Some have a hard time soldering to the thin end tabs. Tip: Fold over the tab, put the stripped wire end in the fold, squeeze with pliers to make a firm mechanical connection. Then, the soldering is easy.

BTW: the battery is a good one, I made a mistake and ordered one when I needed two. My decades old shaver is young again. One more thing, while "inside" I noticed some plastic gears - a bit of powered graphite on them made them run smoother.