About two years ago I started using Sanyo’s rechargeable eneloop batteries. These relatively inexpensive Nickel-Metal Hydride (Ni-MH) cells are available in both AA (単3形) and AAA (単4形) sizes. They are low self-discharge cells that keep their charge for months when not in use. I’ve bought boxes of 8 cells of either type, for use in flash lights, bike blinkies, helmet lights and Bluetooth keyboards.
They are initially more expensive to buy than regular alkaline (primary) cells, but you only need to re-use them about three times before they work out much cheaper than primary cells, while you can actually recharge them hundreds of times before they start losing significant capacity.
Here are some nice gadgets that will take them, which I found sold in convenience stores here Japan.
These little cases (by alicty.co.jp) take power from two or three regular alkaline AA or Ni-MH AA cells and provide a USB port for powering mobile phones and other small gadgets with a USB power cable. As you would expect, the three cell version is slightly more powerful, looking to my Google Samsung Nexus S as an AC charger (i.e. it provides more than 500 mA). For the two cell version, the phone shows “charging (USB)” as the status, i.e. it can draw up to 500 mA. The two cell version has a USB-A socket (female) for generic USB cables while the three cell version comes with an integrated micro USB (male) cable. A very similar concept has been around for a while as the MintyBoost.
The nice thing is, if you carry enough pre-charged eneloop cells with you, you can swap cells as needed and have virtually unlimited power. You could even buy primary cells to top up if desperate (one set came bundled with each device), but they would end up costing you more than re-usable eneloop cells in the long term. I’ll carry some Ni-MH cells as spares on long bike trips or hikes, which could come in handy with these little cases.
UPDATE 2012-04-04: I also tried using this adapter with alkaline (primary = non-rechargable) AA cells and it goes through them quite rapidly. Alkaline AA batteries have a notoriously poor performance in high drain applications because of their high internal resistance. You’re much better off sticking with Ni-MH batteries such as Sanyo Eneloop!
It says on the pack that a set of 3 AAs will boost the charge state of a smartphone battery by 30-40%, i.e. it would take you about 3 sets (9 cells) to fully recharge an empty battery. Or put another way, if the phone lasts 5 hours on one charge doing whatever you’re doing, you will consume a set of fresh AAs every 100 minutes to keep it topped up. To provide 500 mA at 5 V (2.5 W) on the USB connector at 80% efficiency would draw 3 W from the batteries, or 700 mA at 4.5 V (3 x 1.5 V). At that kind of load, an alkaline battery might only supply a quarter of its rated capacity, which is normally measured at a much smaller load (which is OK for alarm clocks, TV remote controls, etc. but not high powered electronics like digital cameras or smart phones).