Battery and Charging Issues

Date: 5 Mar 2016


This article focuses on battery and charging issues in standard cars. The principles are similar for modified cars but keep in mind battery size, placement and charging needs may vary.

Car does not start, engine turns over but does not fire and turnover is laboured. This indicates a battery fault.

In standard cars the battery is charged by the dynamo.

The regulator controls when the dynamo charge needs to go to the battery.

Charging faults often show up as a flat battery (often meaning care will not start) or unresponsive electrical items such as lights or windscreen wipers.

Failed or failing charging will often show up with a flickering or permanently on red lamp on the dashboard. Sometimes the dashboard lamp does not illuminate yet the battery is repeatedly run down.

There are four potential faults:

  1. Failed battery
  2. Failed dynamo
  3. Failed or partially failed regulator
  4. Wiring fault


Fan belt

Check fan belt is tensioned. There should be about 1 inch (25 mm) play in the belt when pushed on the centre between water pump to dynamo/alternator pulley.


To diagnose your battery:

  • Charge battery with an overnight with an external charger
  • Take battery to a battery centre and request a discharge test (in my experience this usually results in battery death so is a last resort)

If battery cannot start car after being charged and then left then replace.

If battery, after charging, can start the car then

  • Leave battery disconnected from car overnight

If battery can still start car then it is probably ok and there is a charging issue.

Charging system diagnosis

  • For this you will need an ammeter with +/- 40A tolerance (a dial from another car will do, if one is not installed in your car)
  • Start car and check ammeter needle moves into the positive (charge) position

 If it is at zero or always in discharge position then either regulator or dynamo are faulty.

Checking the dynamo

These often fail if the fan belt is loose of when the internal brushes wear down and stop touching the brass commutator

  • Check the fan belt tension, it should move about an inch when pushed in the middle of the longest span

If belt is too loose then dynamo pulley will not turn the belt will just slip round, adjust belt tension

If the belt is too tight, then the dynamo will turn but the front bearing will wear out in time.

If dynamo is turning:

Check dynamo is generating volts

  • Connect a voltmeter between earth and output on rear of dynamo
  • Check you are clear of fan
  • Start car and check voltage

Checking the regulator

Once dynamo checked, last resort is to change regulator box, there is some method for testing it but I do not know what it is.

Checking the wiring

Finally, if none of the above solves the charging issue, check all the wiring for faults.


Battery Repair

Modern batteries are sealed units and normally not repairable.

Replacement round post type 038 batteries are readily available. Type 037 square post batteries are harder to source (refer post on batteries).

Older batteries are serviced by topping up the acid with distilled water.

Dynamo Repair

Lucas dynamo’s are quite easy to dismantle and work on.

  • Remove dynamo disassemble and check brushes are in contact with commutator
  • If brushes are very worn spring pressure may not be sufficient or spring may be shorting to earth. Replace the brushes.
  • Whilst the dynamo is apart check commutator is clean and clear of brush carbon. Also check bearing for excessive wear.

If replacing a dynamo, remove the pulley and cooling fan assembly as well as the woodruff key (little metal thing) as these may not be supplied with the replacement unit.

Regulator Repair

Regulators are difficult to diagnose and even more difficult to repair. If suspected faulty it is best to replace.


Approximate prices from: 3 Jan 2023

Battery Type 037 or 038 (refer battery article)
EL56Lucas Type C40 dynamo
Electrical: EL56 C40 Dynamo (
Club Spares£58.56
Electrical: EL1 – Voltage regulator (
Club Spares£20.50
CG13Fan belt
Cooling: CG13 – Fan belt (
Club Spares£5.76


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