Calculating No. of Extra Batteries

Hi Simon,

You've tackled a whoppingly complicated challenge there, and my (WPB) hat's off to you.

My apologies for not knowing much about your Fenix, so my first question is what's the meaning of their run-time #s? If it's a regulated light, is it the time it takes to drop out of regulation–or down to the next lower level–or is it the time to exhaustion in each mode? If not regulated, is it the time to exhaustion or the time it takes to drop to a set minimum output?

I've tested regulated headlamps that glow literally for days after their batteries are nearly exhausted, and at some point arbitrarily end the test before they go completely dark. Some makers include this "twilight time" in their runtime specs while others do not.

A couple other considerations: Do you start every trip with a new battery or do you leave it in until exhausted? (Li cells are expensive enough to use to the bitter end.) At what point do you decide to switch out a weak cell, and what activities might trigger your doing so? (e.g., Night hiking requres much more light than cooking or reading.)

Further complicating things, in my experience batteries deliver greater total life used the way most of us do–intermittently. Batteries rebound unless they're never given the chance to.

My suggestion is rather than a complex formula relying on dubious data, keep a log of your usage and battery life over several trips. You're probably best served by planning based on empirical data. Even then, there can be summer nights when you use it five minutes, tops and other nights when the sun sets two hours from camp. One seldom knows!

Hi Rick,
Thanks for the input!

The Fenix lights are regulated, and–so I've read–run pretty much within the 100-80% range, then flatline quickly without any twilight mode. The runtime numbers from the manuf. would eventually be replaced with more realistic (empirical) nos. from review websites and/or personal or experiences of others.

For different kinds of trips and even seasonal swings, I would alter the usage hrs./day. "Used" batteries could be considered dead or dying, and could be worked into the calculation (by adding an extra trip day, to be safe). Years ago during the airing of one the Eco-Challenges, a team blasted a teammate for bringing a Costco-sized pack (36?) of AA batteries! I'm nowhere near that ignorant, but it'd be nice to know quickly whether one spare AAA is enough, or maybe I should bring two…

Also, with a drawer full of lights (headlamps, coin-cells, etc.) and new ones arriving every now-and-then to satisfy my "addiciton," the table would force some restraint when trying to stuff a bunch of batteries in the pack :-)

In the end, common-sense and personal experience would have to be used by me or anyone else using the spreadsheet. Fun, though, to watch the numbers change as scenarios are played out. But as you point out, it's really hard to estimate how much runtime you'll really need…

My experience with the fenix lights is that they come pretty close to the manufacturer run time, and they do go from nearly full intensity to nothing pretty quickly. I ran several batteries through mine to get a good baseline runtime to know what the "capacity" of the battery was in real life. I don't use a spreadsheet… because I rarely need more than one.

I figured this out by monitoring how much I used the flashlight on several trips. I found that there were three different use patterns.

1) Normal… solo trips, three season, no hiking in the dark. I hardly use the flashlight. Less than 2 minute on full. Less than 20 minutes on low.

2) Trips with others. Haven't done enough to be sure… but it looks like 1.5 hours on low, and around 30 minutes on high.

3) Night hiking… highly depended on how many hours I am hiking after dark, how much moon is out, and it I am on a trail. More than 1/2 moon on a trail… the flashlight gets used less than 1 minute every 15 minutes. Cross country with no moon, flashlight is on more or less continuously, mostly on high.