Building my first power bank (questions) - Page 1
Forum section: electronics / beginners

author: jerryk
author_rank: Regular Contributor
timestamp: March 03, 2025, 08:30:16 pm
content: This is my first power bank and am looking for any suggestions on what brands and types of output to include. It's based on theINR21700-50Ein a 3S7P configuration. BMS is theDaly Smart BMS 3S12V60Awith bluetooth.All this is enclosed in aplastic enclosurefrom Aliexpress. I want to install a robust output scheme and am looking for any suggestions as to brand and type to maximize the versatility of the power bank. I also still want the standard 12V lugs on top to run various high amp devices like air/water pumps, electric fishing reels and the like. Waterproof outputs would be real nice but I'm also trying to keep the cost reasonable.Here are some of the output types I'm considering.Specific questions:1. Any suggestions on reasonable quality output brands and types?2. Should I use multiple layers of nickel strip (using .10mm) to achieve the desired output of 60 Amps?3. Can I get away with keeping the bluetooth antenna inside the enclosure?I'll be using my Kweld to spot weld the cells and if you guy's have any general suggestions I'm all ears.Jerry

author: Jwillis
author_rank: Super Contributor
timestamp: March 04, 2025, 03:37:29 am
content: 0.1mm is not enough for continuous 60A discharge. Of course it depends on the width of the nickel strip as well. 8mm wide 1mm strip min for 60A continuous. That will be difficult to weld. Its not just the 18mm between the batteries but the final tab and connection to terminals. At a more reasonable 12A .1mm x 8mm nickel would work OK.

author: jerryk
author_rank: Regular Contributor
timestamp: March 04, 2025, 07:54:13 pm
content: I'm lowering my expectations of carrying capacity of this power bank but still want to get the most current delivery given the rating of the BMS. If I can pull off 60 amps that would be great but I doubt I would ever need more than 30 amps in reality. I'm considering either two or three layers of the base grid base nickel grid strip and/or bridge the series with additional .15x8mm strips. The base nickel grid strip is actually .15 mm x 9mm (with thermal relief slots). Using the estimate of 15amps per mm2that would give me 20A per layer as an estimate. I don't know if layering nickel strips is truly additive in carrying capacity. I'm definitely looking for any ideas on how to arrange the strips that I have in order to get me close to 60 amp goal or at least understand the true carrying capacity when I decide on the scheme of the nickel strip installation. How do I connect the negative B- terminal to the nickel strip B- end. I have seen videos where the B- wire is separated into three braids and soldered along the negative nickel strip. I'm curious if I can just drill a hole in the strip and fold it over and bolt it directly. The 9mm width may make this difficult. Any suggestions on how to make this connection safe would be greatly appreciated.Photos include a couple of screen shots of an amazon vendors methods to increase current capacity by just bridging the series connection and a chart showing their estimate of various nickel strips.I have added a picture of a 60 amp fuse that has only .9mm2at it's narrow point and I'm curious how this works. Must be some sophistication in the design.As always - appreciate the help.Jerry

author: Jwillis
author_rank: Super Contributor
timestamp: March 05, 2025, 08:40:23 am
content: I've built similar 18650 power packs. My estimations are rather conservative because you need to take into consideration of not just whether the metal strip can handle the current but also the temperature rise of the metal. I try to keep things to around a 10 degree rise or lower over ambient of 25 degrees. This insures that the strips won't melt the surrounding plastic. Specifically the battery covering.You can use a PCB trace calculator to give a ball park estimate of how the metal strip will react to different currents.These of course are for copper traces, but if you stay on the conservative side it will work as an estimate for nickel.https://www.advancedpcb.com/en-us/tools/trace-width-calculator/Type in the current requirements and thickness of the strip. For the length I put in 18mm because that's the approximate length of strip between each 18650 cell of the 7 parallel cells. I try to keep around 10 degrees over ambient. Which would be 35 degrees total temperature of the metal. That keeps the surrounding plastic safe from melting.Use the Results for External Layers in Air for the width of strip.Just make sure you set the measurements to what you prefer. millimetres, inches, Celsius, FahrenheitMy calculation came out to 7.64mm width, so 8mm strip of .15mm nickel should be OK for 30AmpsYou can play around with the calculator to give an Idea of what to expect with different thicknesses and currents.