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sorry in my comment I said "resin" I meant "rosin". Anyway use flux-free solder and do not use the separate flux.
Very helpful. I would suggest a small follow up to make this process even easier: 1. File the battery ends a bit with a metal file or sandpaper. This helps the solder adhere. Just a little filing/sanding is enough, just take off a little bit of the shine. Batteries have very smooth/shiny ends for good contact and solder has a tendency to slide off without filing/sanding. If you are using shiny leads then file/sand them as well. 2. Use resin-free solder aka flux-free solder (the kind where the resin/flux comes in a separate bottle in the solder package). Sometimes called "silver solder". The resin/flux causes the solder to slide off the battery and also can blacken the battery reducing conductivity.