Dave Plowman (News) wrote:
In article ,
spamme0 wrote:
I'd like to hear more about the welder design.

I messed around with low-voltage transformers, but never
got consistent welds. Was VERY sensitive to contact
resistance. About half the welds didn't stick.
The other half blew thru the tab.

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.