Episode 38: January 11, 2013
by Lee Falin, PhD
A few days ago I needed to replace the batteries in an electronic game. So I went to the place where our family has always stored our batteries, the place where every generation of Falin has stored it's batteries for as long as I can remember: the kitchen freezer.
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As I was wiping off the thin layer of frost from the batteries’ contacts, my wife asked me why we always kept the batteries in the freezer. At first I was tempted to attribute her lack of knowledge of Falin battery lore to the fact that she had only been part of our frozen battery dynasty for a scant 10 years, hardly long enough to truly appreciate the subtleties of battery storage.
But as I thought about how to answer such a question, I was confronted with a startling realization: I had no idea. Surely years of tradition, an entire dynasty's adherence to the principles of safe battery storage couldn't be wrong! I sighed as I realized there was only one thing to do: I had to find out the truth for myself. Does storing batteries in the freezer really extend their shelf life?
Of Cells and Batteries
To get to the truth of this important matter, it’s important that we fist have a basic understanding of what batteries are and how they work.
As I mentioned in my previous episode on capacitors, a battery is a collection of one or more cells capable of generating electricity. There are lots of different types of these cells, but they all work according to the same basic principle.
Part of the cell consists of chemicals that undergo a special reaction called an oxidation reaction, which is a reaction that causes atoms to lose electrons. (For more on oxidation reactions check out my episode on The Science of Fire).
The other part of the cell consists of a different set of chemicals that undergo what is called a reduction reaction. In reduction reactions, atoms gain electrons.
Quick and Dirty Tip:If you’re confused about which type of reaction is which, just remember the handy mnemonic: OIL RIG - Oxidation Is Loss, Reduction Is Gain.
As one part of the cell has an excess of electrons, and another part of the cell has a deficiency in electrons, the logical thing to do would be to send the extra electrons from one side of the cell to the other side. This is exactly what happens when you hook both ends of the cell up to a circuit. The excess electrons flow out of one side of the cell (called the anode) through the circuit and into the other side of the cell (called the cathode).
Over time, these chemical reactions can result in changes to the materials that make up the cathode and anode, resulting in the cell losing its ability to carry out the chemical reactions needed to produce electricity. When this happens, we say the cell (or the battery of cells) is “dead.”
I’m Just Getting Warmed Up…
Even when the cell isn’t connected to a circuit, these chemical changes still occur, though at a much slower rate. “Self-discharge” is the term used to describe how cells slowly lose their “charge” or their ability to carry out the reactions that produce electricity, when they’re just sitting around on a shelf somewhere. How fast a cell self-discharges depends on a lot of factors, including the chemicals used inside the cell and the temperature outside.
It turns out to be true that putting batteries in colder temperatures can slow the self-discharge rate of their cells, however just how effective this is depends on the type of battery. Modern alkaline batteries have such a low self-discharge rate that storing them in the freezer is nearly ineffective. Lithium batteries (both standard and rechargeable) are even better.
On the other hand, rechargeable nickel cadmium (NiCd) and nickel metal hydride (NiMH) batteries have really high self-discharge rates that are drastically reduced by storing them in colder temperatures.
However, most battery manufacturers recommend against storing batteries in the freezer or refrigerator for several reasons. First, excess moisture can significantly degrade battery life by causing corrosion around the anode and cathode contacts. In addition, the extreme cold temperatures can cause seals in the battery to rupture, which greatly reduces battery performance. Finally, a cold battery has to be at room temperature before it can operate at maximum performance.
So it turns out that as usual, my wife’s intuition was correct. My family has been shortening the life of their batteries for years all because we didn’t understand the science behind batteries. Hopefully this episode will save your family from a similar fate. Now please excuse me while I go and take the batteries out of my freezer and put them in a drawer where they belong.