The actual problem was not an intermittent switch. It was the last thing I'd have expected -- one of the welds on the battery pack had come loose!
Without turning this into a megillah... Any suggestions on repairing it without damaging the cell? I don't have welding equipment. I do have an EDSYN pencil iron, and a 100W RadioShack gun.
Please don't turn a straightforward question into a tsimmes. (But you will.)
-- "We already know the answers -- we just haven't asked the right questions." -- Edwin Land
It happens. Spot welding takes practice.
Test the broken flat wire to see if it can be soldered. Keep the battery as cold as possible (i.e. wet sponge). Plenty of liquid flux and heat. Work fast to keep the heat affected zone to a minimum.
Mazel Tov. You can also build your own spot welder with a big fat cazapitor. Search Google and YouTube for "capacitor discharge spot welder". For example:
If you don't want to build the entire circuit, just charge up the biggest capacitor bank you can throw together. One lead goes to the battery terminal. The flat wire goes is placed on top of the terminal. The electrode is some kind of heavy (#10 awg) solid copper wire. Compress this sandwich together, and apply power at the capacitor bank. Hopefully, it will weld the flat wire, and not the capacitor terminals. In theory, the stainless leads with have a higher resistance than the copper electrodes, so the stainless will heat up, not the copper.
Last resort is to drag it down to Batteries Plus or some similar vendor that has the proper spot welder.
Very last resort is to just slop some epoxy on the flat wire and battery terminal, and hope that it holds.
-- # Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060 # 831-336-2558 # http://802.11junk.com jeffl@cruzio.com # http://www.LearnByDestroying.com AE6KS
...
There is a conductive epoxy that can be used to fix this sort of thing...
John :-#)#
--
(Please post followups or tech enquiries to the newsgroup)
John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9
Call (604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."
Yes. But I like to double-check.
Pretty much what I thought. Thanks.
Hadn't thought of that. I could stick it in the refrigerator for a while, too.l
Great! A man after my intellectual heart.
I've got some electronic-flash caps lying around.
I never thought of that! It might be worth a coupla bucks!
Unless it's silver-filled epoxy, no way.
I invented conductive epoxy 50+ years ago. I didn't patent it, though. And probably someone else had beaten me to it.
Spot welding is a LOT harder than it sounds. Just to use some round numbers that you can scale for your application... If you expect to get 10,000 amps out of 10V, you can't have more than a milliohm of resistance. That's the cap esr + the switch + the wire + the contacts
An extra couple of milliohms in the system will render the welder impotent. And the weld will be extremely sensitive to the contact resistance between the weldments and thus the pressure applied and the contact area and, and , and. With a low-voltage AC welder made from a microwave oven transformer, I was getting about 20% good welds. The commercial CD welder with the same homebrew weld head was closer to 100% good.
You really want something that controls current rather than voltage. Then, most of the variables end up in one place, the weld. Using a higher voltage and some intentional, controlled series resistance can make it much more reliable.
Many battery tabs are made of zinc. Easier to weld than stainless. Hobby-Store 5mil brass is very easy to weld, but may be too high resistance for a dust buster.
Having said that, you can, with practice, make usable battery tab welds with a capacitor bank, a pressure controlled contact system and a switch made out of a nail. Charge the caps, apply weld pressure, slam the nail across two BIG wires. I was using 4/0 wire. You get one weld per nail, but nails are cheap.
You didn't say which wire. if it's the negative, you may be able to fix it by hose-clamping the wire to the battery case.
I'm gonna get yelled at, but I would not solder a battery. Can be done with practice. Trying it for the first time on the only battery you have is fraught with risk. Separator material inside melts pretty easily. I wouldn't try with a gun. I'd use a 50W pencil with a MASSIVE 1000F tip and very short heat application. You're heating a lot of mass quickly. Gun can't hack it. YMMV
I remember successfully doing it once or twice before. But you're right -- it's not a good idea. Solder doesn't bond well with most battery cases, and the case is huge heat sink that makes it hard to make a quick solder joint.
Charlie Fraught and Joe Risk?
I lucked out. The guy at the local B+ store said he'd do it for free if I came in early tomorrow morning.
I don't mean slopping the epoxy in between the battery terminal and the flat wire. I mean push the two wires together, and slop some epoxy on top in order to just hold them together. The Dustbuster doesn't draw all that much current so I suspect a simple mechanical connection will work. Kinda like the contacts for D cell batteries in kids toys, flashlights, games, etc, where a welded contact isn't necessary. If you're not sure if it will work, and don't want to deal with permanent epoxy, use Duct Tape to hold the flat wire in place.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
I agree. I spent about 5 days tinkering with mine trying to get it right. Every time I added more capacitance, things improved. The basic trick is to make sure the resistance of the joint to be welded, is higher than all the other resistances (and ESR of the capacitor). Mine currently has two 3F 20v caps in parallel. Something like this:
Good point. However, I have no way to measure the peak current or the current necessary to do the weld. I just kept adding capacitance until it worked.
Well, the idea is to dissipate the power in the connection, not in the capacitors, SCR, or contacts. As long as the capacitor is huge, the SCR oversized, wires beefy, and the contacts can handle both the current and the heat, it should work.
Apples and oranges. There's enough resistance in the transformer winding to probably make the welder useless. If the primary winding was made from copper buss bars, it might have a chance, but not a copper wire wound xformer. A big capacitor has an internal resistance, but that's MUCH less than the resistance of a wired transformer winding.
Agreed. Voltage doesn't matter. It's the current that does the heating. If you shove XX Amps through YY ohms of resistance, it's going to heat I^2*R (Ohms Law). Whatever voltage it took to deliver that much current is largely irrelivent.
I haven't seen any zinc tabs yet. I've seen tin plated steel and stainless. Brass should work for a Dustbuster as they don't draw all that much current.
I used a copper contact knife switch. I welded the contacts a few times, but was able to break it apart, file it clean, and do it again.
One more idea. Single sided resitance spot welder as used in auto body shops.
However, you'll need an arc welder for this to work.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Also flat wire made from nickel or various nickel alloys, which is difficult, but not impossible to solder.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
But there's a spring applying pressure.
The problem (from my point of view) is oxidation. I figure the Dustbuster draws ~1amp, so I need a clean, tight, connection.
Regardless, I'm going to have it welded tomorrow morning.
You're an idiot to suggest epoxy. The contacts will oxidize and then there will be no connection, might take a couple of months before this happens, but it will happen.
You should measure it. I was too lazy to get out the big power supply, but 3A would hardly get mine started. I've replaced the batteries in mine twice and finally gave up. The load current is very hard on batteries. Especially since people don't stop when it slows down. They just keep on trucking pumping
5A backwards into the weakest cell. And the constant current charge 24/7 is also very hard on the battery. Bad design.
Interesting. Those welds are way better than anything I could get using a microwave transformer and handheld electrodes. Spring loaded parallel electrodes jigged to an arbor press improved things dramatically.
Yep, at the time I did the experiments, I didn't have a digital storage scope or anything that could measure 10,000 amps. My welds got dramatically better about 10 minutes after I got the DSO and could study what was happening.
Yes, but... CONTROL is critical. Some tiny resistance in the path makes the current in the weld slightly less dependent on the contact resistance. When dumping a cap into the weld, I found that the a small difference in electrode pressure made the difference between no weld at all and a hole in the tab...and no weld at all. Keeping it in the sweet spot was tough.
Your apples must be different from my apples. My transformer had a two turn secondary of copper wire an inch in diameter...4/0 wire if I remember correctly. I carefully controlled an integral number of full cycles of 120VAC into the primary. Took about six cycles to weld. Primary current was about 30A and made the house hum on a 15A breaker.
I'd like to measure the current profile of some of the homebrew welders linked from this thread. No way a 50 amps is gonna be useful. People get away with a 50A SCR in the secondary only because the single-pulse peak current rating is WAY bigger than 50A. I've been using a 40A SSR on the primary side.
Yes, as long as you can control the current. Some resistance in the path makes the current less dependent on the resistance at the weld. You trade significant wasted energy for repeatability.
Might want to measure that current. As I recall, mine was over 5A.
The weld current profile can also be important. You need to match the energy in with the thermal time constant. A capacitor has peak current at the time the contact resistance is highest.
The advantage of a dual-pulse welder is that the first pulse normalizes the contact resistance somewhat. And with some effort, you can use the voltage/current profile of the first pulse to match the characteristics of the second pulse to a specific weld instance.
My CD welder claims to be able to put 7V into a milliohm. They dump the cap into the primary of a transformer. That inductance gives the current profile a softer start and spreads the energy out over a few milliseconds.
That ain't gonna work. The manual says hold it for 2 seconds. the battery will be toast by then. And you have zero control over what's happening. You can't wait for a battery weld to turn red.
Bend the flat wire over a few times and you'll get some spring pressure.
Apply duct tape for contact pressure. Bury the battery pack in cellophane wrap for water, dust, and debris proofing.
Too easy but good luck anyway.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Same basic principle as electrocution.
This is why I always run it at High (which uses all the cells), and stop a bit after the motor starts to slow down. As a result, the battery pack is in excellent condition.
Ok, I'll bite. Stainless or Nickel oxidize? What electrolyte is in epoxy that will attack stainless steel or nickel? Meanwhile, I can buy epoxy paint, epoxy coated rebar, conductive (Aluminum doped) epoxy, epoxy covering junction coating for cheap PCB packaging, etc. I can also buy quite a few fiberglass marine hulls and decks, which are held together by epoxy and have stainless deck fittings. No corrosion warnings on any of the epoxy cans, bottles, and tubes that I can find. It's certainly not hydroscopic so there's no electrolytic action. The stainless and nickel are fairly close on the galvanic series:
Sigh. Where is Dr. Barry L. Ornitz when we need him?
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
It's too short. And I want a "permanet" fix.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.