Fixing lipos is a pain, especially if the tabs are al and have been "spot welded" together. You have to spereate them and be careful not to damage the tabs, or snap the soft al. I use a single edge razor blade to break the welds by rocking it slightly while wedging it between the tabs where they are welded.
You will need zinc solder and the appropriate flux to get it to stick to al tabs. Cheapbatterypacks.com sells both and are not too bad on price.
I have repaired at least 25 maxamps packs (maintaining my investment
), and built/combined at least 40 other cells (maybe more) into packs. I like to use a sharpie to mark the polarity ( + & - ) on the cells on both sides near the tabs. Makes it easier to get them assembled right. Cover the tabs with good electrical tape, and make sure to protect the ends of the lipos. I like to use some cardboard on the non tab end, and usually some soft plastic on the tab end.
A small pair of needle nose pliers helps to hold the tabs together as the solder cools, that helps make the best connection. Just be careful not to short over the other tabs...