Trust me thefreeman, get flux. Get a single bottle if you just aim to try a little, it's cheap and lasts a good while. First of all, even if a cell looks clean to the eye it may have some layer of debris or rust. Also, once you mess with the cell coating or you need to solder over something with any sort of oxidation layer, you're gonna have a hard time. That flux is liquid and makes the job stupid easy:
Thanks everyone for the responses!
Prior to posting here, I DID search for aftermarket replacement packs. The best price I was able to find was $65 per pack. That isn’t terrible, but I like to have at least 3 on hand for multiple tools plus a spare, so I’m looking at $200 plus shipping for 3 packs. (nevermind the fact that the packs I was able to find had very mixed reviews)
In my mind that’s not a wise investment in a 10 year old set of tools when I could upgrade to new brushless technology and brand new batteries for $300-450 depending on what tools are included.(granted replacing my entire set would be even more expensive, as it includes 5 bare tools)
Based on your suggestions; (and the fact that my packs in their current state are basically worthless) I am planning on attempting to rebuild my packs with brand new high drain cells, using Barkuti’s suggestion of low temp soldering and the materials he has linked. I figure I’ll start with one, and if I am successful, do the rest.
As with most projects, I should’ve addressed this issue much sooner, due to the fact that I am in the middle of doing the electrical portion of a sizeable addition to my home, and it would be really nice to not have to load batteries in the charger on such a regular basis. 
For illustrative purposes:
I re-built that pack a year and half ago using solder wick. I used tape for extra insulation during soldering, to make sure the wraps wouldn't get damaged. It ended up being unnecessary. The flux, being liquid and pretty fluid, gets under the wraps and causes some superficial oxidation on the cells. Despite I cleaned them before rewrapping, some rust remnants can still be seen. If I were to dismantle the pack again, I'm pretty sure a nice bunch of additional rust pecks would be seen due to my second flux application to rebuild the pack. It is completely inconsequential, though.
this was the thread that I first saw it on:
I built up a 10S4P bike battery using that approach (I have pics but I’m too lazy to find a pic hosting website to post them) and then tore it back down again to sell the cells when I realised the battery didn’t work (long story). Took ALOT of effort to pull up those strips. Basic idea is that the nickel to battery terminal spot weld blows through the thin copper and pins it in place between the two.
I made a couple of 18V li-ion battery packs using the old crappy Sanyo GA cells from my bike battery using that approach. One used a “50A” BMS and balance board, which runs my drills and lower draw stuff fine, but shuts down on my hand vacuum and leaf blower. The second one is just batteries, no BMS, and is just fine - used it at the weekend to clean up the yard. Does have some sag with the leaf blower, but that’s the shitty old cells, not the connections.
Is it oversized enough?
And after pack it better
Barkuti,
The solder wick + low-temp solder is an interesting possibility. The main issue I’m thinking of is the relatively small size of solder wick - the widest I’m familiar with is 3.0mm, and maybe 0.1 or 0.15mm thick? (I’ll measure tonight maybe). Per the ampacity chart above, that wouldn’t make it any better than the 10x0.2mm nickel strip I currently use. Are you familiar with larger solder wick sizes?
Matt,
Thanks for the link, I’ll read up on it later.
Yuval,
Nice! I have a bastardized old Dustbuster similar.
I don’t see any reason to not just solder solid copper wire with low temp solder. Plus solder wick is expensive.
A solid copper wire has a few things working against it:
edit to add image for item 3. Right is preferable. Though looking at the image makes me realize you could potentially hammer a solid copper wire flat, although that cold work is going to work harden it and make it brittle - whether or not that is significant I don’t know.
Solder wick is a lot narrower than nickel strips and you can use many pieces together. Here is five bits/straps (3 below and 2 above) of 2.0 × 0.25 mm solder wick (the thickness can't say 100% for sure, no nice caliper here):
The wick is wrapped in kapton tape by the center, between the cells.
The conductivity of 2.0 × 0.25 mm solder wick is very close to that of AWG20 wire (nearly same section). So, in any case, I'm doing better than with nickel strips.
Now, whether solder wick is expensive or not, I don't think so (or at least not much more expensive). You just have to buy it at the right merchant:
As I say above if the wick's thickness is 0.25 mm:
In any case due to shape wick sinks heat slightly better than equivalent wire size, so its amperage rating can only be higher.
P.S.: My wick is actually slightly thicker than 0.25 mm, probably more like ≈0.33 mm, and just a tiny bit less than 2 mm wide.
Thu, 02/20/2020 - 19:01
Just the type of thread I was looking for during my research before buying a cordless power tool. I found a local deal for a current and well-reviewed Hitachi drill which comes with older (first available in 2017) 1.5 Ah batteries while more recent listings come with a 3 Ah battery. Amazon.com
This makes me want to stay away from cordless tools. I definitely don’t want to solder LiIon cells, wrangle with bricking electronics, or risk safety issues.
Naturally companies won’t support a model indefinitely, at which point replacement parts become inflated in cost or simply unavailable. It would bother me for a perfectly usable tool to become “bricked” for this reason. It seems not only predictable but unavoidable.
So, sadly, I’m inclined to skip the deal I found, and frankly any cordless tool. If I could replace cells safely myself, it would be interesting. Please tell me I’m wrong.
This thread already has many examples of how to do this stuff, phouton. It is, however, far from a 5 minute job, at least for me.
I've yet to see a drill with smart stupid self-defusing electronics like in laptop packs. But it doesn't matters, there are plenty of generic, non-stupid BMSs for sale (eBay, AliExpress) which do a pretty nice job.
They make money trying to force you to upgrade with the battery thing, they know the batteries will go weak after some time. Two key things here: properly overhauled batteries, with quality cells and proper electronics, and a voltage limiting charger (like I did for the PSR 1200) which along with care can ensure a much longer life for any battery.
The only problem I see with the above drill choice or yours is that it doesn't have hammer. Precisely, yesterday I made two holes in my living room roof with the battery overhauled Bosch PSR 1200 I have at hand, and heck. I had to sweat, and aid me with another bit and a hammer at hand to knock inside the hole frequently.
Sun, 03/15/2020 - 08:51
Thanks, Barkuti, the method and materials you suggested seem like the better DIY solution I’ve seen so far. It’s still a bit worrisome in terms of safety, at least for me without being more familiar with the subject. Plus, the concerns about soldering vs spot welding remain, especially for power tools with lots of vibrations.
Good eye. The drill I found does have a hammer functionality. The listing I posted is the version of the same thing without hammer mode but it illustrates the 1.5 Ah batteries that I’m less enthusiastic about.
0 problems, rest assured. I fellow mate of me has two overhauled batteries (with Turnigy sub-C cells), and uses its drill professionally (report of 300+ holes 
in a day).
Pic of an overworked second repair/rebuild I did for him, he blundered by leaving to rest a fully discharged battery:
Well, at least we know it’s possible. I’m still on the fence whether the hassles of cordless are counterbalanced by any significant benefit, especially for occasional use. Corded just seems more reliable, less headache, and less costly over the long term.
If you do the math you will see this is always a bad idea. When Dewalt or Makita makes these batteries, its a one way transaction. You will have to break the case open, possibly rendering it unusable again. Then you run the risk of damaging the control board. Then you have to unsolder the old batteries out and do the same for the new batteries going in. Unles you are an electronics technician, this is dangerous.
Absolutely positively not worth the time and effort.
Your best bet is putting it on ebay, recouping some of the money and get a new set.
The new sets are cheaper and better.
If you still have NiCAD tools, time to dump/sells and buy new.
If you have lithium tools, you really should get authentic packs. The batteries in power tools get abused. The batteries you install and the installation methodology you use to install them can prove catastrophic.
Can it be done, yes. Is it recommended, NO.
Unless you have a specific need for a different brand, just get the Ridgid cordless tools and be sure to register for the lifetime service agreement (LSA). The batteries are covered, so when they crap out, swap them out for new ones.
I just rebuilt a B&D 20V pack for my dad. 5 x Sanyo UR18650W2 (OEM) replaced with 5 x Moli P26A. New strips, spot welded. He was finally able to blow most of the backyard clean on a single charge 
. Be careful not to short anything out when removing the strips off the old pack, try and reduce the potential energy store by fully depleting the pack as much as you can before you begin.
Here is the next pack rebuild…An older Polarion 4S2P pack…
Will likely go with either the Moli M35A or the upcoming Moli P30A (for a potentially larger safety margin)
Yes and No.
You have to buy Ridgid from Home Depot to get the lifetime service. You also have to register the tool AND they have to approve you. Meaning, if you violate any terms to the agreement, they wont approve you. Even then, I don’t think they will be giving you free batteries indefinitely.
Bottom line, rebuilding battery packs is risky business. NiCAD for example is dated technology. Dont bother. As for lithium, buy a new pack from the manufacturer. Lets face it, if the original pack lasts 5 years, you replace it one time and you get another 5 years, by then the tool is obsolete or too worn out to use.
Now on the other hand, if you want to do it to prove it can be done, there is absolutely nothing wrong with that.