The other night there was a power outage in our area, so naturally all of the flashlights are brought out so everyone is able to see. There have been these crank flashlight hanging around for years, and I actually cannot remember the last time they held a charge. They would light up when you crank them, but the batteries were long gone, or so I thought.
I cracked one of them open and discovered that they have coin cell lithium batteries. Obviously from sitting with no charge for probably years they have reached the damaging point of the cell. I decided to see if I could get them going anyway. I connected the output of the motor to my voltmeter and found that it can produce up to 20V while cranking, but 5-20V seems to allow an output of ~3.8V to the lithium battery; that is the usable input voltage. I also realized that since they are lithium batteries, you would have to stand and crank these for a long time to create any sort of lasting charge.
My solution was to just solder in some connectors so I could connect my own 5-20V charge instead of relying on the crank. That way I could plug them in for extended periods of time and maybe bring the battery back to a state of holding a charge. There is a simple bridge rectifier in the circuit off of the motor so that it would convert to the correct voltage even while turning the generator either direction. I just followed this to a capacitor that goes to the rest of the circuit and soldered my leads on.
I took one of my 12V wall warts and made a usable connection for the light and left them charging for about an hour; they started holding a charge again. The crank still works for when the power goes out for extended periods of time but until then they can be charged. Saved some electronics from the trash. I’m also thinking of soldering a much larger lithium battery in there for longer usage. Ill have to scavenge one from somewhere. We have three of these and I know the battery is done in one of them, so it may get it. They also make great lithium battery charging circuits for projects as well.