I like LiFePO4 batteries. They have a rather flat discharge at around 3.2V, which is ideal for powering 3.3V devices without a regulator. You can also use them in devices that take 2 standard AA cells by using a blank shunt in the 2nd battery slot since 2 fresh alkaline cells in series provide 3.2-3.3V. And since they are readily available in the 14x50mm AA size, you can use cheap AA holders for them in electronics projects.
When it comes to chargers, things can be a bit problematic. LiFePO4 batteries should be charged to 3.6V, rather than 4.2V like regular lithium-ion batteries. A good charger costs $10-$15, but charging at a high current will reduce the number of recharge cycles. The Soshine batteries I bought indicate on the label a standard charging current of 300mA to 3.6V. Rather than search for a charger to fit the bill, I decided to make one.
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