I conditioned new batteries and charged them in a Sanyo NC-MDR02NU charger. This is the best compact, lightweight, reliable charger I could find that works on interational voltage rates (100 to 240 volts). It weighs 76 grams. It charges two fully discharged batteries in slightly less than 4 hours.
Eneloop batteries lasted 19 hours and 22 minutes, and the 2700 mAh batteries lasted 25 hours in the Dakota 20 using the same test methods I used for alkaline and lithium batteries. The 2700 mAh batteries are obviously superior when using the GPS for a long time, but if you're just using it for short periods of time followed by long periods of disuse, the Eneloops would be a better choice since they don't lose much charge while not being used.
Next comes the weight savings analysis. Which battery will save weight on a hike, disposable lithium or rechargeable NiMH? This depends on two things: the total legth of the hike, which dictates how many disposable batteries would be needed, and the maximum amount of time between opportunities to recharge batteries, which dictates how many rechargeable batteries would be needed. In general, longer hikes with more opportunities for recharging favor using rechargeable batteries, and shorter hikes with less opportunities for recharging favor disposable batteries. The following table summarizes the results. I used 10 hours per day of hiking time.
Weight with charger Recharge frequency Lithium equivalent ————————————————————— —————————————————— ———————————————————————————————— 2 batteries 136 grams 25 hours, 2 days 10 batteries, 150 hours, 15 days 4 batteries 196 grams 50 hours, 5 days 14 batteries, 210 hours, 21 days 6 batteries 256 grams 75 hours, 7 days 18 batteries, 270 hours, 27 days 8 batteries 316 grams 100 hours, 10 days 22 batteries, 330 hours, 33 days
Weight with charger Recharge frequency Lithium equivalent ————————————————————— —————————————————— ———————————————————————————————— 2 batteries 136 grams 25 hours, 2 days 20 batteries, 300 hours, 30 days 4 batteries 196 grams 50 hours, 5 days 28 batteries, 420 hours, 42 days 6 batteries 256 grams 75 hours, 7 days 36 batteries, 540 hours, 54 days 8 batteries 316 grams 100 hours, 10 days 44 batteries, 660 hours, 66 days
Disposable batteries are simpler to use because you won't have to change batteries as frequently and you don't have to worry about remembering to recharge your batteries at night and making sure not to forget the charger in the morning!
One more point I should mention is that lithium batteries don't lose as much power in cold weather as NiMH batteries do.
In sumary, assuming weight savings are your first priority (true for me), I think I would only want to use rechargeable batteries when it's a really long hike and I am sure I will be able to charge the batteries at least once every two days. For any long hike on the Great Wall I don't think rechargeable batteries are appropriate. If cost savings is more important to you than weight savings, rechargeable batteries are a viable choice for a long hike. I hope this analysis will help you to decide when to use which type.
Of course, the weight savings analysis assumes you won't be able to buy lithium batteries during your hike, which would be true on the Great Wall but not so, for example, on the Appalachian Trail.
My next step will be analysis of solar chargers...