Why the time limit? If the H51 is good for weekend trips, why wouldn't it also be good for week-long trips?
--MV
A few reasons...
An even number of batteries is easy to rotate and charge. A single battery means bringing a fancier charger and/or keeping better track of my batteries. Still, without a fancy charger I'd charge the batteries unevenly. Over the course of months the imbalance could get pretty bad, especially since I'd want to charge them at every opportunity. What charging strategy do I use to keep cells as balanced as possible and how many would I carry? I certainly don't ever want to be without a light. With the HP10 I would be able to go weeks or months between charges, so charging would be rare and any imbalances during the trip would be minor. I guess this isn't a big deal if disposable lithiums were used.
Also, I use between 50 & 100 lumens to hike at night. That would drain a small headlamp like the H51 quickly and possibly force me to change batteries in the middle of the night...or as I experienced one night, using the maximum output to keep scaring away a curious cougar all night.
And to get back to charging imbalances... I also wanted to have the chance to share batteries with my gps momentarily...just long enough to write down my coordinates. The negative effects of this practice wouldn't be as bad with the HP10...I could use the left bank one time, and the right bank the next...that's easy enough for me to handle, even when I'm dead tired after walking uphill for most of 20 miles.
And by "long hikes" I meant hikes that takes weeks to months to finish, although I'd stop in a town about once a week...although it'd be nice if I only had to stop long enough to pick up food.
As far as convenience goes, I'd probably have to change the batteries in the H51 every 3 days. That's one hour of hiking every morning. A little more for killing time around camp, and very little for reading. I'm not sure if 30 lumens is enough for me to hike comfortably in, so I'd probably be stuck with the H2 setting.
Now back to short trips...
On short trips I don't mind carrying an extra cell or two, and charging isn't a problem since I'll be back home soon where I have a nice charger that can handle charging individual cells. Or if I carried lithiums I could put them into a remote until they go completely dead. It's unlikely that I'd carry a used lithium on a trip.
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One more thing... The perfect light for me would use 2 AA batteries. It would provide a nice compromise of battery life and convenience. Unfortunately a dual AA battery with 200 lumen output doesn't exist yet, or at least I don't know of it.