On 11/17/2017 10:13 AM, Jeff Liebermann wrote:
On Wed, 15 Nov 2017 19:56:22 -0800, Jeff Liebermann
wrote:

My ESR meter is in my office so I can't measure the resistance of the
battery right now. I'll try to remember to check tomorrow.

I don't think there's a problem with pushing current through the
magnet if there is enough spring pressure on the contacts.

The 6mm dia x 1mm thick magnets show about 0.02 ohms measured with the
original Bob Parker ESR meter. It was fairly difficult getting a good
connection which required using two strips of nickel flat wire to get
a decent connection. I also had to apply some pressure to get a
reliable connection to the meter probes. Even so, the resistance
never climbed over about 0.30 ohms.

That was my concern. I could not get a reliable connection, as mesured
by a VOM, with casual contact to the magnet. I had to use pointy probes
and press very hard.

If you get some square magnets, you can wrap a piece of battery tab
material around them. Leave some sticking out so you can
solder a wire or put a clip on it. Works for charging all types
of batteries with magnet-attractive connection points. And the current
doesn't go through the magnet or depend on the surface plating.

I don't have any square magnets and most everything that I could find
in the right size on eBay is round, but I'm sure they exist.

I'm not recommending square magnets for contact extenders in use.
Problem I solved was the lack of battery holders for charging cells.
If you wrap battery tab strips around the magnet, you can stick it
on any cell connection that's magnetic. The square magnet self-centers
in the rectangular cavity you created in the tab material.
Also works on most of the
power tool batteries that I have.


I'm (slowly) building a better spot welder suitable for welding tabs
onto batteries.

I'd be interested in your welder experiments.
I have a small arbor press with a 1/2" arbor. Turns out that a 1/2"
copper pipe cap fits nicely over the end. I soldered a circuit board to it
that holds spring-loaded contacts made from square brass tubing
and tips made of big copper wire. Gives some consistency to applied
pressure.

Bigger problem was consistency of applied energy.
I tried to use a microwave transformer.
I have some tips for dramatically increasing the reliability
of the weld from "useless" to marginally acceptable...sometimes,
if anybody's interested.

I tried discharging caps.
I could get some very nice welds, but the consistency was abysmal.
Don't think I ever built a battery pack with 100% good welds.
The voltage was just too low. Tiny variations in contact resistance
created MAJOR variations in weld quality.

I tripped over a small CD spot welder on ebay for cheap.
The thing delivers a pulse of energy, so it's much more tolerant
of contact resistance. It can put 7000 amps into a milliohm.
That terminated my work on the microwave transformer welder.

There are some relatively cheap CD spot welders on ebay.
I looked into a few of them. Although I never got any real info,
I surmised that they're still trying to switch relatively low
voltages.
A real CD spot welder switches 600V or so into a stepdown transformer.
I think the magic is in the design of the transformer.

I plan to buy some flat nickel wire at about 10 mm
width, and make replacement button tops in a small bench press, which
would then be spot welded to the top of recycled batteries.

Or, I could be crude, and just spot weld one end of a nickel strip to
the battery, and zig-zag the strip to simulate the button top. With
luck, it might act as a spring.

That's what I do. If the cell was removed from a pack, there's already
enough tab left to fold over for a tip. You don't really need to zig-zag
it, just fold it over once and put a blob of solder underneath to raise
it to the desired height. Soldering the blob on both sides reduces the
resistance,
but risks overheating the cell top.

Zig-zag just gives it more opportunity to
fold to the side and cause problems. If you need a spring, it's often
part of the battery holder.

Hobby stores sell sheets of brass that can be cut to any size/shape.
Very easy to weld. I wouldn't use 'em in high current applications,
but they can work well at low current.

Sigh... yet another project.