Uses for old, but good cell phone batteries?

I have recently come into possession of several 3.7v cell phone batteries. I have toyed with the idea of using them to power a mag light type (D cell) flash light. Any ideas as to carry this out? Or just any ideas for re-purposing them?

wtd17 years ago
Charging's simple - put the battery in the phone, charge it, take the battery out of the phone and use it. Don't know why you'd build a separate charger....
The hardest part is gearing up a circuit to charge them. I've got a big big box of lead-acid cordless phone batteries (50lbs of them). I haven't come up with a great use for them myself... No matter what idea I come up with, it always seems like I'm grabbing a regular 9v or something instead.
frollard8 years ago
You need a special circuit to protect them - from overcharging and undercharging. Other than that, you can use them at their rated voltage. search (ibles or google) for li-po battery charger.