Batteries do not make electricity; they store it. Therefore, a fully-discharged battery does not gain juice from the ether simply because of an increase in ambient temperature. That said, the myth that warming up a cold battery causes it to work better has some grounding in scientific fact. External cold has a distinct effect on a car battery’s ability to perform: At 80 degrees Fahrenheit, a battery is at its optimum; at 32 degrees, it can only supply two-thirds of that optimum; at 0 degrees, its performance is down to 40 percent. Partially discharged batteries that do not start an engine when extremely cold, therefore, may do so if they are warmed up.
Car batteries do not mysteriously discharge in cold weather, even in extreme conditions, nor do they mysteriously recharge when they are warmed up. Increasing the ambient temperature, however, can resuscitate reduced performance to a workable level.
Why the Cold Makes Batteries Fail
Since a battery has reduced ability to perform in adverse conditions, symptoms that do not present in warm weather can do so in the cold. A parasitic -- sometimes called a vampire -- drain, such as a warning light or alarm system, that does not affect the battery charge level in the warm, can drain it in the cold. An internal battery short that doesn’t affect performance in the summer can do so in the winter. The cells in a fully discharged battery can freeze; if this happens, the battery will not take a full charge again and must be replaced.
If the battery has never had a full charge because the alternator or generator is not functioning as it should, that partial charge may be adequate to start the engine in warm conditions. Making frequent stops and starts, such as a multi-store shopping trip, especially in adverse conditions that dictate heaters and headlights be used, can deplete a car’s battery and not give it time to fully recharge.
Understanding Cold Cranking Amps
The figure noted on a battery as a CCA value is a measure of its cold cranking amps; this is an industry term that defines that battery’s ability to perform in cold conditions. It references the battery’s ability to deliver at least 7.2 volts for 30 seconds at 0 degrees Fahrenheit. The greater the CCA value, the better it performs in cold conditions.
It is not uncommon for vehicle operators in extremely cold places to remove the batteries from their vehicles at nighttime and keep them in heated indoor areas. Dedicated battery heaters are sold in regions susceptible to extreme cold; these plug into 110-volt outlets and warm the battery while the vehicle is not in use.
An entirely flat battery is not receptive to a jump start; this is why the two cars must be left connected by jumper cables for a few minutes before the running one can start the dead one. Batteries that appear dead because they are extremely cold, and thus cannot be jump started, can become receptive to a jump start if they are warmed first.
Paying It Forward
Due to the performance percentage loss detailed above, even a fully-charged battery may not be able to provide sufficient current to start the engine in extreme cold. In this case -- in the opposite of what may seem obvious -- taking a little charge out of the battery can make it work. Switching on the headlights causes the battery to function; current passes through it and in doing so, creates some warmth. It may be that after a minute with the headlights on, the battery can turn over the engine.
Word of Caution
Batteries contain acid, and they give off fumes that are both flammable and explosive in the course of normal operation. Artificially heating an extremely cold battery by any other method than moving it to a warm space could be dangerous. Never lower a cold battery into a tub of warm water, set it near a radiant heat source or use a hair dryer over the case.
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