Basics of Battery Capacity Ratings

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With the rise in global warming due to excessive use of fossil fuels on the plant, calls are becoming increasingly louder for renewable energy sources that are less damaging to the environment and sustainable in the long run. Batteries are a common feature of renewable energy sources such as solar systems as well as a common feature in both cars using fossil fuel derivatives and electric cars which are being touted to replace fossil fuel cars in the feature.

Given the role batteries play in our everyday life, there is the need to understand battery capacity ratings which are commonly used.

What is the Capacity of a Battery?
Battery capacity is the amount of electrical energy a battery can deliver when fully charged. The capacity of a battery is determined by factors such as size, number of plates, the number of cells and the strength and volume of electrolyte.
Common battery capacity ratings in use are:
1. Cold Cranking Amperes (CCA)
2. Reserve Capacity (RC)
3. Amp-Hours (AH)
4. Power (Watts)



Cold Cranking Amperes (CCA)
This capacity rating applies to the ability of the battery to provide the required energy to drive a prime mover e.g. car engine. In this case, it refers to the ability of the battery to provide energy to crank an engine during starting. This will entail a large discharge in a short time. The CCA rating of a battery specifies in amperes the discharge load a fully charged battery at 0°F can deliver for 30 seconds while maintaining a voltage of at least 1.2 volts per cell 



Reserve Capacity (RC)
This describes the ability of a battery to provide emergency energy for a given time to meet certain load demands should the battery charging system fails. This will require adequate battery capacity at normal temperatures for certain period of time.  The RC rating of a battery specifies in minutes, the length of time a fully charged battery at 80°F (26.7°C) can be discharged at 25 Amps while maintaining a voltage of at least 1.75 volts per cell

Amp-Hours (AH)
The Amp-Hour (AH) rating of a battery is the most popular and commonly used rating of a battery. It is often called the 20-hour discharge rating. The Amp-Hour rating of a battery specifies in amp-hours, the current the battery can provide in 20 hours at 80°F (26.7°C) while maintaining a voltage of at least 1.75 volts per cell.

Amp-Hour (AH) = Current x Time (Hours)

A battery that delivers 10Amps for 20hrs has a capacity of = 10 x 20 = 200AH.

Power (Watts)
For prime mover applications where the power is required to provide cranking power, its capacity can also be rated in watts. The power rating of a battery in watt is determined by multiplying the current available by the battery voltage at 0°F (-17.8°C)