Car battery shopping comes down to matching two specific numbers to your vehicle, more than comparing brands or prices alone.
Cold Cranking Amps (CCA)
CCA measures how much current a battery can deliver for 30 seconds at 0°F (-18°C) while maintaining a minimum voltage — essentially, how much power it can put out to start a cold engine, which is the moment a battery is under the most strain. Every vehicle has a manufacturer-recommended minimum CCA rating, usually found in the owner's manual or on a sticker under the hood; buying a battery with a CCA rating below that minimum risks unreliable starts, especially in cold weather, while a higher CCA than required isn't harmful, just not necessary.
Group size determines physical fit
Group size is a standardized code describing a battery's physical dimensions and terminal layout — it needs to match your vehicle's battery tray and cable configuration to fit and connect properly. A battery with excellent CCA and capacity specs is still the wrong battery if the group size doesn't match what the vehicle is built for; this is a fit specification, not a performance one, and both need to be right independently of each other.
Reserve capacity matters more with heavy electrical use
Reserve capacity measures how long a battery can power essential electrical systems if the alternator stops charging it — relevant if a vehicle has a lot of aftermarket electronics, or as a general buffer against a failing alternator being caught early rather than leaving you stranded. For most standard vehicles with typical electrical loads, this matters less than CCA and group size, but it's worth checking for vehicles with significant added electrical equipment.
AGM vs. flooded lead-acid batteries
Absorbent Glass Mat (AGM) batteries are sealed, more resistant to vibration, and generally required for vehicles with auto start-stop systems, which cycle the battery far more often than a conventional engine. Traditional flooded lead-acid batteries are less expensive and sufficient for vehicles without start-stop systems, but generally shouldn't be substituted into a vehicle designed for AGM, since the electrical demands of start-stop systems can shorten a flooded battery's life significantly faster than in a conventional vehicle.
The one thing people forget
Check the battery's manufacture date, usually printed as a letter-and-number code (like "A24" for January 2024), not just the warranty length on the box. Batteries can sit in inventory for a while before being sold, and one that's already a year or more old at the time of purchase has effectively used up some of its useful life before it's even installed.