Replacing rechargeable battery packs in consumer kit is a bane I suspect most of us share. You can't "solder" to individual cells. And, you usually can't slip a battery holder in place to make the connections for you.
Sure, you can buy premade packs in a wide variety of configurations. But, these often come at a premium in price (to pay for the added labor of their fabrication)
*and* you can't just pick them up "off the shelf" (well, *some* configurations are available like this but not "in general").
Has anyone come up with a clever way around this problem? Is there a poor man's approach to welding tabs onto batteries? Or, some other conductive fastener that doesn't put the cell at risk in the process?
You can buy most cell sizes with tags already welded on - so you can then solder them together. From most decent electronics suppliers.
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Dave Plowman dave@davenoise.co.uk London SW
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For only one spot weld though, so lowish current purposes. Connect both parts each to one side of a high current transformer (with a thermal cutout somewhere) with some cigarette paper separating the two parts of stainless steel. Press the 2 parts together until current breaks through and incinerates the paper. Check for integrity and repeat if weak.
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You might try replacing the heating element of a large 120 volt powered soldering iron with a pair of spot welding points. I'm thinking of the type with a 1 pound transformer, with a trigger switch and usually a light to illuminate the work area - they create large amounts of low voltage current. Buy using just the right size area of contacts, one might get lucky enough to make a one second spot welder that could be just the ticket for doing this. This is how it's done in the first place. The end does get hot, but cooler than soldering. You can solder if you do it very fast.
How about getting the manufacturers to change all those custom sized NiCad batteries to standard sized cells so we could just change them out at a local Radio Shack instead of buying expensive Digi-Key one off replacements?
One option is to buy the pack at "Bats R Us" place where they spot weld them as needed. Sometimes a good price can be agreed upon, other times you are lucky if they take money instead of your first born.
Yes, that was the "premade packs" option I mentioned. These are almost always "specials" -- with added cost and lead times.
You can also buy "single cell packs" (i.e., a cell with tabs) but assembling these into complete batteries usually results in a battery pack that is too large to fit into the space available. (in addition to being more expensive than "regular cells")
Understood. The kind used to solder two lengths of 12AWG together :>
The problem with soldering is not knowing how much damage you have done to the cell. So, you go to all the trouble of building a battery pack -- only to discover a month later that it is crap.
Many of the cells *are* standard sizes. Though often without the protruding "nubs" (positive end of cell).
I'd rather wind a small generator by hand than deal with their outrageous pricing schedule!
Digikey is a reasonable alternative but that's a week (at least it seems they have eliminated the minimum order surcharge -- so, you don't have to delay ordering that *one* item until you have come up with a list of other items to purchase at the same time!)
In terms of numbers made I'd say sub C is a standard size - pretty well every cordless tool I have uses them. But supply and demand determines aftermarket prices for bare cells.
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Dave Plowman dave@davenoise.co.uk London SW
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Some battery users need to weld lots of cells together on a regular basis, and they sometimes build their own welding equipment. I found numerous online examples of home-built welders when I searched for battery pack welding or some similar terms.
The types of welding processes used are resistance welding, and capacitor discharge spot welding. Resistance welding is performed by a low voltage, high current secondary winding on a large transformer. The winding may be made from large gauge cable and may only be a few turns. A timer circuit is used to energize the transformer for very brief periods, some tenths of a second. The high current passing between the 2 tips creates the spot welds. Capacitor discharge welding is performed by a large bank of high value electrolytic capacitors (rated/capable of rapid discharge) which are charged, then discharged thru the tips applied to the strap. Some commercial versions are microprocessor controlled with numerous features.
Fixtures are often used to apply pressure to the tips, for more consistent welds.
Nickel strip is available in coils (McMaster Carr), and straps are cut to length, then welded to the cells. Spot welding is typically done as 2 spots at once, with 2 tips pressed against the strap simultaneously. When the current passes from one tip to the other, 2 welds are made. This is then repeated, providing 4 spot welds per cell contact, for very reliable connections.
Cells purchased with tabs already welded to them are fairly easily trimmed, then joined with solder, which doesn't heat the cell as much as trying to solder directly to the cell. Fiberglas tape can be used as an insulating gaurd between straps and cell cases, and to cover straps after assembly.
I've only encountered one piece of test equipment where the battery compartment was so tight that a new pack made with tabbed cells almost didn't fit.
Yes, you can if you work fast, prepare the surface, use a very big and hot iron, and keep the pack cool (water dunking). I've done it with reasonable success with NiCd and NiMH packs. Total failure with LiIon which really don't like the heat.
Plan B is a do-it-thyself spot welder:
Hmmm... broken web site. Anyway, it's just a big fat capacitor and two electrodes. The trick is to not weld the electrodes to the battery or terminals. I use TIG welding electrodes but stainless nails will also work. Plenty of plans on YouTube and Google videos:
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Exactly. And they'll probably be significantly cheaper than from a retailer.
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IME, standard cells take regular solder easily, if you file off the plating. You need to use a real soldering iron (ie; a station) & work fast though.
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W
. | ,. w , "Some people are alive only because
\|/ \|/ it is illegal to kill them." Perna condita delenda est
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I fiddled with this for a decade or so and have some thoughts.
What you want is constant energy into a constant area of a repeatable material.
Low Voltage Spot Welder
You can replace the secondary of a microwave oven transformer with two turns of the biggest wire that will fit the core...don't forget to take out the magnetic shunts.
Couple of issues...you have to build some logic to turn the transformer on AND OFF at zero crossings of the mains. You can't just leave the transformer stuck at one corner of the B-H curve, or you'll eventually be digging bits of your Solid-state relay out of the ceiling when the core saturates. The residual magnetism also makes a bad repeatability problem worse.
This is not an energy welder. The amount of current you get is critically dependent on the total resistance in the secondary circuit. The amount of heat depends on the current and voltage at the weld junction. With careful control of electrode position and pressure, I was able to get about
50% pretty-good welds. But the other 50% ranged from no weld at all to blasting the tab completely away.
I never addressed the long-term effects of six cycles of
40-amps on a 15-amp 120VAC primary circuit.
I discovered that .005" brass that you get at the hobby store is much easier to weld than nickel. I never examined the electrochemical issues of brass. Resistance is higher, so probably not viable for high-current applications.
If you're gonna experiment, a good current probe and digital storage scope are essential. I had been messing with this thing for months. When a digital storage scope fell into my lap, I had significant improvement in a matter of hours. There's no substitute for knowing what's going on.
I was experimenting with ways to stabilize the weld when a $15 CD spot welder fell into my lap.
A CD welder is discharging the total energy stored in a cap into the weld. Since the open-circuit voltage is higher, the resistance of the weld junction is less critical. Most of the energy gets dumped thru a transformer into the weld over a wider range of contact resistance. And the leakage inductance of the transformer softens the initial transient. Good-weld percentage went way, way up.
Some have suggested you can discharge a cap without a transformer. My experiments quickly dismissed that possibility. If you can get repeatable contact resistance, you still need a switch. Closest I came to a switch was a pair of spring-loaded nails that blew itself up on every weld...but nails are cheap. And you need a LOT of caps in parallel on huge copper strap to get the peak current. My CD welder puts out 7000 AMPS. If you could reliably get 10A from each cap, that's a lot of caps in parallel.
A car battery will weld like gangbusters, if you could figure how to control it accurately.
About the time I figured out how to weld tabs, reality hit. You can replace the cells in a laptop computer pack, but you can't make it work. Many either lose their brains completely when discharged or remember that the cells were dead. Symptoms range from "still dead in the computer" to "still has the capacity of the old dead cells." Had one pack with a pic processor inside that I could look up. Resetting the pic reinitialized the battery, but that's rare.
Have a heavy metal can with a heavy lid available for the times when you accidentally create an internal short in a lithium battery. An old pressure cooker works well.
Wear safety glasses. Hot bits of nickel are not compatible with clear vision. I still have nightmares about the time a NiCd blew up in my face and splattered hot electrolyte all over my glasses.
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