Choosing a car battery can be a confusing experience, especially if you want to take the time to cross-shop instead of buying the first thing on the shelf that fits your car. You'll need to consider many factors, including the cold-cranking amps, group size, and reserve capacity. However, another crucial factor worth considering is the battery's manufacture date.

You might think your car battery will remain relatively fresh and pristine until you start using it, but the reality is much more complex. Understanding a little bit about battery chemistry can help you understand why the manufacture date matters when choosing a battery and when deciding if it's time to replace your car's old battery.

How Lead-Acid Batteries Live and Die

Most modern internal combustion engines use lead-acid batteries to run the starter motor and power accessories. The alternator provides constant electrical power while the engine runs, but the battery still acts as an energy store to deliver smooth, consistent power to your car's electronics. Some cars use AGM (absorbent glass mat) batteries, which are advanced lead-acid designs.

As you might have guessed from the name, lead-acid batteries use a chemical reaction between lead plates and acid (typically sulfuric acid) to generate electricity. This reaction is reversible, so you can partially discharge and recharge your car's battery. Unfortunately, this reaction is inevitable. In other words, your battery will slowly discharge power even when not in use.

You can damage a lead-acid starter battery in several ways, but the most common is a deep discharge. Allowing your car's battery to discharge too far will result in internal damage, permanently reducing its capacity. However, normal usage and discharge will slowly reduce the lead's ability to react with the acid, eventually leading to a battery that no longer functions and must be replaced.

Why Your Battery's Manufacture Date Matters

The inevitable nature of the internal chemical reaction in your battery means that the clock starts ticking from the moment your battery leaves the factory. An old, unused battery might still function since a slow partial discharge is less intense than a deep discharge but will have a noticeably reduced lifespan. Likewise, an old battery in a car you rarely drive will still eventually fail.

In practice, these facts lead to two important conclusions:

• Plan to replace your batteries five years (or seven years for AGM batteries) after their manufacture date.
• Buy replacements with relatively recent manufacture dates.

If you aren't shopping for your battery in a physical store, always purchase from a reputable supplier, and don't be afraid to contact the store and ask about the age of their batteries. Buying batteries with more recent manufacture dates is an excellent way to ensure your replacement lasts as long as possible. 

For more info about car batteries, contact a local company.