Virtual Mechanic

SAE Cranking Current
The Society of Automotive Engineers (SAE) defines starting power as the “SAE Cranking Current” or Cold Cranking Amps (CCA) which is the internationally recognised industry standard to determine battery starting capability.  The cold cranking test is conducted at – 18C to simulate very cold, difficult to start conditions.  The battery’s ability to perform is measured by the amount of current the battery can deliver over a short period of 30 seconds (referred to as the 30 second rate) while maintaining a voltage equivalent of 1.2 volts per cell or higher.  Therefore, a 12 volt battery must maintain a voltage equivalent of 7.2 and 3.6 for a 6 volt battery.

Reserve Capacity
A battery’s reserve capacity must sustain a minimum electrical load for ignition, headlights, windshield wipers and defroster under cold winter conditions in the event of a charging system failure.

The reserve capacity rating represents the number of minutes at 25C a battery can supply a load of 25 Amps and maintain a voltage of 1.75 volts or higher per cell.  (10.5 volts for a 12 volt battery of 5.25 volts for a 6 volt battery).

Amp Hours
No longer in popular use as a standard for rating automotive batteries, the “Amp Hour” rating represents the current a battery can supply for 20 hours.  For example, a 50 Amp hour battery supplies 2.5 Amps for 20 hours.  2.5 Amps x 20 hours = 50 Amp hours.  The standard was abandoned because it fails to rate the starting capability of a battery and its ability to power a typical accessory load.

The Amp hour rating does have benefits with regard to cycling batteries.  A battery’s efficiency varies depending on the rate of discharge.  The rating for cycling batteries uses Amp hours at three separate rates of discharge (see table).

20 hr 50 Amp Hrs (2.5 amp x 20 hrs)
  5 hr 41 Amp Hrs (8.2 amp x 5 hrs)
  2 hr 34 Amp Hrs (17 amp x 2 hrs)

Plates per Cell
In the past, it was assumed that “The more plates the greater the power”.  Therefore, “An 11 plate battery is more powerful than a 7 plate battery”.  Plates per cell has been discarded as a measure of battery performance because the size of the plates is more relevant to performance than the number of plates.  The area of plates determines starting power and the weight of active material determines reserve capacity.

A large number of thin plates that are small in surface area are likely to have less cranking power and less reserve power than a small number of thick plates that are large in area.
This article is found in the Virtual mechanic CD Rom
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By Darren Gow-Brown, Melbourne Australia