I typically use my car only once a week to go buy groceries (about a 10 minute drive). As it is starting to get colder I'm worried about the battery. I was told by a mechanic on this website that as long as the drive is 30 minutes I shouldn't worry about the battery, but I live in a college town where everything is only about a 10 or 15 minute drive away. In general I've never needed to travel more than 20 minutes. I've been told to idle the car for a few minutes, but I've read that starting the engine requires more energy and idling does not help to recharge the battery. Are there other ways to protect my battery? How concerned should I be? My car is used, but the battery was replaced this year.
A car battery relies on a chemical reaction in order to produce usable power or current. Cold weather is known to slow this process down which wreaks havoc on car batteries, resulting in them dying much quicker than when in warmer weather. As you may know, when you drive your car your car’s alternator is busy charging the battery. This generally requires the RPM’s to be at normal operating range (depending on the car, around 1,500-2,000) consistently. Since this is not the case while idling, idling does not help nearly as much.
You are correct in that starting the car uses more power than simply idling will generate. You may want to consider investing in a small battery charger known as a "trickle charger" if you anticipate the car sitting for long periods of time. This is a small battery charger that maintains a slight charge on the battery when sitting for long periods of time. They generally will cycle on and off when the battery is charged or needs a charge.
Should you decide to purchase one or if the battery ends up needing any sort of service, I would recommend having an expert, such as one from YourMechanic, come to your location to perform any needed repairs and assist you in keeping your battery in top shape.