A battery pack conundrum. Please give advice!

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DavidEF
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A battery pack conundrum. Please give advice!

Hey guys. I bought some cells to make a battery pack for a DeWalt drill. I also bought the BMS for it, and a CC/CV board to upgrade the charger. I was planning to try to solder the cells together, but on second thought, I decided it might not be a good idea. For one thing, it’s to be a 5S3P pack. So the connections will be numerous. I’m concerned that while soldering one cell, the heat might follow the copper to another connected cell and melt the joint. Also, the heat itself can be a problem if it sinks into the cells. So, I’m thinking that I really need to spot weld the cells together using the standard Nickel strips. But I don’t have a spot welder or the money to buy one.

So, should I go back to my original intent, and solder the cells together using strips of copper? If so, how should I do that? What would be the most effective way to get all good joints and not dump too much heat into the cells? Also, is there a chance that it might be easier/better to solder wires instead of copper strips?

Or, should I just hold onto the stuff and hope to have the funds some day to buy a spot welder? Right now, that doesn’t look like it’s going to happen anytime soon. I’d really like to get this pack done. But, if it really is that important to wait, I suppose I’ll have to wait. I don’t want any loose joints or bad cells from too much heat, or any other problems. I need this pack to be super reliable for years to come.

Is there another option that I’m not thinking of? The cells are flat top Panasonic 2040mAH NCR18500A’s. They’re going into a defunct DeWalt 18V XRE battery pack that will already need to be modified so they fit. If you can tell me how to make it work, I’d appreciate it.

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HorizontalHunter
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To prevent the heat from affecting another solder joint you need to put a “heat sink” in between the two joints to absorb the heat. I often use a simple alligator clip or a pair of hemostats.

In general heat and batteries don’t go together and I haven’t done it. As to whether it’s a good idea or not is another question; I am sure that other members will chime in.

Bob

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Bob Loblaw
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If you have a batteries+ near by you should ask them. I know we can’t rebuild li-ion packs for insurance reasons but tacking on the leads to each individual cell and letting you connect them shouldn’t be an issue.

DavidEF
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Bob, I don’t know if there is a Batteries+ near me. I might look that up in a little while. I do know there are a couple battery pack builders/re-builders around. I’ve seen their shops. I was thinking that might be costly though. How much do you think they’d charge to put tabs on both ends of each of 15 cells?

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raven38571
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I don’the have a link but I have read about making a spot welder using an old microwave transformer. Check your local recycling Center, they may let you have one for free.

Halo...
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raven38571 wrote:
I don’the have a link but I have read about making a spot welder using an old microwave transformer. Check your local recycling Center, they may let you have one for free.
Yea, but only do this if you have experience with high voltage that will kill you. Plus have confidence in your judgment. If not, do you like living?
raven38571
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We mod lights and direct drive them, knowing that one little mistake could turn into a pipe bomb. But we all educated ourselves and take precautions. Same will need to be done when making a spot welder.

dchomak
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At least here in CT, there is a law against re-building Li-Ion battery packs. Or so they tell me at Batteries +.
They have the equipment to do it and rebuild the NiCads.

DavidEF
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raven38571 wrote:
We mod lights and direct drive them, knowing that one little mistake could turn into a pipe bomb. But we all educated ourselves and take precautions. Same will need to be done when making a spot welder.

Yeah, I’ve read about different ways of making spot welders. Other than the microwave transformer, there is also a way using supercaps. But, I’m not planning on making a spot welder at this time. I do have lots of experience with line voltage and some experience also with high voltage, since I’m a licensed electrical contractor. But, I don’t have any experience welding. I’m not so much worried about the high voltage killing me as I am the cell venting. I’d like to first learn how to weld using professionally designed and built equipment. Maybe after that I’ll one day build a spot welder. Then again, if I can’t figure anything else out, and can come across a microwave transformer, maybe I’ll give it a try.

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DanielM
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make it a lipo and no more soldering

Old-Lumens wrote:
I love modding, but I don't have much use at all for flashlights in general.
DavidEF
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DanielM wrote:
make it a lipo and no more soldering

Ok, great! Where can I find a lipo that is ~18V @ 6000mAH or more and fits in 40mm x 48mm x 120mm space, and price is less than $100USD?

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Ronin42
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dont over think it. use good copper wires (you know what to do with thoes). no one says you need to finish all the cells at the same time, do thermal batches if needed.

use a hot iron and short time to avoid heat soaking. heck i even had a wet towel nearby to help cool the cell i just finished when i soldered a battery pack togther.

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Never did LiIons but I’ve soldered NiMh and NiCad packs. I learned to buy the ‘tabbed’ cells which makes it a LOT easier to deal with. Soldering stainless was still a bugger though; I had to re-do maybe 1 in 5 connections after everything cooled down. I’ve got a NiMh build to do soon as my packs for work are pretty worn now.

The trick is to solder the tabs to the cells on both ends first (orient them correctly for how they will sit in the pack) then solder the tabs together afterward. Makes it twice as easy. I used two needle-nosed pliers with the handles wrapped with rubber-bands (one on each cell) when doing the final tab connections for heatsinking. Hemostats would get too hot to unclamp them without burned skin which I prefer to avoid. Make sure the pliers don’t short the cell- put a piece of cardstock in the appropriate places to forestall that problem.

Phil

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So, a solder connection to the cell(s) can work fine and be reliable? That’s what I needed to know. Thanks guys! RBD, thanks for the tip about cooling the cells with a wet towel. And thanks SawMaster for the tip about using needle-nose pliers for heat-sinking.

SawMaster, I would have bought the cells tabbed if I could, but I wanted the highest capacity, and these don’t have tabs. Maybe, if I’d researched a bit more I could have found them with tabs, but I was also trying to avoid fakes if possible, so I was limited in where I would buy from. I have a hard time trusting Chinese retailers to sell me genuine cells, after all the horror stories I’ve seen on BLF. The cells I bought were Panasonic 2040mAH NCR18500A recommended by Werner in this thread. If you know where I can get those same cells with tabs, I’d love to know it. If this works out, I may get another set of 15 and do it again.

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SawMaster
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DavidEF wrote:
So, a solder connection to the cell(s) can work fine and be reliable?

Yes, but remember these weren’t LiIon so whether it can be done safely is still unanswered. What I do is coarse file or sand the spot to be soldered, just enough to roughen it for a better mechanical bite. Flux the spot, then using a pinpoint-tip butane torch apply heat and use electronic solder, applying and removing the heat the first moment it melts. Try to kind of rub it in. Leave to cool. Soldering the tab material is the same. Using pliers or hemostats hold tab to cell and pinpoint heat similarly, removing heat at first melting. Just melt the two together, no more solder is needed than what’s there. It helps to have your hand resting on something as the slightest movement will break the bond. Hold steady for one minute after removing the heat, let cool, then try pulingv the tab loose with moderate force at a right angle t the cell axis- do not pull from the end. If the tab stays put it should be OK.

The joint will probably appear to be “cold-soldered”- grayish and grainy, not shiny- but it will be OK as long as it passes the ‘pull test’. To get the solder shiny takes a lot more heat and I have ruined cells trying to achieve that. It’s also helpful to have a vise or clamp to hold the cell so it doesn’t wobble around while you’re working on it. Pad that with cardstock so it doesn’t act as a heatsink and use the least pressure possible.
I would still prefer spot-welding as it’s a hugely better connection and likely safer to do, but not having that resource is what drove me to try soldering. TBH I’d even go to the extent of building a spot-welder before trying this again since I wasn’t aware of how to do that when I went down this path.

So yes, it can work but it’s touchy- you’re working on the ragged edge of overheating to get a minimally usable solder joint and you’re going to have several reworks from pulled tabs before you get one pack built so it’s frustrating too.

Phil

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A few months back I decided to rebuild a few battery packs for my dad. I started doing some research and watched tons of videos. It doesn’t take long before you can tell who is wrecking cells and who is doing it right. The principle seems to be the same as soldering electronics. Lots of heat/power fast is best to keep the work from over heating. However, the scale changes quite a bit with soldering cells. Meaning a lot of heat for an led connection is not nessessarily a lot of heat for a battery connection. The guys that are doing it right are using very large irons and only holding the iron there for 1-2 sec or so. Take a look at this video:
https://youtu.be/IelXfcrIXCc
I’ve practiced on laptop pulls with my 60w soldering station. If I use a large tip, turn the heat all the way up and let the tip get food and hot, I can make a nice joint on the battery without overheating it. I start with the tip in the battery for about two seconds, then add a bunch of solder and pull the tip away just enough that it is still heating the solder, but not the battery. After another three or four seconds of heating the solder, I can pull the tip away and leave a nicely tinned spot on the battery. When applying the tab, I heat the solder and squish it together quickly. Idk… I don’t have a lot of experience, but this is what I gather from my limited research. Ideally, I would have a high watt iron with lots of surface area like the one in the video.

All that said… I just built an mot spot welder. It’s not as hard to get a good weld as I thought it would be. I was planning on an elaborate control circuit with controlled pulses, but I found I can give a quick tap to a standard relay and make a very nice weld with just a very small amount of bluing at the contact points. Anyway…

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Thanks, PPDB22. I need all the help I can get. Even though I have a tendency to be a know-it-all, as lots of people on BLF can attest, I really don’t know all that much about this stuff. I’m still very much a newbie with electronics and LEDs and soldering. I’ve gotten to the point that I can put a new driver and LED into a flashlight, but that’s still about the best I can do.

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Beachlogger
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What about starting with nos laptop pulls?