How a Battery Works
I can never remember the charge % for a battery at specific voltage settings so I decided to put this article on the site. This is not very technical but gives a good overview on how a battery works.
Voltage
The standard automotive battery in today's vehicles is 12 volts. Each battery has six cells with 2.1 volts. A car battery is considered fully charged at 12.6 volts.
When a battery drops voltage, even a small amount, it makes a big difference. For instance, when a battery drops from 12.6 to 12.0 volts, its power drops from 100% to 25%. At 12.4 volts, a car battery is 75% charged. At 12.2 volts, it's 50% charged. A car battery is considered charged at 12.4 volts or higher. It is considered discharged when it's at 12.39 volts or less.
Voltage is produced by a chemical reaction. Inside a battery there are positive and negative lead plates that sit in a liquid called electrolyte solution. Electrolyte solution is a mixture of water and sulfuric acid.
When this solution interacts with the lead plates, there's a chemical reaction. This chemical reaction creates pressure that we call "voltage."
The pressure (voltage) pushes the current out of the battery's negative terminal through the "load", the equipment drawing the power. The current returns through the positive terminal.
Chemical
To achieve the chemical reaction that creates voltage in an automotive battery, the electrolyte solution inside the battery must have the correct mix of water and sulfuric acid. As you know, when a car battery is at 12.6 volts, it's charged at 100 percent.
At 12.6 volts, the electrolyte solution is 65 percent water and 35 percent sulfuric acid. This is the ideal combination!
Sometimes electrolyte "breaks down" and the acid moves onto the plates. So, there's less acid in the water. Whenever the percentage of acid in the solution decreases, the charge drops.
Cranking Power
As the temperature drops, the cranking power required by the car increases. However, as more cranking power is used, the amount of battery power available decreases.
Cold Cranking Amps (CCA) is critical for good cranking ability. It refers to the number of amps a battery can support for 30 seconds at 0°F until the battery voltage drops to unusable levels. For example, a 12 volt battery with 600 CCAs means the battery will provide 600 amps for 30 seconds at 0°F before the voltage falls to 7.20 volts (six cells). The higher the CCA, the more powerful the cranking ability.
If you live in a cold climate, you should consider the CCA rating when choosing a battery. If you live in a very hot climate, you don't need as much CCA.