Speaking of power, while you're comparing car batteries, compare when a battery was shipped or manufactured. A battery loses power while sitting on a store or warehouse shelf. There are two ways to know the age of a battery: a shipping date code or a manufacturer date code. A shipping date code either will be on a sticker or heat-stamped into the battery. You'll notice a letter and a number. The letter notes the month. For example, the letter A tells you it was shipped in January. The number notes the year and only uses the last number of the year. For example, 2 would refer to 2012, and 3 to 2013. A manufacturer date code uses the same numbering system to mark the year; however, the day will be marked by the day of the year. For example, 2 150 means the battery was manufactured on 2012, May 30. Make sure the battery you buy is less than six months old. This information won't be on a battery comparison chart, but it's an important factor to consider as you do side-by-side comparisons.