Wot Pete says.
BUT... If it's a tagged battery pack (and they usually are), the tags are attached to the individual cells of the battery with spot welds. These are made by a highly specialised machine, because the battery contains liquid and can boil if overheated (too hot or hot for too long)
Furthermore, there is serious risk of shorting out one or more of the cells if they are loose when you attempt any repair. This can be highly dangerous with Lithium Ion cells because (a) they have a habit of bursting into flames, and (b) the contents are themselves pretty nasty.
If you can't do it or are nervous, get a tool repairer to do it instead. To solder onto a battery casing successfully, various things are critical:
- It needs to be brass or zinc or nickel-plated. You cannot solder to stainless steel at all easily. So roughen the surface a bit and see.
- You need a lot of heat, applied very fast and for a short time.
- I've managed to solder directly to dry batteries (zinc-carbon and Duracells) and to NiCds, but never dared to risk it with Lithium Ion cells. With those, it's RISKY/DANGEROUS. Make your own assessment of risk, and if you don't like the idea, do not attempt it, because of the risks of explosion/fire, etc.
- If there is a tag left sticking out from the cell, you can probably solder to that safely. I'm not sure if I'd risk soldering onto the cell itself nowadays.
Here's my suggestions:
Collect together these things:
- A big-ish soldering iron with a suitable stand - do NOT rely on getting it to sit horizontal on the bench. The gas torch sort might even work better, but I've never tried one so can't speak from experience. I _have_ used the Portasol butane/catalyst pen type, and a big soldering tip on one of those can be excellent (more heat than most electric irons and quite controllable. It needs a big tip. both in overall mass and usable area.
- A kitchen blowlamp (the sort for browning creme brulee). This is to pre-heat the tip of the iron just a bit more, to get extra energy into it.
- LaCo plumbing flux, and 60:40 lead plumbing solder (NOT lead-free). Other flux may work, but that's what I use. Note that in my experience, lead-free solder "poisons" leaded solder a bit. If it's on the pieces to be joined, try to remove it mechanically by abrading most of it away before you start. If there's enough slack in the wire, nip the end off and start with a fresh bit. In my limited experience, electrical solder doesn't work well for bigger joints (even Ersin multicore, which used to be my gold standard electrical solder) , because of the flux oxides - you do better with separate flux and solder, like water plumbing.
- Metal cleaning materials: some fairly coarse wet+dry paper, and/or some 000 grade (or finer) steel wool.
- Kitchen roll and a saucer of water to dampen it as necessary.
- Insulating tape and/or masking tape: If they aren't already, tape up the cells not involved in this so that they can't short against each other, and the same for any straps between them. Keep the tap away from the end of the cell you'll be working on.
- A scrap bit of thick-ish wire and some 15mm copper pipe, to practice on first.
- Isopropyl alcohol for degreasing and flux removal.
- A flat file of medium coarseness, for your soldering iron tip (may not be needed).
- A fireproof wastebin into which you can dispose of alcohol-soaked kitchen paper (temporarily - take it outside ASAP!).
- Some heatshrink sleeving that's big enough to go over wire and tag (if it's still there). Use a piece that's long enough to cover the joint and a bit of wire, but short enough so it can be slid far enough along the wire that it can't heat up while the joint is being soldered. 3/4" or so is usually a good compromise. You'll need several pieces (for experimentation), and a sharp knife (to cut them off if they shrink when that's not intended!). Toolstation sell pre-cut lengths in plastic boxes (bright colours too!), which you can cut down further to what you need. It's handy stuff to have around.
- Some contrivance to hold the battery pack still while you solder to it. I'm lucky: I used to work for a lab equipment manufacturer (decades ago!), and I have some retort stands and clamps, of the chemistry lab style. They're excellent for this sort of thing. You may need to apply some pressure with the soldering iron initially, so you don't want stuff sliding around or tipping over on the bench.
- Ideally, a Kevlar oven glove, for the hand holding the wire - burned fingers are inevitable otherwise! You might get away with work gloves, I guess.
Doing it:
You need ventilation, partly because of the IPA and because you don't want to breathe in soldering fumes! But it also cools the joint, so don't arrange huge airflow! I use old PC fans on the bench sucking air from close to where I'm working, and leave a window open (12V bench power supply is a very useful thing!).
1. Experiment:
The copper of the "test" pipe will act as a heatsink, making it hard to make a good joint to it with just a soldering iron, as they can't supply enough heat normally. That's why it's a good thing to practice on. Ordinarily, you'd just leave the soldering iron on the joint for longer (novices never apply enough heat for long enough when soldering), but it's inadvisable in this case, as you risk boiling the electrolyte inside the battery (if you're soldering to the case).
- Prep the two components thoroughly (wire and pipe): clean to brightness with the abrasive(s), then degrease, then put a smear of flux on the bright surface (temporarily prevents oxidation). For thicker wires I usually strip back the insulation with a Stanley knife blade or similar - don't cut strands. If you haven't got back to bright wire, you will need to clean it, and there is a risk you may get a "dry joint"*.
- Every time you need to use solder, flux the end of the solder roll, by dipping and wiping off excess on the edge of the pot. LaCo absorbs moisture, so keep the lid on when you're not working (the moisture boils off at 100 deg C, so it cools the joint - not what you want!).
- Tin the soldering iron tip: dip it in the flux, then apply tinned solder to it, so that it's well 'wetted' If the solder forms globules in one part of the tip, you need to let it cool, file it clean and bright and try again. You can also mechanically remove oxides etc. by cleaning it in a pad of wire wool while it's hot (but mind yer fingers!). Work at this until it wets well.
- Tin the wire well first: Dip it in the flux, then dip the end of the solder and tin the wire by heating through the solder onto the wire. If it's stranded, make sure all of it is bright with solder, and you have no individual strands loose, nor un-tinned, nor separated from the rest by lines of burned flux.
- Tin the copper pipe: Again, heat through the solder onto the pipe, and hold the iron on until you can see the solder flowing on the pipe around the iron's tip. Aim for a well-tinned area not much bigger than the soldering iron tip or the part of the wire you'll be soldering down. You don't want to leave a lot of solder on there, as it increases the thermal mass you have to heat for the joint. you want the solder bonded to the copper, nothing more really.
- Finally make the actual joint: Clean and tin the soldering iron: you can get most crud off it by wiping it with a damp bit of kitchen roll - the steam removes the flux residue. Let it recover its temperature, and re-flux it and the wire, by dipping in the flux very briefly. Solder through the wire onto the pipe, and add in tiny amounts of (fluxed) solder wire from the side if needed. Once it goes molten, count five seconds (max.) with the iron on the joint, before removing it.
- You MUST hold the wire and pipe pressed together and absolutely still while the solder solidifies (this is the part when you burn your fingers!). The joint is molten around 180 deg C - far too hot to touch directly - but the heat also travels up the wire, so it will heat up unpleasantly while you're holding it. If using bare fingers, you have to endure this, without twitching!
2. Review the experiment
If your soldering iron can put out enough heat, this will work fairly easily. If not, you have a few choices: get a bigger soldering iron; pre-heat with a blowlamp (just the tip, and just before doing the joint!); give up altogether. If you do decide to pre-heat using a blowlamp, be very careful with the isopropyl alcohol, and any tissues moistened with it - highly flammable vapour and liquid!
You're trying to heat the joint fast enough to get a good joint, so you can
stop heating before much heat can conduct into the cell of the battery (or your experimental copper pipe). It should happen in a few seconds - you're fighting the thermal conductivity of the materials, and working against you is that the wire needs to "fat" because of the current normally flowing.
Test the test joint mechanically, by tugging hard on the wire. It shouldn't really pull away from the copper pipe - if stranded, I'd expect individual strands to break before it separates. If you can pull the wire off intact, it's probably a failure.
. . .
If you can't get this down pat with your experimental setup, my strong advice is to give up and admit defeat. Lithium Ion cells, are just too dangerous to have accidents with.
Assuming all is good, it ought to be obvious how to approach the real thing. But to reiterate: it may be your soldering attempts aren't working because the cell casing or the tag is the wrong material in the first place. If so, you're probably hosed. You might fit a tiny "Lucar" connector to any sticking-out tag and mechanically crimp them together (and cover with heatshrink afterwards (put it on the wire first!), but it may be impossible.
Finally the heatshrink: don't forget it goes on first when doing the real thing. once the joint is made, the heat from the main part of the soldering iron is enough to shrink it (if the iron is held underneath the heatshrink, very close to it). Hot air paintstrippers work very well, and shouldn't hurt the battery. Shrink over the fattest part of the joint first, or the middle if it's fatter at each end.
. . .
OK, too much information from me as usual!
I have done this successfully several times in the past, for a cordless drill, and broadcast equipment batteries, and re-celling rechargeable flashgun batteries (a horrid job!), but never with Lithium Ion cells (only NiCds). I
would try it, if I was comfortable I could get enough heat onto the joint,
and only the joint, quickly enough to be safe and make a good joint.
Only you can make the risk assessment for yourself..
E.
*in your power tool this can be a problem, as the wire will be cut to length already (you can't shorten it). consider replacing it altogether (you can get cut lengths of wire of the right gauge by the metre from most good old-style independent car accessory shops - forget Halfords!).
If the other end terminates in a crimp connector, by far the best approach is to crimp a new connector to your new wire - the accessory shop can probably do this for you (get several done so you can trim to length at the bench), and start with it slightly longer than before (assuming there is room). Failing that, you can solder to the old crimp connector with care, and this is better than going straight onto the switch or motor, if there is a choice).
