One of my favorite project batteries is this 7.4v/4.4Ah Li-ion from BatterySpace. It’s basically four 3.7v batteries (approximately AA sized) packed in heavy shrink wrap.
The original is the white and green on the left. I needed it to fit the form on the right so it will sit nicely in the project box for the animatronic tail. So I cut it open, and found a little more complexity than I expected.
The batteries are in two packs of 2 each, wired in parallel; the packs are then wired in series. They connect to a little PCB that adds charging and short protection.
The trick is that the batteries have foil tape for the contacts, and I couldn’t figure out how to solder to the foil. I made 3 copper plates with solder blobs for contacts. All 4 batteries connect on one end (two negative and two positive) with a square copper plate. The solder blobs on the plate press into the battery contacts, and I taped it tightly. I soldered a wire to the plate which connects to the common pad on the PCB.
On the positive and negative ends, I did something similar with two rectangular copper plates, one taped across the two positives, the other across the two negatives. Lots of tape prevents shorts, and now I have a battery that fits nicely in the box.
Wylbur's Inner Geek 