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Electric Stunt => Gettin all AMP'ed up! => Topic started by: Matt Brown on August 28, 2023, 09:00:41 PM
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I’m somewhat invested in building some 18650 packs for new planes. I bought 50 molicel P28B cells, a battery spot welder, heat shrink and a variety of nickel strip for battery inter-connections. I made some poor assumptions and now not sure how to proceed. My first assumption was that .2 x 8mm strip would be fine to weld and handle the current. Second was that the spot welder would be fine for .2mm thick strips. It welds but it’s marginal at best. I started looking into how much current these strip are good for. I found that the .2 x 8mm strips are only rated for 12 amps. Not near enough for my planes. I was told at the Nats that a guy was making packs for others but he was soldering them rather than welding. I’ve never liked soldering on batteries but I found a few instructional videos to watch. Maybe that will help.
Anyone else building their own 18650 packs? Anything you can educate me further on the subject? I’ve bought the cells so they are going to be made into packs one way or another!
Thanks, Matt
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Contact Will DeMauro. He can help get you started with lots of pictures and build info.
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First I used an online calculator to find the requiring metal cross-section for my needs, e.g.: https://energoprom.net.ua/en/calc-cabel/calc-secheniya/
Then I made 8 x 0.5 mm tabs out of copper sheet.
Finally I used regular 150W iron to solder the terminals.
Vitalis
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That is the first cable cross section calculator I’ve seen that made sense! From that calculator, .2mm nickel strips definitely are not going to be sufficient! Need to find some of those copper bars and solder them.
Thank you Vitalis!
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https://batteriesamerica.com/products/battery-bars-sub-c-size-10-pcs?_pos=2&_sid=8e7499201&_ss=r
These bars are designed for Sub-C sized cells (22mm OD) and might work on the 18650. They are gold plated and have a claimed 100amp carrying capacity.
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If you are making up 6S packs, note that Will D. has been using strips of .008 K&S tinned steel for his series connections. It solders well and has worked generally flawlessly for him and the folks that have used his batteries. Look at the top-20 at the past few nats and you'll see names of many guys using his batteries and/or his methods and tutorials. It may not seem like it is material that jives perfectly with an ampacity chart, but in actual use, it seems to work just fine. This is a great bit of data because it lets you know that the material options for the series connections are more plentiful and probably less critical than we think. Adequate is adequate. Will has a test stand that he uses to gather motor, prop and battery performance data. His method works and the packs he builds discharge equally cell to cell. I have built packs with braided copper and plated copper bars and they also work fine.
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Good to know that Brent! I’ve always thought those charts have serious safety factor built in. I know my RC helicopters pull 50-60 amps regularly with peaks over 150 amps on 10ga wire that are only spec’d to 30 amps or so. They never have any problems. I’ll see if I can’t find some of the tinned steel.
Matt
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Two things come into play to determine current carrying capacity. Cross section and length. All that matters is total resistance of the strap. The bigger the cross section the lower the resistance. The longer the strap the higher the resistance. Cell straps are short so by nature they are low resistance. Of course the type of material also factures in. Just saying. D>K
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This should work best. It should be easy to solder too. 3/16 or the 1/4 should be just fine.
https://www.amazon.com/Tinned-Shielding-Protection-Screening-Sleeving/dp/B0BBZL7L3J/ref=sr_1_3?crid=V9VETXD76HCJ&keywords=Braided%2BTinned%2BCopper%2BSleeve%2B3%2F16&qid=1693840118&s=industrial&sprefix=braided%2Btinned%2Bcopper%2Bsleeve%2B3%2F16%2Cindustrial%2C88&sr=1-3&th=1
On another set of idea here is the question of how much A average do we really demand in the air. Say you use 6S and 2200mAh for a 5min 30s flight that comes up with 20A average in flight. At his point I want to mention the thin tabs that connect the batteries and start to wonder if the simple tin covered Nickel tabs are enough to get the job done and we might be too worried.
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Two things come into play to determine current carrying capacity. Cross section and length. All that matters is total resistance of the strap. The bigger the cross section the lower the resistance. The longer the strap the higher the resistance. Cell straps are short so by nature they are low resistance. Of course the type of material also factures in. Just saying. D>K
Perfectly said. The best strip I found is .2mm x 6mm, which is about .008 thou.
P28B is powerful, I have gotten away with .15mm x6mm, and the overall IR went down just a little.
I have used .1mmx6mm, don’t go there, 😭😭😭😭😭 trust me.
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I got sidetracked on my pack building project. I know Will and others are soldering the tabs. What is everyone doing to prep the ends of the cells to accept solder? I saw one video where the guy used acid flux and it worked fantastic to get the solder to instantly tin but I don’t want acid flux on my batteries. Others have mentioned grinding on the ends of the cells. I’m not real thrilled with that idea either. I’m getting close to tackling this again, hopefully in the next week or so.
Thanks, Matt
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I would not say I grind the cells. I use a small stone wheel on the end of my Dremel to "scuff" it up just a bit. I really don't grind on it or even get it hot. It works well and I have no problem with solder flow. This method is easy and fast. This is the Dremel part I use. Dremel 1/8 In. Silicon Carbide Grinding Stone
Item No.SKU# 83322. This can be found in many places on the web including Amazon which is probably where i bought mine a few years ago. I'm sure anything similar will work just as well.
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I would not say I grind the cells. I use a small stone wheel on the end of my Dremel to "scuff" it up just a bit. I really don't grind on it or even get it hot. It works well and I have no problem with solder flow. This method is easy and fast. This is the Dremel part I use. Dremel 1/8 In. Silicon Carbide Grinding Stone
Item No.SKU# 83322. This can be found in many places on the web including Amazon which is probably where i bought mine a few years ago. I'm sure anything similar will work just as well.
Thanks Will!
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As Will D. mentioned, just a basic light scuffing of the surface followed by a wipe with acetone or denatured alcohol works great.
I have used Chip Quik liquid flux with good results on several packs. A tiny dab is really plenty.
https://www.chipquik.com/datasheets/CQ4LF-1.0.pdf
https://www.amazon.com/gp/product/B095KV4MDJ/ref=pe_27541350_747766620_em_2p_7_lm