ThreeAAAto18500LiIon - Keithl wiki

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Replacing Flashlight 3 AAA Alkaline Battery Cartridge

with One Rechargeable 18500 Lithium Ion Battery

9 LED plastic flashlights using 3 AAA alkaline batteries in a round battery holder are inexpensive and reasonably tolerant of abuse.

Sadly, they use up alkaline batteries fast, which are more expensive than the flashlight. As the batteries wear out, voltage drops below 1.2V, the flashlight dims, and becomes unusable while there is some energy left in the batteries. Worse, old alkaline batteries fail and leak alkaline gunk that corrodes the innards of the flashlight.

Fortunately, an 18500 rechargeable lithium-ion cell is the same length as the battery holder, holds twice the energy, does not drop voltage nearly as much, can be charged in an external charger, and reused many times.

It is slightly smaller diameter than the battery pack, but if wrapped in a flat hair elastic (three loops, mind the twist), it fits snug, and slides out easily for recharging.

I am using (in 2024) JESSPOW 18500 rechargeable batteries from Amazon, 1600 mAh, about $4 each. With button top, not the flat top batteries. In theory, these will charge at "1C", 1.6 amps in ~1 hour, but the Keenstone charger (also from Amazon) I am using charges "only" up to 1 amp, and is set to 0.5 amps out of the box. It should be set lower for smaller capacity batteries.

Some fools charge much smaller batteries with this charger; higher current than 1C will roast the batteries. I'll leave it at 0.5 amps and hope both the battery and the charger last longer, and keep charged spares handy.