Is it safe to use lithium batteries in LED flashlights?

You may have heard that LED flashlights using AA lithium batteries run hotter and die sooner. Is there any truth to this? 

The short answer: There could be an issue, but likely only with "cheap" LED flashlights—that is, ones that lack circuitry to limit the current flowing through the LED. There should be no problem using lithium batteries in a well-designed LED flashlight that contains current-limiting circuitry inside to protect the LEDs.

We're not talking about rechargeable lithium batteries like those used in notebooks, phones, cameras, tablets, and high-end flashlights. Consumer Reports does not test these individually (although we do test battery life for portable devices and tools that have built-in lithium batteries). We're referring to single-use or primary-cell lithium batteries (actually lithium-iron disulfide, but only industry experts call them that). These come in AA size, and are the type we test and report on in our Ratings. 

Be sure to visit our buying guide and Ratings of AA batteries.

In theory, single-use AA batteries, whether lithium or alkaline, should work wherever 1.5-volt batteries are called for. Technically, that's not always the case. Here's why.

If you load a battery, its voltage will "sag" (reduce for a short time), a reaction called internal resistance. As the battery is used and ages, internal resistance increases, and it takes progressively less of a load for the voltage sag to occur.  

Fresh lithium batteries have a slightly higher voltage and less internal resistance than alkalines, so they'll allow more voltage to reach the LEDs, which means nice bright light—while it lasts! Higher power into the LEDs means the LEDs will run hotter and die sooner. In practice, though, after a short time, the lithium AA battery voltage will sag to 1.6 volts, which is safe enough for most flashlights and other devices. 

Be sure to read the device's user instructions—if there are any.

—Rich Sulin