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I've been looking Maxwell ultracapacitors and have noticed despite their very low voltage, they also have a very low ESR. The BCAP3000 for example has an ESR of 0.00029Ohms, a maximum peak current of 2170A for one second and a short circuit current of 9,300A! This is just the range of current I'm looking for to make a coilgun or railgun and I think the low voltage will certainly be good for a railgun. Also the low voltage will mean it's easier to charge and safer to handle. Although how would you switch it without causing a voltage drop as every mV will count? So do you think it would be worth buying a used one on ebay, or just stick to ~400v caps?
Here's a video of one showing it's insane discharge:
It says 0.29mOhms = 0.00029Ohms and that lines up with the rated short circuit current of 9300A (4th from the bottom, second page) I=V/R I=2.7/0.00029 I=9310.34A
I think the problem would be keeping the coils/rails low resistance, if they were even 10mOhms it would make the current too low to be useful.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Don't forget about inductance! 9,000 A is not that much current for a railgun, and you need volts per henry to get the current started fast.
Suppose the resistance of your whole circuit loop (including switch and ultracap ESR) is 0.001 ohms, so your peak current is about 2,500 A.
Also suppose the inductance of the whole loop is 1 microhenry (including ultracap ESL, which is unspecified because negligible in ordinary service, whose charge and discharge times measured in seconds). Well, V = L dI/dt, so it will take about a whole millisecond for the current to reach 2,500 A.
If you reduced the resistance, peak current would be higher, but would take proportionally longer to build up. Think about the internal ballistics -- the time scale while your projectile is between the rails.
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Ultracapacitors?
