The cold-solder battery design was a failure, in sense that making cold-solder joints is really messy, takes 24 hours for the cold-solder to join mechanically and the joints themselves leak and require extreme dexterity. On top of that the cold-solder is expensive and very hard to work with.

The solution would be to make spot-welded designs from coin-cell batteries. I have ordered a simple and really cheap spot-welder (it's just a battery with two copper leads and a trigger) and a roll of nickel strips. The upside of the spot-welded design is that I can also design very quickly any arbitrary battery configuration, and the profile is only 100 microns thick. The cold-solder required extra thickness for the silver epoxy, so that's that.

This will also allow me to start work on the rechargeable battery pack with a solar harvester. That's a separate mini-project, so I keep adding to the list, mais c'est la vie.

Stay tuned for the battery pack!

Cheers.