Flowlights are optimized for rechargeable AAA batteries. We recommend high-quality rechargeable NiMH batteries which you can find on our website here: http://www.flowtoys.com/powerup/batteries/
Generation 2 flowlights can use Lithium-ion batteries, though you need a specific kind. Some AAA size lithium cells include on-board low-voltage protection circuitry, but these are too long to fit in a flowlight. The cells that you can use in a flowlight are NOT protected, but the flowlight has built-in protection.
Generation 1 flowlights cannot use Li-ion batteries. If you have Watermelon lights or older flowlights, we recommend rechargeable NiMHs or high quality alkaline batteries instead.
Important note: If inserted in the wrong direction, the reverse polarity of Li-ions can ruin your flowlight. Always insert your battery with the positive "head" of the battery pointed towards the pointy "head" of the flowlight.
If you have any questions, please feel free to contact us!