Li Ion replacement

Think like all spot welds you need the correct current for the correct time.

I flat-out disagree with that statement. Sure, you can play semantics with the words and claim it means anything you want...but current/time control doesn't solve the repeatability problem in the face of process variation.

If your resistance is zero, all the current in the world won't make a weld. If your resistance is high, low current will blast holes thru the medium.

What you need is local temperature rise to fuse the metal together.

A current/time controlled weld is VERY sensitive to resistance. If you have repeatable resistance and machine enforced pressure, you can set the current/time for the particular setup and make good welds...until something changes.

If you raise the minimum resistance, the variable portion becomes a smaller percentage and welds get more repeatable. .005" sheet brass that you get at the hobby store makes tabs that are much easier to weld. I never used it in a battery pack because I didn't want that added resistance and I was too lazy to look up the corrosion effects due to the dissimilar metals.

A CD welder delivers energy and is much more forgiving of process variations, ie pressure and resistance changes in a hand-operated homebrew setup making welds on old used batteries.

A REAL battery tab welder hits it twice. The first shot stabilizes the contact area and measures the resistance. The second shot uses results from the first measurement to optimize the weld energy.

Re the discussion on making connections to battery cells: the summary of the content so far is that soldering is dangerous, weldingis the way to go, but is not trivial.

There's another possibility that I've seen read about but have no experience with: conductive glue. From a commercial battery website I saw

"Wire Glue - electrically conductive adhesive

Wire Glue utilizes the latest advances in microcarbon technology to bring you a highly conductive glue at a fraction the price of competitive products which use precious metals such as silver.

Wire Glue allows you to make low voltage AC and DC electrical connections without soldering or heating.

There is no requirement to measure out and mix hardeners. Simply stir and apply the glue to your surface and let it cure overnight. By morning, you will have a reliable, permanent and electrically conductive bond!

Cost-effective replacement for silver-loaded epoxies.

This lead free formulation is ideal for bonding all types of low voltage electrical connections.

Wire Glue is not intended for high voltage or high current applications. In high power circuits Wire Glue may become hot and give off burning smells."

From another page on the same site:

"Can you make the tabbed cell I require?

We do not add tabs to order... Order a bare, untagged cell from us and then carefully remove the existing tags from your dead cell. Apply the Wire Glue to your cell's surface and to the tag, pop them together and let it all cure overnight. By the morning your problems will be solved."

This also suggests that you should be able to get similar but more expensive silver-based glue. What current either will handle sensibly I don't know - a data sheet would help.

HTH

Be nice if there was some guidance there. Most rechargeables can supply very high current.