You can use nichrome wire that is thick enough to support itself in a big loop. You just need to be able to adjust the voltage relative to the length and diameter of the wire such that the amp draw is correct to get the right heat. In other words… thicker and/or longer wire needs more voltage to produce the same heat. I use a car battery charger plugged into a box with a dimmer switch. Note: the dimmer switch goes before the charger not after.
We use this and it’s been great! Many different levels of voltage from 12v up to 24v.
Thanks for your replies - I’ve been noodling on this. I think I’ve figured out how to mount it in a loop such that I don’t short out the wire and I don’t burn the crap out of my fingers. I’m going to order some thicker nichrome wire than what I have in stock (which was for a 3D printer build platform) and try to use a bench power supply to drive it.
I’m thinking 22 gauge - thoughts? I haven’t actually done the math to figure out how much current I’d have to push through it because math is hard.
Failing that the car charger+dimmer switch sounds like fun
Looks good but pricey. I used nicrome wire (also called resistance wire) in a wooden frame and attached a ordinary 24V 2-3A power supply.
Took me about 1 hour and 10$ s to build.
Worth noting is that the resistance wire gets longer when heated. So some kind of spring is required to make the wire tensed at different temperatures.
i used for the wire fishing line , and my small rc lipo charger has the option: hot wire, not all lipo charger have this but it is good idea to check and read the manual something
May years ago when I did a hot wire project, I used a re-curve hunting bow, Strung backwards.
I got the wire from a old space heater. Car battery as the power source. I used a long length of the wire as a variable resister to meter the power. I would use alligator clips to make connections. It worked really well.
Doug, I remembered this post, I have a pelican like case that failed my leak test. I did the test similar to your test. This is what I am doing. It might help you. Add a tube in to the box so you can blow a small amount of air into the box and create a positive pressure inside. If yours is like mine it wont take much pressure before you hear the air leaking out of the box. As log as there is positive pressure in the box, no water will get in. So I added a strap around my box, It made a significant difference. Still only holds about 1 psi, but that is twice as good as it was. The straps are dirt cheap. I have the details if you want to go that way.
Thanks @MAC, that had been my intention to embed a camstrap into the inset where the battery box is to go and use it to hold the box in place.
But I also have an active air pressurization system in place that consists of a small air pump , a water resistant air inlet thingie (which should work fine so long as the board doesn’t get completely inverted, which I think it can’t on account of the weight of the motor and wing) and an air outlet tube that goes from the battery box (where the air pump lives) to the Driver box and provides positive pressure in both. I have a pressure gauge on the mainboard but probably air will leak out at about the same rate that the pump will bring it in if I’m unlucky. If I’m lucky I’ll save some milliamps.
Thank you @MAC, @Alexandre and @cyberfoil for feedback on the hot wire issue. I have ordered a roll of Nichrome 20 ga. It’ll take some amps to get it warm, but it should stay rigid enough at temp to allow me to scoop out foam in the 2" diameter form that I’m after.
I’ve been fighting the flu for almost a week now and I’m super cranky when I’m sick, but I did, bit by bit, finally get all the batteries welded before the welder died.
No, I’m not kidding, my BRAND NEW welder really died, but fortunately just shortly after I finished with the above packs (which represent 1,080 individual spot welds).
But before it died it really well. It actually had too much power for the thickness of Nickle strip I was using.
The welder has 2 outputs: the standard thingies sticking out of the front and a connector for a handheld. The thingies sticking out of the front stopped working, but the handheld still works. I opened it up and there was no obvious melting so either they are on 2 different sets of wires on the transformer and one set failed, or they are on the same set and there’s something much simpler wrong. I haven’t made the time to dive into it yet.
But now all I have to do is bolt the banks together, wire them up and the battery box is almost done. After that it’s hot wire followed by fiberglass layup.
I’ve been thinking, though, that I might have to drink the BMS cool-aid. Prior to this I have always divided my packs into 2 banks of 5 (all my past packs have been 10S, this is my first 12S) and using a B6AC to charge them. But the B6AC is a pretty timid charger - it doesn’t normally like to go above about 2.7A even if you tell it to, so in the case of the 10S 20700 pack on the eMTB it takes the better part of a day and a night to fully charge it.
But, in the reading I’ve done, most BMSs are crappy and don’t properly balance charge the pack and don’t monitor the overall state of the pack, just the state of each bank within the pack.
Apparently the good BMSs are the ones that contain a BM3451 chip. I haven’t started my search, but my constraints will be this chip and able to support 12S at up to 8A, which happens to be the maximum current of my only other charger.
If you have suggestions, please let me know.
I’ve been shopping BMS’s too. Smart BMS’s seem pretty rad since you can monitor individual cells on your phone. Been thinking of getting some of them.
So I spent a bit of time on BMSs. Nobody likes them.
The cheap ones don’t balance very well and a cursory look at the BM3451 based ones indicates that they can’t handle a high Series pack (like they want 3S).
So I’m looking at this iSDT
Two things I like about it are that it’s high current (the Turnigy a bit less so) and has 2 6S channels, so if I split the pack right down the middle I can charge both sides simultaneously.
The iSDT also has a USB charge port so I can top up the remote as well.
An external charger such as the above solves another problem, and that is I didn’t really allow for space in my battery box for a full-on BMS board.
I will need to make a custom split charge connector, though, but it will be internal so doesn’t need to be waterproof.
I’m leaning heavily towards the iSDT.
Addendum: the iSDT is actually cheaper at Amazon, and 1 day shipping.
I ordered a bunch of these:
12S, 48v, 60a. I’ll divide my battery pack into 3 parts so I’ll need 3 BMS’s. Not smart BMS, but seems to do the job I need for cheap.
For my lipo i used 2 of ISDT : https://hobbyking.com/en_us/isdt-t6-lite-600w-charger.html with 2 power supply 20A 12v
Worked very well ( balancing is not extremely precis but it was fine ) since i have them , for my 18650 i slipt it in 2x6s
ISDT have also a 8s
Hot wire cutter is complete and works. My bench power supply only delivers 5A so it barely has enough power to get the wire through the foam but if I go slow enough it works fine (you can see the test cut in the foam block)
So Basically what you’re looking at is a 20AWG nichrome wire loop, 2 heat insulation screwterm blocks, then behind them those 2 little blue wires are high-temp wire which comes off the nichrome and attaches to standard 18AWG wire (OR/GN). All installed on a nice piece of oak.
I’m weighing the option of getting a bigger supply or just calling it good enough. I rarely call things good enough when the alternative involves acquiring giant prehistoric rheostats, but this time I might have to resist, since the clock is ticking.