Battery capacity ratings are established by the Battery Council International (BCI) in conjunction with the Society of Automotive Engineers (SAE). Battery cell voltage depends on the types of materials used in the construction of the battery. Current capacity depends on several factors:
- The size of the cell plates. The larger the surface area of the plates, the more chemical action that can occur. This means a greater current is produced.
- The weight of the positive and negative plate active materials.
- The weight of the sulfuric acid in the electrolyte solution.
The battery’s current capacity rating is an indication of its ability to deliver cranking power to the starter motor and of its ability to provide reserve power to the electrical system. The commonly used current capacity ratings are explained in the following sections.
The ampere-hour rating is the amount of steady current that a fully charged battery can supply for 20 hours at 80°F (26.7°C) without the terminal voltage falling below 10.5 volts. For example, if a battery can be discharged for 20 hours at a rate of 4.0 amperes before its terminal voltage reads 10.5 volts, it would be rated at 80 ampere-hours. This method of battery rating is not widely used.
Cold Cranking Rating
Cold cranking rating is the most common method of rating automotive batteries. It is determined by the load, in amperes, that a battery is able to deliver for 30 seconds at 0°F (-17.7°C) without terminal voltage falling below 7.2 volts (1.2 volts per cell) for a 12-volt battery. The cold cranking rating is given in total amperage and is identified as 300 CCA, 400 CCA, 500 CCA, and so on. Some batteries are rated as high as 1,100 CCA.
Cranking Amps (CA) is an indication of the battery’s ability to provide a cranking amperage at 32°F (0°C). This rating uses the same test procedure as the cold cranking rating or CCA discussed earlier, except it uses a higher temperature. To convert CA to CCA, divide the CA by 1.25. For example, a 650-CCA-rated battery is the same as 812 CA. It is important that the technician does not misread the rating and think the battery is rated as CCA instead of CA.
The reserve-capacity rating is determined by the length of time, in minutes, that a fully charged battery can be discharged at 25 amperes before battery voltage drops below 10.5 volts. This rating gives an indication of how long the vehicle can be driven, with the headlights on, if the charging system should fail.
Battery Size Selection
FIGURE. The effect temperature has on the cranking power of the battery.
Some of the aspects that determine the battery rating required for a vehicle include engine size, engine type, climatic conditions, vehicle options, and so on. The requirement for electrical energy to crank the engine increases as the temperature decreases. Battery power drops drastically as temperatures drop below freezing. The engine also becomes harder to crank due to the tendency of oils to thicken when cold, which results in increased friction. As a general rule, it takes 1 ampere of cold cranking power per cubic inch of engine displacement. Therefore, a 200-cubic-inch displacement (CID) engine should be fitted with a battery of at least 200 CCA. To convert this into metric, it takes 1 amp of cold cranking power for every 16 cm3 of engine displacement. A 1.6-liter engine should require at least a battery rated at 100 CCA. This rule may not apply to vehicles that have several electrical accessories. The best method of determining the correct battery is to refer to the manufacturer’s specifications.
The battery that is selected should fit the battery holding fixture and the holddown must be able to be installed. It is also important that the height of the battery not allow the terminals to short across the vehicle hood when it is shut. BCI group numbers are used to indicate the physical size and other features of the battery. This group number does not indicate the current capacity of the battery.