My favorite online store (aliexpress) offered me these 16340 batteries recently, and I found some battery holders and a charger.
They are 16mm x 34mm [the 18650 in the photo is 18mmx65mm, so these are a little thinner and a lot shorter.] They claim to be 5800mah, but I assume that’s a figment of someone’s imagination. Still, as our little trains don’t gobble power like an RC car, they should work fine, and give me some options for smaller locos.
I was thinking of making a small cube, with the BMS (Battery Management System) pcb on top. Here’s a horizontal 18650 pack next to the stack:
Finally, I ordered some more 18650s and they turned up with BMS already built in (confirmed by the vendor.) They will just fit in my pack even though they are a little longer.
The real question is whether I still need a BMS. I wasn’t even sure if it was a good BMS, so I left the single battery with a couple of Aristo light bulbs providing a drain, and after a night or so the lights went out and the battery turned off. It does accept a charge again, so I’m inclined to believe its a BMS.
The big question now is how they work as a 4-pack in series. I think they will produce the usual voltage until one of them decides it has gone too low. Whether they will all charge at the same time, and whether they will charge until one decides it is full remains to be seen.