What Is Reserve Capacity in a Battery?

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Reserve capacity is an important factor in battery performance.

Battery usage and maintenance is a critical matter in automobiles and other devices. Battery performance is a complex concept that encompasses a number of factors, but one of the most important is what's called reserve capacity. Realizing what reserve capacity is allows motorists to make good decisions about purchasing car batteries.

Defining Reserve Capacity

Reserve capacity is one of the ratings or specifications ascribed to a battery. The term is usually encountered when discussing 12-volt car starting batteries. Reserve capacity is defined as the number of minutes a fully charged 12-volt battery at 80 degrees Fahrenheit can provide 25 amperes at 10.5 volts until the voltage decreases.

Importance of Reserve Capacity

When a driver starts his vehicle, the battery pushes out 25 amperes at 10.5 volts in order to make the alternator run. As the alternator takes over to power the engine, the battery provides the same amount of electricity at a steady voltage to provide power to lights, wipers and other accessories. When a vehicle is in proper condition, the alternator replaces the electricity a battery puts out. Should the alternator or the fan belt fail, the vehicle's engine uses electricity from the battery. The time a vehicle operates solely on battery power until battery discharge is a practical measure of reserve capacity.

Differentiating Reserve Capacity from Cold Cranking Amps

A driver in an area with a colder climate will choose a battery based on cold cranking amperes while a driver in a warmer area chooses a battery based on reserve capacity. The difference in judgment is due to the difference in requirements based on climate and ambient temperature. Cold cranking amps are the discharge load a fully charged battery can deliver for 30 seconds while maintaining a voltage higher than 7.2 volts at 0 degrees Fahrenheit. In cold climates, the higher a battery's cold cranking amps, the more easily an engine will start. In hot climates cold cranking amps are not an issue with starting a vehicle.

Measuring Reserve Capacity

Measuring reserve capacity has already become more important than measuring cold cranking amps as demand for auxiliary power is increasing as car models get updated. A recently developed instrument called the Cadex CA-12 can read reserve capacity, cold cranking amps and state-of-charge. Gaining such information gives drivers and battery technicians a better way of determining the time to replace batteries.


Simon Slayford

Simon Slayford is an education professional who wanted to try something different. He has been a writer for over eight years and has been published in local writing magazines such as "The New Zealand Listener." Slayford holds a master's degree in language studies and early literacy from University of Otago.