joules per coulomb
Voltage is a measure of how much energy each unit of charge is carrying.
1 V = 1 J/C. A 48 V system gives each coulomb four times the energy of a 12 V system.
On the vehicle
This is why 48 V mild hybrids can move the same power through much thinner cable than a 12 V system could.
What this means when diagnosing
Higher voltage means less current for the same power, which means smaller losses in the wiring.
Chapter 07
Current — the flow
Current is how much charge is moving past a point each second.
One amp is one coulomb per second.
On the vehicle
A starter drawing 200 A is moving 200 coulombs of charge every second.
What this means when diagnosing
Current is the same all the way round a series circuit. If it is low everywhere, something is restricting the whole path.
Chapter 08
What determines current
Current is set by the voltage available and the resistance in its way.
I = V / R. Raise the resistance and the current falls in exact proportion.
On the vehicle
12 V across a 2 Ω lamp gives 6 A. Add 1 Ω of corrosion and it falls to 4 A.
What this means when diagnosing
This is the whole basis of the voltage drop test: added resistance steals both current and voltage from the load.
Chapter 09
Resistance
Resistance is opposition to flow. Every wire, joint and component has some.
Resistance rises with conductor length, falls with cross-sectional area, and changes with material and temperature.
On the vehicle
A starter cable of 0.05 Ω passing 200 A drops 10 V — the starter only sees 2.6 V and barely turns.
What this means when diagnosing
A joint with 1 Ω of corrosion is invisible at the milliamps a meter uses, and catastrophic at working current.
Chapter 10
Power
Power is the rate energy is being converted — light, heat or motion.
P = V × I, and by substitution P = I²R.
On the vehicle
A 55 W headlight on 12 V draws about 4.6 A.
What this means when diagnosing
Because heat rises with the square of current, a slightly loose high-current joint gets hot very fast.
Chapter 11
Series circuits
One path. The same current passes through everything on it.
Resistances add. The supply voltage divides across them in proportion to each resistance.
On the vehicle
Two identical bulbs in series each get half the voltage and both run dim.
What this means when diagnosing
Unwanted resistance is simply an extra series element stealing its share of the supply.
Chapter 12
Parallel circuits
Several paths. Each branch gets the full voltage and draws its own current.
Total resistance falls below the smallest branch; branch currents add to give the total.
On the vehicle
Vehicle lighting is wired in parallel so each lamp gets full voltage independently.
What this means when diagnosing
Adding loads to a parallel circuit raises total current, which is how a circuit ends up overloading its fuse.
Chapter 13
Open circuits
A break in the path. Nothing flows.
With no current, the full supply voltage appears across the break and nowhere else.
On the vehicle
A broken wire measures battery voltage on one side and nothing on the other.
What this means when diagnosing
Measuring near full source voltage across two probes means you have found the break between them.
Chapter 14
Short circuits
An unintended low-resistance path that lets current soar.
Current is limited only by the small resistance of the cable and source, so it rises until the fuse clears.
On the vehicle
A chafed wire touching the body drops the circuit resistance to almost nothing.
What this means when diagnosing
Fuses that blow instantly point to a short to ground. Fuses that blow after a while usually point to an overload.
Chapter 15
Multimeter fundamentals
The right function, the right socket, and two probes in the right places.
A voltmeter has very high input impedance and draws almost no current. An ammeter has near-zero resistance and must carry the circuit's current.
On the vehicle
Leaving the leads in the 10 A socket and probing a live feed is a direct short.
What this means when diagnosing
Because the meter draws so little current, it can read healthy voltage through a badly corroded joint.
Chapter 16
Measuring voltage
Always across — the meter goes in parallel with what you are measuring.
You are measuring the difference between two points, so your reference matters as much as your probe.
On the vehicle
Red at the supply pin and black at the battery negative tests the supply and the ground path in a single reading.
What this means when diagnosing
Referencing a local ground rather than the battery hides ground faults completely.
Chapter 17
Measuring current
Always in series — the current has to go through the meter.
The circuit must be broken and the meter inserted into the path.
On the vehicle
A parasitic drain test puts the meter between the battery post and its cable.
What this means when diagnosing
Never place an ammeter across a battery or a live load. It is a direct short through the meter.
Chapter 18
Current clamps
Measures current from the magnetic field around a cable, without breaking the circuit.
DC clamps use a Hall-effect sensor and must be zeroed before each measurement.
On the vehicle
A 600 A clamp is the practical way to measure starter draw.
What this means when diagnosing
Clamp one conductor only. Feed and return together produce opposing fields that cancel to almost zero.
Chapter 19
Amp-hours and capacity
How much current a battery can supply for how long.
A 70 Ah battery nominally delivers 3.5 A for 20 hours. High discharge rates deliver noticeably less than the rating suggests.
On the vehicle
A 50 mA drain over 72 hours consumes 3.6 Ah — trivial. 500 mA consumes 36 Ah — fatal.
What this means when diagnosing
Comparing a measured drain against capacity tells you whether the drain explains the complaint.
Chapter 20
Voltage drop — where it all comes together
Unwanted resistance steals voltage from the load, and only shows up under load.
V = I × R applied to the fault itself. No current means no drop, which is why static tests pass bad circuits.
On the vehicle
12.6 V, a 2 Ω load and 1 Ω of corrosion gives 4.2 A, 4.2 V lost in the fault and only 8.4 V at the load.
What this means when diagnosing
Under 0.2 V across a connection, under 0.1 V across a ground. Anything more is stealing performance.
Interactive — resistance fault simulator
12.6 V source, a 2 Ω load, and a corroded connection you control.
- Current
- 5.48 A
- Lost in fault
- 1.64 V
- At the load
- 10.96 V
- Heat in fault
- 9.0 W