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Saz43

Wed Dec 05 2012, 04:10PM

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

Steve Conner wrote ...

The half-bridge circuit dumps the energy back into the DC bus. If you have a capacitor in parallel with your battery, you can view the recovered energy as recharging the capacitor a little bit, but mostly recharging the battery.

Keep in mind that the half bridge will dump close to peak coil current back into the battery (100's of amps), which is probably not very good since LiPos typically have a 1C charge rate. Although it will only be for a millisecond or so, so maybe it's not a big deal. Adding a capacitor in parallel to the battery makes little difference in terms of energy recovery.

Steve Conner

Wed Dec 05 2012, 05:29PM

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

How is this any worse for the battery than the equivalent discharge current? Surely for small increments of charge, the physics work the same both ways.

ben5017

Wed Dec 05 2012, 06:38PM

Registered Member #3315 Joined: Thu Oct 14 2010, 04:23PM
Location:
Posts: 156

While I am still waiting for my order of igbt's and caps to come in, I got to work building the clip and locking mechanisms. It is made from a heavily modified Nerf clip and some 1/2" plastic pipe tees. The clip snaps into place with almost no play what so ever. This system will eventually be optimized and redesigned for the portable V3CC project to come, even though this initial attempt works well. As soon as my igbt's arrive I can test the injector and clip as a complete system.

ben5017

Wed Dec 05 2012, 06:43PM

Registered Member #3315 Joined: Thu Oct 14 2010, 04:23PM
Location:
Posts: 156

Clip pics didnt upload for some reason last post, so here they are

Saz43

Sat Dec 08 2012, 03:16AM

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

Steve Conner wrote ...

How is this any worse for the battery than the equivalent discharge current? Surely for small increments of charge, the physics work the same both ways.

Does it though? Every LiPo I've ever seen specifies a separate charge and drain rate. For example, my LiPos specify a 50C discharge rate, 100C burst rate, and a 1C charge rate, which comes with a warning not to charge over 1C or else damage can occur.

See #6 under charging instructions:

Ben, I admire your use of existing hardware. Keep up the good work.

Hanzie

Sat Dec 08 2012, 04:06PM

Registered Member #6534 Joined: Sat Sept 08 2012, 12:22PM
Location: The Netherlands
Posts: 32

Saz is right, LiPos don't like high charge rates AT ALL. Though some may have a 65C continuous discharge, a charge rate of anywhere over 5 should be very much avoided. They will go boomzies with fire. Lots of fire. ^^
Also, you should always use a special LiPo-charger when charging your batteries as they need to charge differently from nickel or lead based cells.

Ash Small

Sat Dec 08 2012, 04:22PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

.....And with a firing rate of 150 rounds per minute (5 rounds in 2 seconds) the LiPo's are unlikely to be happy with that amount of current being dumped back into them.

Worst case will be fire or explosion, best case will be very short battery life, I assume.

It may well be cheaper in the long run to use non-polarised caps, and an appropriate switching circuit for the batteries, etc. if you wish to re-claim the wasted energy.

EDIT: Assuming 10% efficiency, if you use zeners to 'dump' the wasted energy, you'll need 10 times as many LiPo's for the same number of shots as you'd need if you use non-polar caps, etc. to re-claim the wasted energy anyway.

Steve Conner

Sat Dec 08 2012, 04:42PM

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

I'm pretty sure some brushless ESCs for RC cars incorporate regen braking. If your theory was right, that wouldn't be possible without destroying the batteries.

Yandersen

Sat Dec 08 2012, 07:40PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Hey Ben, yesterday night I've got enlighted by ingenious idea for your battery-driven coilgun! I've invented the circuit which will recuperate energy, shorten the discharge AND charge time for your coils, will consume less of the battery energy and will not charge it at all! See the schematic:

How it works.
As in your current design, coil is driven by IGBT transistor. Howhether, when it is switches off, current will deplete much faster due to discharge onto C2 cap. When coil is completely discharged, C2 will contain all recuperated energy. The less is C2 capacitance, the higher the voltage it will have. Then... Once projectile left the coil, you trigger SCR1 to return energy back to source - C2 will discharge through the coil in opposite direction, charging C1 to the voltage level which is higher than the battery voltage.

Wait a sec, the schematic has a bug - after current depletes, C1 will discharge back onto C2 through D2... Okay, give me some time, I think I know how to deal with this problem...

Hanzie

Sat Dec 08 2012, 07:58PM

Registered Member #6534 Joined: Sat Sept 08 2012, 12:22PM
Location: The Netherlands
Posts: 32

Steve, I've got some RC cars too. (I got way too many hobbies... xD) It's true they use the engine to brake, but as far as I know it doesn't charge the batteries with it. It's usually just very little reversed throttle. Hope that explains stuff a bit. ^^

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