On Fri, 17 Nov 2017 12:18:42 -0800, mike wrote:
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.
I did ok with points, the for higher currents, a flat surface with
many points of contact is better. This is my test fixture for
discharge testing various cells. The clamp is plastic so insulation
is not a problem:
http://www.learnbydestroying.com/jeffl/LiPo/Ultrafire%2018650%20test.jpg
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.
Ok, that should work. Where did you find a small enough square shaped
magnet?
Also works on most of the
power tool batteries that I have.
For power tools, I use the spot welder. I tried battery holders only
once. The vibration from the tool caused are rather interesting form
of intermittent connection.
I'm (slowly) building a better spot welder suitable for welding tabs
onto batteries.
I'd be interested in your welder experiments.
It's nothing special. Cazapitor discharge systems are ok, but I think
I can do as well with a LiIon (or LiPo) battery for power storage. My
big worry is that as the battery discharges, the energy delivered to
the weld will decrease. So, I'm trying to measure the current and
adjust the duration accordingly. I don't have this part working yet.
Another nice thing about using a battery instead of CD is that I can
spot weld as fast as I can push the foot switch. I don't need to wait
for the cazapitor to charge. At this time, it's just a simple eBay
timer board driving a automobile starter relay controlled by a foot
switch.
https://www.ebay.com/itm//111983136980
It took about 3 weeks to arrive. The starter relays I bought surplus
many years ago. Get the biggest you can find. I'll post photos when
I get something that's reliable.
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.
I use two 1/4" copper rods mounted on drilled fiberglass insulators.
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 think the secret is in the ESR or internal resistance of the power
source. New quality LiIon cells run about 150-400 mohms (milli-ohms).
Older cells and eBay crap batteries are much higher. The series
connectors, switch, SCR, wire, probes, and relay all add to the series
resistance.
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.
I do ok with soldering to NiCd and NiMH cells. However, LiIon cells
are stainless steel and difficult solder. With enough flux, I can do
a tolerable job. However, I tend to overheat the cell trying to get a
decent connection, which is both dangerous and irritating when the
battery pack eventually fails.
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.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558