has anyone out there had success having glued a small piece of felt (to the battery enclosement) to extend battery life? i hear tell this measure "keeps the batteries warm" and will therefore facilitate overall performance? curious to know. thanks! l

Having batteries warm may increase their ability to deliver a particular current at a particular voltage. For instance, your car battery can't put at nearly as many amps at 0F as it can at 70F. But that temperaure point varies with battery type and somewhat with the current demands. If you have a low-draw application (many hours of battery life), then it isn't a big issue. A high-draw application (e.g. charging the flash in a camera) will be helped more by warm batteries.

But keeping them warm won't extend battery life. It will only put more of the nameplate voltage into the bulb rather than in internal voltage drop inside the battery.

It would be hard to work much felt into most battery compartments and it wouldn't help a whole lot - the battery is close to ambient temps anyway.

If you want it warm, keep it in an inside pocket. It you want it warmer, toss it in your skivies for a while.

David, are you sure about that? As the battery is used, the electronics will generate heat and the felt should mitigate that heat loss (I'm assuming we're looking at cold weather performance). As the OP was asking, they wanted to maximize performance, not necessarily battery life. The battery may last the same, but keeping it warmer using felt should keep the headlamp brighter than without.

"But keeping them warm won't extend battery life. It will only put more of the nameplate voltage into the bulb rather than in internal voltage drop inside the battery."

David - as a warm battery will put more voltage across the bulb/LED, the light will be brighter. A cold battery will produce a dimmer light, so the effect is the same as shortening the battery life (unless the battery is rewarmed).

Dustin - the electronics in LED headlamps produce negligible heat, and the heat produced by the internal resistance of the battery is also minimal.

OP - felt will make very little difference for very little time. If cold is an issue either use a headlamp with a remote battery pack and keep the batteries inside your jacket, or use Energizer lithium batteries.

thanks all, mighty do appreciate the feedback. to clarify, yes, i'm raising this in question of cold weather performance. i know of several reliable sources whom speak true to this measure. regardless-- perhaps this is cause for experimentation. l

Personally, I would throw in some lithiums and not give it another thought. They perform well in cold weather.