The automotive battery is often taken for granted, however with today’s technology changes it has become the cornerstone of the modern vehicle. In any vehicle, the battery is expected to deliver power to the engine management, ignition, airbag, ABS, stability, audio, heating and cooling systems as well as ensuring the engine starts when required!
The smart battery management systems found in many vehicles require the battery to power these systems independently, with the charging system only activating for short, high power bursts.
Automotive batteries are rated using three criteria. Many of us are familiar with CCA and RC, however most new vehicles now specify an Ah rating. Club Assist batteries nominate all three ratings on their top label, so it is appropriate that we take a few minutes to understand each rating.
CCA – Cold Cranking Amps (SAE)
One of the key roles of a battery is to start the engine. To do this, it must drive the starter motor (which consumes a large amount of power) while maintaining a high enough voltage to power the fuel, ignition and engine management system until the engine starts. Starting the engine involves a high current discharge (measured in Amperes or Amps) for a short period of time (normally <1.5 seconds).
The most common standard is SAE Cold Cranking Amps, which is defined by the Society of Automotive Engineers as:
The discharge load (in Amperes) that a new, fully charged battery at -17.8°C (0°F) can continuously deliver for 30 seconds, while maintaining a terminal voltage equal to or greater than 1.20 volts per cell (for a 12 volt battery, this minimum voltage is 7.2 volts).
Ah – Capacity (20 Hour Rate)
The 20Hr rate is a measure of a battery’s total energy storage capacity – it tells you how big the bucket of energy is! This rating is becoming more popular as more vehicles are fitted with smart battery management systems. The 20Hr rating is defined as:
The amount of power a battery can deliver in a 20 hour period, while maintaining a voltage that is equal to or greater than 1.75 volts per cell
As an example, a 60Ah battery will deliver a constant current of 3A for 20 hours (3A x 20hours =60Ah). Although the 20Hr rate is an actual measure of the energy storage of a battery, it does not give any indication of the cranking capacity of a battery or its ability to deliver power for a typical vehicle accessory load (vehicle accessory loads are much higher).
RC – Reserve Capacity
Reserve Capacity is a measure of how long a battery can sustain a voltage under a constant 25A load. Reserve Capacity is defined as:
The number of minutes that a new fully charged battery at 26.6°C (80°F) can be continuously discharged at 25 Amperes, and maintain a voltage that is equal to or greater than 1.75 volts per cell (for a 12 volt battery, this minimum voltage is 10.5 volts).
As you can see, battery ratings can be quite different and so too can their performance capabilities.