Battery Capacity Testing
An automotive battery is a rechargeable SLI or starting, lighting, and ignition battery. It’s a 12 volt battery made of 6 cells connected in series. Each of the 6 cells produces 2.1 volts and wiring these cells in series produces the 12.6 volts required by the automotive electrical system.
A battery’s current capacity ratings are used to measure its readiness to deliver cranking power to the starter motor as well as reserve energy for the electrical system.
CCA: The cold cranking amps or cold cranking rating determines the load or amperage the 12 volt battery can deliver for 30 seconds at 0 F without falling below 7.2 volts.
RC: A battery’s reserve capacity is the number of minutes that a fully charged battery can be discharged at 25 amperes before it drops below 10.5 volts.
AH: The ampere hour rating measures the amount of steady current a battery can supply for 20 hours at a certain amperage at 80F without cell voltage falling below 1.75 volts.
Battery Capacity Tests: Perform a battery load test to see if service or replacement is necessary. Determine the load by retrieving the CCA from the battery case and reducing this number by half. The discharge rate is one half of the battery’s cold cranking rating. Apply this load to the battery for 15 seconds. The battery’s voltage must remain above 9.6 volts at 70 F. It’s relative to temperature, the lower the temperature, the lower the voltage.
A specific gravity test determines the weight of a certain volume of liquid divided by an equal amount of water. A fully charged automotive battery has a specific gravity of 1.265. As the battery discharges the specific gravity becomes lighter and more like water, closer to 1.000. The specific gravity of a battery is a good indication or its state of charge. Use a hydrometer and adjust to temperature by adding .004 for every ten degrees above 80F and subtracting .004 for every10 degrees below 80F.