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Projectile Magazine plus Power Source?

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Turkey9

Sat Dec 18 2010, 12:05AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

It seems that one of the most limiting factors preventing medium to large power coilguns going from bench top experiments to hand carried recreation oriented devices is battery power. I was thinking, could the power be incorporated into the projectiles similar to cartridges in normal guns? Say, each projectile had a small battery in it that could charge the caps for each shot. That would be cool but would create a big challenge in the size of the projectile and complexity of design. Instead, how about a battery in the cartridge with enough power to charge the caps until the magazine is empty.

The idea is to have a boost converter to take the battery voltage and output a constant voltage to the charging circuit. This way, the charge time won't drop as the battery drains. If rechargeable batteries are used, the cartridge gets plugged into a charger and can be reloaded!

If the battery is going to be rechargeable, it can't drop below about 1V or the cell will be damaged if NiMH are used. That gives a total energy available of 3520J with two cells (2450 mAh) dropping in voltage from 1.2V to 1V. And with 70% efficiency in the charging circuitry, enough power to have 8 300J shots. I imagine if Lithium Ions were used even more shots could be had.

What do you think? Good idea? Worth perusing? How can the most energy be extracted from these batteries?

Thanks!

rp181

Sat Dec 18 2010, 12:29AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

how would it react to the magnetic pulses?

Turkey9

Sat Dec 18 2010, 12:36AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

Are you thinking if the battery is attached to the projectile and gets fired also? I think it should be fine as it is chemical. The magnetic field shouldn't affect the battery.

It might be easier of the batteries are part of the magazine and stay with the mechanical assembly once all the projectiles are gone. That way there will be no interaction with the magnetic field.

Sorry if what I said doesn't make sense, I'll clarify anything that's unclear!

...

Sat Dec 18 2010, 12:40AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

The batteries would probably be fine, if worse comes to worst just put the batteries at the bottom of the magazine so they they are a long ways away from the coil.

Also, I am not sure how you got 3520J, but a 2.45A/hr NiMH cell stores almost exactly 10KJ of energy, assuming you are drawing the rated current (higher discharge rates will reduce the energy available).

I am curious, where do you see people having problems with the batteries dying? Unless you need hundreds of shots, a decent sized NiMh pack (or LiPo if you can afford it) should last plenty long.

Turkey9

Sat Dec 18 2010, 12:53AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

It's also something of size and weight. Big battery packs are heavy and bulky. This is also something of a thought experiment where 100s of shots would happen. With my coilguns, it seemed like I was always replacing batteries and in my opinion it would be cool to just plug a magazine in and keep experimenting.

I got the energy because NiMH get permanently damaged if discharged below about 1V. If alkaline batteries are used, they can be discharged to zero and performance over many charge cycles is not an issue.

Saz43

Sun Dec 19 2010, 05:08AM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

Why not use LiPoly? You'll get an even better energy density than NiMH. And with a smaller battery it would be easier to fit it into a magazine.

I think the idea to store the battery in the magazine is a great idea for a portable coilgun. My portable gun (1.25kJ) ran out of battery power after about 10 to 15 shots. If I wanted to boost the charging power and go for semi-auto operation, it would have been quite lame to have to replace the battery before the ammo ran out. One thing to keep in mind though, is that charging circuits (boost converter, ZVS) actually stop functioning as soon as the input voltage dips below a certain point, not once the battery cells are exhausted. My booster quit giving a full charge at ~10V in and my ZVS quit oscillating around 10-11 volts.

Are you going to build something like this? You should-there aren't enough good portable coilgun designs IMO.

Turkey9

Mon Dec 20 2010, 06:23AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

Saz43, I think I will go with LiPoly for the reasons you stated.

The charger stopping at a certain voltage is a problem I'd like to fix. That's why I'm thinking I'll have a boost converter or something that will regulate the batter charge to something like 15V. That will then be used for the main cap charger. I know that circuits like the Joule Thief thing work with very small input voltage. Does anyone know if this is possible? Say, using an input of 8.46 (2 LiPo cells in series) to 6V and regulating that up to a constant voltage? I think I would want a cap charge voltage input of around 50V.

...

Mon Dec 20 2010, 06:50AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Turkey9 wrote ...

I got the energy because NiMH get permanently damaged if discharged below about 1V. If alkaline batteries are used, they can be discharged to zero and performance over many charge cycles is not an issue.

Not to nitpick, but I think you misunderstand how rechargeable batteries work. The A/hr rating for a battery is the usable life in the battery, generally specified down to 1V for a NiMH cell. There is almost no energy stored in the 0v-1v range, due the battery chemistry, but for a 2.5 A/hr cell, you get 9000A/s of juice, so for an average 1.1V output voltage you get just under 10KJ of usable energy.

Of course manufactures often exaggerate these values, and they are often quoted at a 1C discharge rate (who uses NiMH cells at a 1C charcarge rate

), so you might not get every last joule that you should have.

It should work just fine in any case, I may have to suggest this to my coil-cannon building friend just for the coolness factor

Turkey9

Mon Dec 20 2010, 08:31AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

Wow thanks for the corrections; I was way off. This also means that there can be way more shots than I though for a single magazine.

I believe that normal lithium batteries have a very constant discharge voltage and a sharp drop when they're dead, is this the same for LiPoly cells?

Steve Conner

Mon Dec 20 2010, 11:22AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yes. Li-Ion and Li-Poly are the same chemistry, just in a metal can or a plastic bag respectively. They are nominally about 3.6V per cell: 4.2V when completely full and 3.0V when empty. An undervoltage lockout is required because discharging below 3.0V can shorten the battery's life. I've designed battery management systems that accept either.

Li-Poly come in normal (for MP3 players etc) and high discharge (for R/C toys). You'll certainly want the high discharge kind for a coilgun power supply. I only have experience with the normal kind, the high discharge ones might be different. I know that the higher the discharge rate, the less sharp the "knee" at the end.

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