How (and when) to buy a new car battery
Question: I think I might need a new battery soon. A friend rode with me and commented that my engine didn't crank over as fast as it should as we departed early one morning. Does this sound right? Can you give me some help on how to deal with this? I don't recall when it was replaced last; it was probably a long time back.
C.M. Answer: Your friend sounds like a great person to have onboard! Symptoms of an aging battery are slow cranking, particularly during a cool morning start-up, after a long period with doors open or headlights left on for a bit. A noticeable change in headlight brightness from engine idle to faster is another symptom.
Just to be sure it’s truly time to replace the battery, there is a quick and easy test called a conductance test, which requires a diagnostic tool. Many auto parts stores and repair facilities offer this as a free service, hoping you’ll follow up with a battery purchase, if needed.
Other possible causes of a noticeably weaker battery include corroded/loose terminals, excessive draining of battery energy while parked, insufficient charging of the battery or starter problems (if poor/no cranking is the only symptom). These are easy to check, if symptoms warrant it. Batteries usually show aging symptoms during colder weather as their performance is reduced and starting system requirements are higher.
Here’s some advice if you do need to shop for a new battery. Your particular automotive battery has a group number (usually two digits, all vehicles have one) as specified by BCI (Battery Council International). This number indicates physical size, terminal type and orientation, hold-down method and other characteristics.
In addition to the group number, carmakers specify a CCA (cold cranking amps) rating for a vehicle depending on engine size and the quantity of electrical accessories. CCA is the number of amps (quantity of electrical current) the battery can deliver for 30 seconds at -17 C, while maintaining at least 7.2 volts — kind of an odd yardstick but useful for comparisons. Depending on a battery’s physical size, plate composition and design characteristics, CCA can range from about 400 to 1,000. Typically, a physically larger battery has a higher rating, and it doesn’t hurt to go larger if it will fit. Be sure to meet or exceed the recommended CCA rating.
Reserve capacity (RC) might also be specified. This is a measure of the battery’s ability to deliver current without being recharged, perhaps while the vehicle is parked or driven with a charging system failure. RC is the number of minutes the battery can deliver 25 amps of current without the voltage falling below 10.5 volts (12.6 volts is the normal no-load voltage of a fully charged battery).
Replacement batteries can also be optimized for climate. Cold weather reduces performance and hot weather shortens life. Another possible choice is a pricier AGM (absorbed glass mat) battery instead of a traditional flooded cell battery. This design is spill-proof, more vibration resistant and longer lasting, and it doesn’t self-discharge as quickly during storage, among other advantages.
When battery shopping, take a look at the warranty offered. There’s a full replacement time (typically two to three years) often followed by a prorated period, perhaps another three to four years. I look for the best full replacement period, as the pro-rated warranty limits replacement choices.