I made two 7-cell NiMH battery packs with 2/3AA KAN 1050 cells from cheapbatterypacks.com. Standard warning: don't try this at home! The batteries arrived after 3 business days. Here are 7 cells, alternating plus and minus ends up. The 7 cells will be connected in series, plus to minus, plus to minus, etc.
This picture shows almost everything I used to make the packs:
Electricians tape, stranded copper wire, rosin core solder, wire strippers, 100-watt soldering iron.
I sanded the ends of the batteries using a sanding attachment in the drill press. The solder won't stick well to the new battery top:
I taped the batteries together as a 7-cell block. Then I heated a pool of solder on the tops of two adjacent batteries, always minus to plus or plus to minus. My little soldering iron (25 watts) was not powerful enough. It quickly cooled when I touched it to the battery. Using my old 100-watt iron, I heated the center of the tops for about 3 seconds each. I did not heat the batteries any longer than necessary. I was careful not to melt the plastic at the edge of the battery. The metal edge is the battery can, the negative end of the battery. I exposed the metal on the edge of one battery and taped it up. I striped and "tinned" a piece of stranded copper wire by melting a little solder on the wire where it would connect to the batteries. Then I put wire on the solder-topped battery and held the iron on for 3 seconds. The pool of solder looked "wet" and shiny when it cooled. I pulled on it to be sure the connection was good.
When all the solder connections were made, I checked the voltage from the first plus to the last minus, holding the red voltmeter tester on the plus and the black tester on the minus. This gave me a reading of about plus 7 volts (not charged yet). I cut the connector off an old, weak NiCd pack. I stripped the insulation off the very tip of the red and black wires and scraped the bare wire with a file to expose copper since solder would not flow onto the silver stranded wire. I soldered the red wire to the plus end and the black wire to the minus end. Having no heat-shrink wrap, I taped the batteries tightly with electricians tape. Done:
I checked the voltage again after charging, red to plus, black to minus, got about 9.5 volts for the 7-cell pack.
This pack gave me a long, steady flight in my park flyer.