March 29, 2008

Batteries Dying In Cold

Stupid Question ™
Aug. 9, 2004
By John Ruch
© 2004

Q: If people keep batteries in the fridge to keep them fresh, why is it bad for car batteries to be out in the cold?
—Frosty, Chicago, Illinois

A: We’re talking about different kinds of batteries kept at different temperatures for different reasons.

There’s actually no need to keep modern batteries in the refrigerator. And back when it may have been a good idea, the concept was to warm them up again to room temperature before using them—something hard to do with a car battery. The same cold that prevents energy from leaking out of a battery also prevents us from extracting it for our own devices.

Batteries slowly discharge tiny amounts of energy even when they’re just lying around, until they finally die. (This is called “self-discharge” or “local action.”) Temperature can quicken or hinder that process, since heat is a key ingredient of the chemical reactions inside a battery.

The point of storing old-style batteries into the refrigerator was to cool them and slow down the chemical reactions, thus slowing down the energy leakage and extending the batteries’ shelf life.

The chill of a winter night (well below your refrigerator’s comfy 45 degrees or so) will similarly extend your car battery’s life, but that’s a negligible benefit. Self-discharge is not the winter problem with car batteries.

A brief primer on batteries: A battery is a series of chemical cells that store energy for later release. The basic elements of a cell are a substance that emits electrons during a chemical reaction (the cathode); a substance that picks up electrons (the anode); and a substance that triggers or serves as the medium for a chemical reaction (the electrolyte). The cathode, anode and electrolyte can be made of a variety of substances, usually some type of metal.

By sticking the battery into an electronic circuit (like any battery-operated machine), we can pull electricity out of it.

The typical car battery is a lead-acid battery. Its cathodes are plates of lead and its anodes are lead dioxide, with water-diluted sulfuric acid as the electrolyte.

Lead-acid batteries have a very high self-discharge rate and usually die within a few years, even though they are quite large. They are also very temperature-sensitive. Their typical peak efficiency temperature is around 75 degrees Fahrenheit. At temperatures in the 100-degree range, the batteries will start self-discharging at even higher rates.

But lead-acid batteries don’t work well in cooler temperatures, either. The electrolyte thickens up and electrons don’t flow as smoothly (in fact, it can actually freeze entirely, damaging the battery). The energy capacity drops sharply when you get around the zero-degree mark. In below-zero temperatures, a car battery will often produce less than half its normal electrical current.

That’s a special problem because car batteries are a special type of lead-acid battery. They are designed to provide a single quick burst of energy to start the motor, and then get recharged by the alternator. There is rarely enough juice for more than a couple more tries. On a cold winter day, when the battery’s capacity has plunged, you may get only one or two cranks before the battery dies—or it might not produce enough juice to start the motor at all.

The first generation of household batteries for flashlights and whatnot did have a fairly short shelf-life due to self-discharge. These so-called zinc-carbon batteries may have benefited from refrigeration, though the self-discharge really sped up at temperatures of about 100 degrees and higher. Like a car battery, their capacity also plunged at low temperatures (below 50 degrees or so) but a quick warming with the hands would fix that.

The standard household battery today has a manganese dioxide cathode, a zinc anode and water-diluted potassium hydroxide as an electrolyte. It operates well under a very wide temperature range, from about 0 to 130 degrees. And even at room temperature, its self-discharge rate is very low. I have a pack of Energizers laying around the house that boasts a 2010 expiration date.

Refrigerating such batteries would undoubtedly lengthen their shelf life by slowing self-discharge, but probably to a negligible degree due to the stability of its materials. No major battery manufacturer recommends refrigerating household batteries; they merely suggest keeping them dry and at room temperature.

Batteries based on lithium operate well in truly incredible temperature extremes, from 40 below zero to 150 degrees. And they have very little self-discharge because of lithium’s tendency to develop an inert, oxidized surface when the battery isn’t being used. Temperature hardly affects that miniscule self-discharge one way or the other.

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