Battery stuff

General Science Bit

A battery's state of charge can be measured by the specific gravity of the acid and the temperature.

The SG of the battery acid varies with temperature and charge as the acid-lead solubility changes.

SG20 = St + 0.007 (t-20)

St = measured SG
t= temperature of electrolyte (the acid)
SG20 = corrected SG to 20'C

SG 1.11 to 1.13 discharged
1.23 to 1.25 70% charged
1.27 to 1.29 fully charged

Acid loses sulphate on discharging and oxygen is evolved into the electrolyte and water is formed also diluting the acid.

Dilute or not, battery acid is the realm of mass-murders to tread very carefully working around batteries as they can give off hydrogen (as in Hindenburg disaster?)

Nice and easy battery hydrometers are available.

Battery cut-off, FIA spec

A normal battery cut-off switch just disconnects the battery. This can allow the engine to keep running (on the alto), which is not permitted in racing.

FIA type cutoffs have another lower current contact for the alternator output wire. Breaking this as well as the battery feed will take LT feed to 0V and cut the engine.

If the Master switch is a true FIA approved type it has 3 switches within it. Two of the switches (battery lead and ignition feed) are normally closed, the third is normally open. The normally open switch will ground the alternator via a protective resistor causing the field effect to collapse, therefore no electricity is produced. On a carb Mini, the switch cuts power to the LT side of the ignition and the fuel pump supply.

On fuel-injected cars, the normally closed switches are wired direct to the ECU Feed. This shuts down the ECU therefore shutting down the fuel pump and the ignition and injector; the main switch isolates the battery.

Batteries, testing and health

Car battery testers measure the voltage under a small load.

A healthy (lead acid) battery with (some or more) charge will have a very low internal resistance so the current through a resistor, resistance R will be I=V/R.

If the battery is ill (or discharged), internal resistance will be higher (chemical bit: less ions to carry the charge for the current) so the value for the current will be lower (and the voltage at the terminals will
be lower as well).

If the battery is reading 10V (or lower) unloaded then it's likely that the plates in one (or more) of the cells have warped and shorted out the cell (each cell is 2V). If the battery is reading 0V, then something has gone open circuit somewhere.

07 June 2003