How I finally put my surplus batteries to use - steps that actually worked

I ended up in a similar spot a couple years back when I cleared out a warehouse job site and came home with a crate of 32700 cells, mostly from a discontinued solar lighting order. The date codes were all over the place, some from two diferent production years, so I did what you said and sorted them before doing anything else. Took a Sunday afternoon with a multimeter and a notebook, grouped them by resting voltage and capacity test results on my little hobby charger. Ended up with three usable groups and one group that had drifted too far apart to trust togather.

The matched groups became a 12v box for my shed lights and a backup pack for the chest freezer in the garage. The freezer one has paid for itself twice during outages allready, ran it about nine hours last spring without breaking a sweat.

On the selling side, I agree local is the only sane path. I had a guy drive forty minutes for one of my leftover groups because he wanted them for an ebike build. Cash in hand, no shipping drama, done in ten minutes. One thing I'd add on storage, I write the resting voltage and the date right on the tape I put over the terminals. When I check them every few months I can see at a glance if anything has crept down more than I'd expect.

I totally agree with the method of sorting those cells by resting voltage first. It makes all the difference wether you end up with a usable pack or just a bunch of dead weight. I ran into a similar situation when I cleared out some old industrial gear and ended up with a bunch of different cell sizes. Grouping them by internal resistance instead of just voltage helped me decide which ones were truly viable for deep cycle use. Using an ohmmeter for that pre-sort was crucial. My setup was for a small off-grid setup for a remote pump, and having those stable groups meant the battery maintained its charge even in very cold weather. I learned quickly that a cell's performance in the field doesn't always match its resting specs in the garage.

I acutally ended up with a small pile of different size cells, and my initial thought was just to toss them, but I realized I coud use them for a tiny off-grid lamp setup. I found that matching them by just resting voltage wasn't enugh; the internal resistance differences were making charging uneven across the whole group. It took forever to balance them out. If you're planning a simple pack, don't forget to check the amp-hour capacity difference between the batches too. I wouldn't wanna put them together just because they were the same voltage but totally different in capacity.

Not sold on writing the info on tape you stick over the terminals. Any adhesive left behind when you pull it off ends up on the contact surface, and down the road when you actually build with those cells you're cleaning gunk off every one of them. I keep mine in plastic storage trays with the label on the side of the tray instead, one tray per group. Nothing touches the terminals and the info is still right there when I do my check. Cheap bins from the dollar store have held up fine for a few years now.