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I got a Tekkeon TekCharge MP1550 AA-battery-to-USB¹ charger for a backpacking trip. (Know what’s really useful pretty much anywhere? An iPhone.)

AA batteries are much more versatile than common lithium-ion battery-extender packs. You can buy AA batteries nearly anywhere that sells anything, and they hold a surprising amount of power. And as long as you can keep feeding them AAs, they’ll keep going, letting you carry quite a bit of charging capacity if you’re willing to bring a lot of batteries.

There are three common types of AA batteries today:

• Standard alkalines: “Normal” batteries, not rechargeable. They provide good power, but only under ideal conditions. In cold temperatures, or in applications that draw a lot of power quickly (like charging an iPhone), their capacity diminishes severely. They’re great for applications that draw little power over long timespans, like in remote controls.
• Lithiums: Similar to alkalines, and also not rechargeable. They cost much more and perform no better in slow-drain applications. But they perform much better in high-drain applications and cold temperatures, and they’re extremely lightweight.
• NiMH rechargeables: They’re extremely heavy and don’t hold as much power, but they’re rechargeable. Older NiMH batteries lost their charge quickly, even when not in use, but the new low-self-discharge type (including Eneloops and the AccuEvolutions² I tested) solves that problem. They’re also much heavier than alkaline or lithium cells.

To test them in my iPhone 4, I discharged it to 20% (when the low-battery warning dialog pops up), then charged it back up to 100% or as far as it would go until the voltage was too low to keep charging the iPhone.³ This was not perfectly isolated, since I left the radios on and occasionally used the phone lightly during the tests. The numbers are meant to be estimates.

So here’s each battery type and the number of 20-100% charges they were able to achieve in the TekCharge MP1550, plus a popular traditional-style iPhone extended battery, the generic Monoprice lithium-ion iPhone extended battery⁴.

Two options seem to make the most sense: iPhone-specific lithium-ion batteries like the Monoprice or the more popular 3GJuice, Richard Solo, or Mophie products are best for rechargeable use, especially in situations in which you can recharge them often and only need an occasional boost.

But for long trips⁵ away from power sources, non-rechargeable lithium AAs are by far the best — as long as you don’t mind spending money on each set and throwing them away afterward, which are admittedly two major downsides.

It’s hard to make a case for using NiMH rechargeables to charge an iPhone, especially for backpacking, where weight is important: seven lithiums weigh just under what four NiMHs weigh. So you can get about 3.6 times the iPhone-charging capacity per gram from lithiums than from NiMHs — but you can’t recharge them.