Car batteries have two jobs: starting the engine and powering other vehicle systems. Choosing the right battery for your needs requires understanding the numbers on the battery label.
CCA, cold cranking amps, is a good place to start when shopping for standard lead-acid car batteries. Other important numbers include reserve capacity and battery group sizes, which are standardized specifications.
Volts
Your battery stores volts—the electrical charge that powers all of your car’s electrical systems and accessories. Without proper voltage, your engine wouldn’t crank over and you wouldn’t be able to power things like the lights, radio, and sensors that help your vehicle operate at all times.
The optimum car battery voltage reading is between 13.7 and 14.7 volts, boosted slightly by your alternator. If you check your battery and it reads lower than that, it signals the need for a replacement.
In addition to the volts, your car battery has an ampere-hour (Ah) rating that determines its capacity and how long it can sustain high levels of current. A higher Ah rating results in more dependable performance, especially when running accessories.
Using a voltmeter, you can check your battery’s voltage at home. Connect the voltmeter’s positive and negative leads to your battery and note the voltage. 13.7 to 14.7 volts indicates the battery has its full capacity. A lower reading, such as 12.2 or below, indicates that it is only operating at 75 percent of its capacity and may be nearing the end of its lifespan.
Car batteries with a lower Ah rating are designed to provide more starting power than those with an upper range, but they also tend to have less cycling capability. Alternatively, you can opt for a battery with a high CCA rating that provides good cycling performance as well.
Most auto batteries are lead-acid with an outer casing that’s filled with acid and has a pair of heavy terminals that correspond to the positive and negative plates in each cell. Some have vents that open to release gas during recharging and can leak acid when overcharged. Newer batteries are often called low-maintenance or zero-maintenance, and they don’t require adding water to replace the sulfate that decomposes during operation. They’re also less prone to deep discharge that can coat the plates with lead sulfate deposits, shortening their lifespan.
Amps
Amps are a measure of how much electric current moves through a battery, and also of the battery’s capacity to supply that current for an extended period of time. Car batteries typically have a “CCA” rating, which stands for Cold Cranking Amps, and indicates how many seconds the battery can crank an engine at 0°F before its voltage starts to drop.
A battery’s “CCA” rating is important because it can help ensure your engine will start reliably, even in cold weather. However, there are other factors that can affect a battery’s performance, such as its reserve capacity (RC). Reserve Capacity is a measure of the amount of time the battery can keep essential electrical components running if the alternator fails and the vehicle is running solely on battery power.
Battery amp ratings are also helpful because they indicate how much electricity a battery can deliver over an hourly period before reaching its cutoff voltage. A battery with a higher amp-hour rating can support more systems and power-hungry features without draining the battery as quickly.
Another way to measure a battery’s output is to hook up five light bulbs in parallel, and then place a voltmeter across the terminals to see how many minutes the battery can sustain the circuit before its voltage drops below 13.5 volts. This can be a quick, inexpensive test to determine the strength of a battery, but it’s not accurate when comparing batteries, as different chemistries have varying discharge rates.
Amp-hour ratings are important to know because they can help you find the right battery for your vehicle’s needs and usage. A larger amp-hour rating can support a more powerful engine or power-hungry accessories, while a smaller ampere-hour rating is better for vehicles with simpler options that require less energy from the battery during periods of inactivity. It’s also useful to consider how often the battery will be charged, as a greater number of charging cycles will wear down a battery more quickly.