How come cold weather is bad for car batteries (sapping their long-term charging potential) but it’s (allegedly, anyway) good to put your AA and AAA batteriess in the fridge “so they’ll last longer”?
few different things going on here. a car battery (specifically, a flooded lead-acid battery) has an electrolyte containing a lot of water (75%) and sulfuric acid (25%.) When the battery is fully charged and all of the sulfuric acid is in solution, that electrolyte won’t freeze until about -70 to -80°F. as the battery discharges, some of the sulfuric acid is converted to lead sulfate on the electrode plates. as the amount of acid in solution decreases, the freeze point rises rapidly. at 40% charge, the electrolyte can freeze at only -10°F. and when the electrolyte freezes, the ice can break the electrode plates and/or push them into contacting each other, damaging the battery permanently.
as for your AA/AAA batteries, assuming you’re talking about alkalines, it has a paste electrolyte which won’t freeze easily. keeping them at cooler temperatures reduces their rate of self discharge.
There’s a major difference between storing a battery in cold temps and using a battery in cold temps. Besides the physical damage mentioned above, chemical reactions slow down in colder temperatures. That’s great for storing a battery. That’s not so great for using a battery. Car batteries need to operate in cold temps all the time, but they’re constantly being recharged by the alternator. Disposable or rechargeable batteries (flashlight, cameras) will suffer greatly in cold temps and stop working even if they still have charge. When the batteries warm up you might find that they’re still fully or near fully charged but they just won’t work in sub-zero temps.
Also, keeping regular alkaline batteries in the fridge does almost nothing useful for extending their life. They already have such low self-discharge rates that you might gain a few extra months out of a 10 year lifespan, but condensation from the fridge might damage them during that time. Unless you live in the desert, don’t bother.
Here’s some temperature data on alkaline, Li, NiMH, silver oxide, and zinc batteries.
As mentioned by others, the self-discharge rate of a battery decreases as the temperature goes down (good), but the performance of the battery also decreases as the temperature goes down. Of course, this doesn’t mean the performance will forever increase as the temperature goes up and up. For all chemistries, the life of the battery starts to decrease as the temperatures goes above 50 °C. And the higher the temperature (above 50 °C), the worse it is on the battery.
I was recently involved in a project for a portable radio transmitter that used a lithium bromine chloride cell. It has an operating temperature range of -55 °C to 85 °C and a self-discharge rate of only 3% per year at 25 °C. Impressive.