not very flashlight related, but I know that there's a bunch of people on here, like myself, who compulsively salvage laptop batteries to add to their Smaug like pile of cells :) So when my wife bought a Coleman mattress inflater from a garage sale for $2 with what we found out later to be a duff battery, it was an obvious destination for some of my cell stash.
Original
pile of replacement batteries. I should have used matched cells (for reasons I'll get into later), but I wanted to keep my matched sets for some powertool packs I'm going to make.
getting wired up
all installed, with balance tap added. I tried a couple of times to add a 2S PCB, but managed to burn holes in both of them (literally, with magic smoke too), so that plan went in the circular file.
sealed up with voltage monitor velcroed on (I plan to use it on my powertool packs too)
balance charging (after I traced a charging problem to a reverse polarity protection diode in the +ve charge wire, grr)
and around 5min of blowing time later
:D
It's like a little shreaking mini-leaf blower. I don't know how much power the motor is consuming (or how long it'll last running at 7.4V!), but it pulls the voltage down on those cells something fierce. On the last mattress, the two banks were reading ~3.3V and 3.45V, yet when I recharged the battery I found that I'd only used 1500mAh of capacity. As total capacity is around 8Ah, that's some huge voltage sag for a 2S4P battery! The original set up must have used less than 6A continuous as that's what the switch is rated for, but the new set up must be pulling more than that.
Anyway, at the end it turned out to be a really fun project, even if it didn't feel that way during parts of it. It's also useful for doing model photoshoots (puffing up the model's hair) and chasing the kids around the house :)