Spent my layover hunting battery parts on an actual street called Battery Street

I've had good luck with market bin cells myself, picked up a pile of old laptop pulls at a swap meet a couple years back for next to nothing. The trick I learned the hard way is to not just sort by voltage once. I label each cell with a paint pen, charge them all up, then let them sit on the shelf for two weeks. Whatever is still fat with charge after that is a keeper. Anything that sagged goes in the scrap bucket, no second chances. Out of about forty cells I only trusted eleven by the end, but those eleven are still going strong in a little power bank I built for camping. Guess that's the real cost of cheap cells, your time sorting them. One thing I'd suggest before you build anything permanent is check the pouches for any puffiness or dents, especially secondhand ones. A soft or bulging pouch is done no matter what the meter says. The ones that are flat and hold voltage are usually fine for low drain stuff like lights or a workbench fan. That testing at the counter detail is a nice touch though. Around here the sellers just hand you a box and wish you luck.

I've gone down the mystery bin road a few times and my hit rate has been honestly pretty poor. Last year I grabbed twenty cells from a flea market guy who swore they came from hospital UPS swaps, and after all the sorting and charging I ended up with maybe five worth keeping, all around 800mAh instead of the 2000+ printed on the wrapper. The hour or two per cell I spent capacity testing them on my bench charger added up to a lot of weekends for very little payoff. What worked better for me was making friends with the guy at the e-bike repair shop two streets over. When he swaps a pack he just bins the old cells, and he lets me take the ones that test fine. The nice part is they're ussually a matched set from one build, same brand and vintage, so the sorting is way less of a lottery. I built a 4s2p test pack out of his rejects last spring and it's still running the porch lights. So for the trip souvenir angle, I'd probaly skip the cells and grab the fun stuff instead, heat shrink in wierd colors, weird connectors, that sort of thing, and source actual cells from somewhere with a story you can trace.

The anticlimax part made me chuckle, though my pedantic side suspects the name is leftover from artillery, since a bunch of cities named streets Battery after coastal gun emplacements, not after anything electrochemical. We have a Discharge Road near me and it turned out to be about a mill race, which was equally disappointing. On the cells themselves, one habit I picked up is checking how fast each one's voltage drifts downward over the first day after charging rather than waiting weeks, since the leakers usually show thier hand almost immediatly.

Pouch cells are a diferent beast from the round ones if you plan to build with them, no spot welder works on those tabs. I got a batch from a repair shop clearout last summer and ended up just soldering the leads quick and hot, and even then I cooked two of them learning the timing. What ended up working better was keeping the tabs long where I coud and using screw-down contacts for anything temporary. Mine wound up running a shed light off a small panel and it's still going a year later. Would love to know what the stall owner was charging per cell out there, sounds like a whole different price bracket.

Gotta push back on the 'no second chances' rule for sagged cells. I used to bin anything that dropped after sitting, untill one winter I got bored and recharged the whole scrap bucket a second time. Maybe a third of them held fine on round two and a handful of those are still in a garden light bank years later. Some of them just need a cycle or two to settle, especially pulls that sat dead for a long time before I got them. I'd rather waste an extra afternoon retesting than toss cells that were actualy sound. Works out cheaper than buying new, even counting my time at lazy weekend rates.