Why Do Car Batteries Die in Winter?

Few things are more frustrating than climbing into a cold, snow-covered car or truck only to hear the dreaded “click-click" of a dead battery. It happens to the best of us. But why does a car battery's life seem to end more frequently in winter? Read on for the reason why.

Car battery

Source | Flickr

The inner life of your vehicle's battery

First, a quick refresher on the science happening inside a car battery. Lead acid batteries are the most common car batteries because they're inexpensive and fairly dependable. They're made of a plastic case that houses a series of lead plates immersed in a pool of electrolyte—a mix of water and sulfuric acid. Each pair of plates makes up one "cell." When fully charged, each cell in a lead acid battery produces 2.1 volts. So, a 12-volt battery consists of six cells.

The lead acid battery doesn't produce a charge, but receives and stores an initial charge through a chemical reaction between the cell's lead plates and the electrolyte. But as the chemical reaction occurs, the positive and negative lead plates are slowly coated with lead sulfate. This process is known as sulfation, and it reduces your battery's ability to hold a full charge. To complicate matters, lead acid batteries experience self-discharge, a natural loss of charge over time. Left too long without a fresh charge, a battery can discharge beyond recovery.

So why do batteries fail in winter?

Extreme heat or cold can increase your battery's rate of discharge, making winter a triple-threat to your battery. All that exposure to summer's heat evaporates the water in the electrolyte, increasing sulfation. Then winter rolls around, and freezing temperatures slow the chemical reactions occurring inside a lead acid battery, further reducing your battery's ability to perform. At the same time, a cold engine and sluggish oil demand more power, while power-hungry features like heat and defrost place more demand on your battery. Although lead acid batteries last an average of four years, they can fail earlier under the right (or wrong) conditions.

Signs of a failing battery

Your battery won't always warn you before it fails, but here are common signs to watch for:

  • Headlights dim yellow instead of white
  • Dashboard battery warning light is on
  • Electronic accessories fail
  • Engine cranks more slowly
  • Dome lights dim
  • Car horn sounds unusual
  • Battery case swollen or cracked
  • Smell of sulfur or rotten eggs
  • Battery is more than three years old

The best way to find out if it's time to replace your car battery is to have your battery tested.

Have you had to deal with a dead battery in winter? Share your experience in the comments.

Last updated June 14, 2017

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