# Why do coil guns contain capacitors?

I have been interested in coil guns lately, and see that they use coils. However, in every design I have seen, their have been capacitors connected to a charging circuit that is connected to a battery. My question is, why can't a higher voltage battery be directly connected to the coils to create that electromagnetic field that drives the projectile?

• What voltages to these coil guns typically require? What peak currents? What currents and voltages are batteries capable of? Commented Dec 3, 2016 at 3:03
• One way to summarize the various answers: batteries have high energy density, but relatively low power density. Capacitors are the opposite. Therefore, to get both the high total energy storage for long-term operation, and the high peak power required for each firing, you need both. Commented Dec 3, 2016 at 15:52

Each battery cell has an internal series resistance and when you stack up the batteries, these series resistances add up, too, limiting the current you can draw. You can load a capacitator with that limited current. The time it takes is not relevant for the function of your coil gun.

When you trigger the coil gun, the capacitator is discharged very quickly at a very high current. It can supply this current because its own series resistance is much lower than that of the battery stack. The height of the current peak is practically only limited by the inductance of the coil you use.

• So are you saying that the reason that batteries are not used is that they simply can't provide high enough current by themselves, and you can't stack them because their internal resistance adds up? Commented Dec 3, 2016 at 3:05
• @joshglen Yes, that is correct. The internal resistance of batteries severely limits the current, and also leads to internal heating and possible explosion of the batteries. Capacitors, having lower internal resistances, are able to dump a huge amount of current into the coil with few undesirable side effects (not as much heating, for example) Commented Dec 3, 2016 at 3:32
• I would also add that some batteries can provide lots of current, e.g. multi-cell LiPo batteries, which can provide 100A or more. The problem is that repeated abuse of such batteries is usually a hazard (read: smoke, fire, explosion). At best they just won't last very long, and batteries are comparatively expensive. Dumping energy into a capacitor, which innately has much lower ESR than a battery, means you can expend that energy very quickly and reliably, since capacitors with sufficiently high capacitance and low ESR are still quite cheap, and designed to perform such tasks repeatedly. Commented Dec 3, 2016 at 10:41

You may find coil guns with caps, for high voltage energy storage, but who said it was better? http://www.deltaveng.com/gauss-machine-gun/construction/

Comparing batteries and caps in terms of times relative to motion in a coilgun or solenoid, (>1ms)

• LiPo batteries have far higher capacitance,
• produce far more Amp-seconds.

For much shorter time constants <1ms to 1ns

• caps can offer lower ESR*C =T values and from both lower C and ESR thus more peak current and dI/dt
• but much lower energy (watt-seconds) , so in effect of little value except decoupling supply from transient dips in voltage, but not enough to sustain current > 1ms unless massive electrolytic or ultracap which if as many LoPo batteries were avail. for same size the batteries are far better.

In short (pun intended) caps are good for millions of charge dump cycles or more vs 500~10k+ for LiPo or lead acid batteries within reasonable discharge cycle depths.

So batteries are better for this application as the DCR of the coils and power source determines the initial high current and electromotive force on the iron projectile.

But do not try this unless you know the risks of exploding batteries in your face.